Truth About Balochistan - By Aisha Baloch: Balochistan – Changing Perspectives

Truth About Balochistan - By Aisha Baloch: Balochistan – Changing Perspectives: From a person with no homeland like Tarek Fatah to every second champion of human rights is worried about the matters in Balochistan.  Bu...

GENERRAL GEOLOGY

I   INTRODUCTION

Geology, study of the planet earth, its rocky exterior, its history, and the processes that act upon it. Geology is also referred to as earth science and geoscience. The word geology comes from the Greek geo, “earth,” and logia, “the study of.” Geologists seek to understand how the earth formed and evolved into what it is today, as well as what made the earth capable of supporting life. Geologists study the changes that the earth has undergone as its physical, chemical, and biological systems have interacted during its 4.5 billion year history.

       Geology is an important way of understanding the world around us, and it enables scientists to predict how our planet will behave. Scientists and others use geology to understand how geological events and earth’s geological history affect people, for example, in terms of living with natural disasters and using the earth’s natural resources. As the human population grows, more and more people live in areas exposed to natural geologic hazards, such as floods, earthquakes, tsunamis, volcanoes, and landslides. Some geologists use their knowledge to try to understand these natural hazards and forecast potential geologic events, such as volcanic eruptions or earthquakes. They study the history of these events as recorded in rocks and try to determine when the next eruption or earthquake will occur. They also study the geologic record of climate change in order to help predict future changes. As human population grows, geologists’ ability to locate fossil and mineral resources, such as oil, coal, iron, and aluminum, becomes more important. Finding and maintaining a clean water supply, and disposing safely of waste products, requires understanding the earth’s systems through which they cycle.

       The field of geology includes subfields that examine all of the earth's systems, from the deep interior core to the outer atmosphere, including the hydrosphere (the waters of the earth) and the biosphere (the living component of earth). Generally, these subfields are divided into the two major categories of physical and historical geology. Geologists also examine events such as asteroid impacts, mass extinctions, and ice ages. Geologic history shows that the processes that shaped the earth are still acting on it and that change is normal.

Many other scientific fields overlap extensively with geology, including oceanography, atmospheric sciences, physics, chemistry, botany, zoology, and microbiology. Geology is also used to study other planets and moons in our solar system. Specialized fields of extraterrestrial geology include lunar geology, the study of earth’s moon, and astrogeology, the study of other rocky bodies in the solar system and beyond. Scientific teams currently studying Mars and the moons of Jupiter include geologists.

II   GUIDING PRINCIPLES OF GEOLOGY

Geologists use three main principles, or concepts, to study earth and its history. The first concept, called plate tectonics, is the theory that the earth’s surface is made up of separate, rigid plates moving and floating over another, less rigid layer of rock. These plates are made up of the continents and the ocean floor as well as the rigid rock beneath them. The second guiding concept is that many processes that occur on the earth may be described in terms of recycling: the reuse of the same materials in cycles, or repeating series of events. The third principle is called uniformitarianism. Uniformitarianism states that the physical and chemical processes that have acted throughout geologic time are the same processes that are observable today. Because of this, geologists can use their knowledge of what is happening on the earth right now to help explain what happened in the past.

A   Plate Tectonics

Plate tectonics is the unifying theory of geology. It was established in the 1960s, making it one of the most recent revolutions in all of science. The theory describes the lithosphere (the outer rocky layer of the earth) as a collection of rigid plates that move sideways above a less rigid layer called the asthenosphere. The asthenosphere is made up of rock that is under tremendous pressure, which softens it and allows it to move and circulate slowly. Plate tectonics is useful in the field of geology because it can be used to explain a variety of geologic processes, including volcanic activity, earthquakes, and mountain building. See also Earth.

B

Geologic Cycles

A second guiding principle of geology is the principle of recycling materials, or using materials many times. All processes in geology can be viewed as a series of mostly closed cycles, meaning the materials of the cycles are found on earth, and very few materials from outside our world are introduced into these cycles. The energy that drives geologic recycling comes from two sources: the sun and the earth's interior. Two examples of geologic cycles are the rock cycle and the water cycle.

The rock cycle begins as rocks are uplifted, or pushed up by tectonic forces. The exposed rocks erode as a result of surface processes, such as rain and wind. The eroded particles, or sediment, travel by wind or moving water until they are deposited, and the deposited material settles into layers. Additional sediment may bury these layers until heat and pressure metamorphose, or change, the underlying sediment to metamorphic rock. Additional sediment may compact the layers into sedimentary rocks. Rocks can also be subducted (sunk down into the lower layers of the earth) by plate tectonic processes. Buried and subducted rocks may also melt and recrystallize into igneous rocks (see Magma). Metamorphic, sedimentary, and igneous rocks may then be uplifted, starting the rock cycle again.

The water cycle is also known as the hydrologic cycle. Phases of the water cycle are storage, evaporation, precipitation, and runoff. Water is stored in glaciers, polar ice caps, lakes, rivers, oceans, and in the ground. Heat from the sun evaporates water from the earth’s surface and the water then condenses to form clouds. It falls back to the earth as precipitation, either as rain or snow, then runs into the oceans through rivers or underground and begins the cycle again.

C

Uniformitarianism

Uniformitarianism, or actualism, helps geologists use their knowledge of modern processes and events to reconstruct the past. The principle of uniformitarianism depends on the "uniformity of laws," which assumes that the laws of physics and chemistry have remained constant. To test uniformity of laws, geologists can examine preserved one-billion-year-old ripples that look very much like ripples on the beach today. If gravity had changed, water and sand would have interacted differently in the past, and the ripple evidence would be different. Also, minerals in three-billion-year-old rocks are the same as minerals forming in rocks today, confirming the uniformity of chemical laws. Uniformitarianism contrasts with, for example, the idea that past events such as floods or earthquakes were caused by divine intervention or supernatural causes. Catastrophism, which calls on major catastrophes to explain earth’s history, is also sometimes contrasted with uniformitarianism. However, uniformitarianism can include past catastrophes.

III

THE GEOLOGIC TIME SCALE

Geologists have created a geologic time scale to provide a common vocabulary for talking about past events. The practice of determining when past geologic events occurred is called geochronology. This practice began in the 1700s and has sometimes involved some personal and international disputes that led to differences in terminology. Today the geologic time scale is generally agreed upon and used by scientists around the world, dividing time into eons, eras, periods, and epochs. Every few years, the numerical time scale is refined based on new evidence, and geologists publish an update.

Geologists use several methods to determine geologic time. These methods include physical stratigraphy, or the placement of events in the order of their occurrence, and biostratigraphy, which uses fossils to determine geologic time. Another method geologists use is correlation, which allows geologists to determine whether rocks in different geographic locations are the same age. In radiometric dating, geologists use the rate of decay of certain radioactive elements in minerals to assign numerical ages to the rocks.

The process of determining geologic time includes several steps. Geologists first determine the relative age of rocks—which rocks are older and which are younger. They then may correlate rocks to determine which rocks are the same age. Next, they construct a geologic time scale. Finally, they determine the specific numerical ages of rocks by various dating methods and assign numbers to the time scale.

A

Relative Time

Geologists create a relative time scale using rock sequences and the fossils contained within these sequences. The scale they create is based on The Law of Superposition, which states that in a regular series of sedimentary rock strata, or layers, the oldest strata will be at the bottom, and the younger strata will be on top. Danish geologist Nicolaus Steno (also called Niels Stensen) used the idea of uniformity of physical processes. Steno noted that sediment was denser than liquid or air, so it settled until it reached another solid. The newer sediment on the top layer is younger than the layer it settled upon. Since this is what happens in the world today, it should also determine how rock layers formed in the past. Crosscutting relationships are also used to determine the relative age of rocks. For instance, if a thin intrusion of granite, called a dike, cuts through a layer of limestone, the granite must be younger than the limestone.

B

Biostratigraphy

In the field of biostratigraphy geologists study the placement of fossils to determine geologic time. British surveyor William Smith and French anatomist Georges Cuvier both reasoned that in a series of fossil-bearing rocks, the oldest fossils are at the bottom, with successively younger fossils above. They thus extended Steno's Law of Superposition and recognized that fossils could be used to determine geologic time. This principle is called fossil succession. Smith and Cuvier also noted that unique fossils were characteristic of different layers. Biostratigraphy is most useful for determining geologic time during the Phanerozoic Eon (Greek phaneros, “evident”; zoic, “life”), the time of visible and abundant fossil life that has lasted for about the past 570 million years. Although fossils exist that are as old as three billion years or more, they are not common. Few fossils exist that are useful for determining geologic age from time before about 1 billion years ago, so biostratigraphy is of limited use in older sedimentary rocks.

C

Correlation

Using correlation to determine which rocks are of equal age is important for reconstructing snapshots in geologic history. Correlation may use the physical characteristics of rocks or fossils to determine equivalent age. For example, the limestone at the top of one side of the Grand Canyon can be correlated to the opposite side of the canyon. Also, ash from a volcanic eruption can be correlated over long distances and wide areas. Fossils are the most useful tools for correlation. Since the work of Smith and Cuvier, biostratigraphers have noted that "like fossils are of like age.” This is the principle of fossil correlation.

D

Radiometric Dating

Another fundamental goal of geochronology is to determine numerical ages of rocks and to assign numbers to the geologic time scale. The primary tool for this task is radiometric dating, in which the decay of radioactive elements is used to date rocks and minerals. Radiometric dating works best on igneous rocks (rocks that crystallized from molten material). It can also be used to date minerals in metamorphic rocks (rocks that formed when parent rock was submitted to intense heat and pressure and metamorphosed into another type of rock). It is of limited use, however, in sedimentary rocks formed by the compaction of layers of sediment. One of the great triumphs of geochronology is that numbers acquired by radiometric dating matched predictions based on superposition and other means of geologic age determination, confirming the assumption of uniformitarianism. Using dated rocks, geologists have been able to assign numbers to the geologic time scale. See also Dating Methods.

IV

GEOLOGIC SPATIAL SCALES

In order to understand geologic processes and to reconstruct the geologic past, geologists work at different spatial, or size, scales—scales that range from microscopic to planetary. In order to work at these spatial scales, they use a number of tools. At the microscopic level, traditional tools include the petrographic microscope, used to identify minerals and examine rock textures. Modern tools for examining rock chemistry and structure include complex scanning electron microscopes, microprobes that can obtain very small geologic or mineralogic samples, and mass spectrometers (instruments that measure the quantity of atoms, or groups of atoms, in a geologic sample). Geologists can also use lasers and particle accelerators for high-precision work, such as in argon-argon radiometric dating, the use of isotopes of the element argon to date geologic samples.

Some geologic features are very large, and geologists must create detailed maps to observe them completely. Geologists use maps to record basic information, to examine trends, and to understand processes and geologic history. For example, a map may record the locations of historical earthquakes, helping to identify faults. Geologic maps can help geologists understand the history of a mountain belt or locate new mineral deposits. On a planetary scale, geologists can map the earth’s surface using data from orbiting satellites. Geologists also make maps reconstructing a view of the earth at some time in the past; such maps are called paleogeographic maps. Geologists who study Mars map the planet’s surface features with the help of images and information from spacecraft probes sent to Mars.

Traditionally, maps have resulted from fieldwork. In the field, geologists locate exposures of rock, or rock outcrops, and features such as faults, folds, or other geologic structures on a base map or aerial photograph. Mapping has improved through the use of remote sensing techniques, such as radar and infrared mapping from aircraft and satellites, and this in turn has helped geologists better understand the earth. Geologists can now determine latitude and longitude positions on the earth by using the global positioning system of satellites (GPS). Map information can now be stored digitally, as in geographic information systems (GIS). Subsurface, or underground, mapping is becoming more common. This technique uses drilled cores and sound waves sent below the ground to map structures such as faults.

V

FIELDS OF GEOLOGY

Geologists have found it useful to divide geology into two main fields: physical geology, which examines the nature of the earth in its present state, and historical geology, which examines the changes the earth has undergone throughout time.

A

Physical Geology

Physical geology can be subdivided into a number of disciplines according to the way geologists study the earth and which physical aspects they study. Fields such as geophysics, geochemistry, mineralogy and petrology, and structural geology apply the sciences of physics and chemistry to study aspects of the earth. Hydrology, geomorphology, and marine geology incorporate the study of water and its effects on weathering into geology, while environmental, economic, and engineering geology apply geologic knowledge and engineering principles to solve practical problems.

A1

Geophysics

In the field of geophysics, geologists apply the concepts of physics to the study of the earth. Geophysics is such a broad field that scientists sometimes consider it a separate field from geology. The largest subdiscipline in geophysics is seismology, the study of the travel of seismic waves through the earth. Seismic waves are generated naturally by earthquakes, or they can be made artificially by explosions from bombs or air guns. Seismologists study earthquakes and construct models of the earth's interior using seismic techniques. Geophysics also includes the study of the physics of materials such as rocks, minerals, and ice within the fields of petrology, mineralogy, and glaciology. Geophysicists study the behavior of the planet’s oceans, atmosphere, and volcanoes. Specialists called volcanologists study the world’s volcanoes and try to predict eruptions by using seismology and other remote sensing techniques, such as satellite imagery. Monitoring active volcanoes is especially important in highly populated areas.

A2

Geochemistry

Geochemistry is the application of chemistry to the study of the earth, its materials, and the cycling of chemicals through its systems. It is essential in numerical dating and in reconstructing past conditions on the earth. Geochemistry is important for tracing the transport of chemicals through the earth’s four component systems: the lithosphere (rocky exterior), the hydrosphere (waters of the earth), the atmosphere (air), and the biosphere (the system of living things). Biogeochemistry is an emerging field that examines the chemical interactions between living and nonliving systems—for example, microorganisms that act in soil formation. Geochemistry has important applications in environmental and economic geology as well as in the fields of mineralogy and petrology.

A3

Mineralogy and Petrology

The fields of mineralogy (the study of minerals) and petrology (the study of rocks) are closely related because rocks are made of minerals. Mineralogists and petrologists study the origin, occurrence, structure, and history of rocks or minerals. They attempt to understand the physical, chemical, and less commonly, biological conditions under which geologic materials form. Mineralogy is important for understanding natural materials and is also used in the materials engineering field, such as in ceramics. Petrology focuses on two of the three rock types: igneous rocks—rocks made from molten material—and metamorphic rocks—those rocks that have been changed by high temperatures or pressures. The third rock type, sedimentary rocks, are the focus of sedimentary geology, commonly classified under historical geology.

A4

Structural Geology

Structural geology deals with the form, arrangement, and internal structure of rocks, including their history of deformation, such as folding and faulting. Structural geology includes everything from field mapping to the study of microscopic deformation within rocks. Most geologic reconstructions require an understanding of structural geology. The term tectonics is commonly used for large-scale structural geology, such as the study of the history of a mountain belt, or plate tectonics (the study of the crustal plates). Neotectonics is the study of recent faulting and deformation; such studies can reconstruct the history of active faults, and the history can be used in hazard analysis and land-use planning.

A5

Hydrology and Geomorphology

The earth's surface processes are the focus of hydrology and geomorphology. Hydrology is the study of water on the earth's surface, excluding the oceans. Hydrogeology is the study of groundwater (water under the ground) and the geologic processes of surface water. As water is necessary for life, hydrology and hydrogeology are important for economic and environmental reasons, such as maintaining a clean water supply. Geomorphology is the examination of the development of present landforms; geomorphologists attempt to understand the nature and origin of these landforms. They may work from the large scale of mountain belts to the small scale of rill marks (small grooves in sand). Geomorphologists commonly specialize in one of many areas, such as in glacial or periglacial (near glaciers), fluvial (river), hillslope, or coastal processes. Their work is important for a basic understanding of the active surface that humans live on, a surface that is subject to erosion, landslides, floods, and other processes that affect our daily lives.

A6

Marine Geology

Geology specific to the ocean environment is called marine geology. Marine geologists may be specialists in a number of fields, including petrology, sedimentology, stratigraphy, paleontology, geochemistry, geophysics, and volcanology. They may take samples from the ocean while out at sea or make measurements through remote sensing techniques. Drilling platforms and drilling ships allow earth scientists to make more-detailed studies of the history of the oceans and the ocean floor. For example, in 1984 an international team of geoscientists from 20 nations formed the Ocean Drilling Program, an outgrowth of the earlier Deep Sea Drilling Program. This program is designed to set up drilling through the top sedimentary layer and the ocean crust in deep-sea sites around the world. This work has helped the field of paleoceanography (the reconstruction of the history of the oceans, including ancient ocean chemistry, temperature, circulation, and biology). See also Ocean and Oceanography.

A7

Environmental, Economic, and Engineering Geology

The application of geologic knowledge to practical problems is the focus of the fields of environmental, economic, and engineering geology. Environmental geology involves the protection of human health and safety through understanding geological processes. For example, it is critically important to understand the geology of areas where people propose to store nuclear waste products. The study of geologic hazards, such as earthquakes and volcanic eruptions, can also be considered part of environmental geology. Economic geology is the use of geologic knowledge to find and recover materials that can be used profitably by humans, including fuels, ores, and building materials. Because these products are so diverse, economic geologists must be broadly trained; they commonly specialize in a particular aspect of economic geology, such as petroleum geology or mining geology. Engineering geology is the application of engineering principles to geologic problems. Two fields of engineering that use geology extensively are civil engineering and mining engineering. For example, the stability of a building or bridge requires an understanding of both the foundation material (rocks, soil) and the potential for earthquakes in the area. See also Engineering: Geological and Mining Engineering.

B

Historical Geology

Historical geology focuses on the study of the evolution of earth and its life through time. Historical geology includes many subfields. Stratigraphy and sedimentary geology are fields that investigate layered rocks and the environments in which they are found. Geochronology is the study of determining the age of rocks, while paleontology is the study of fossils. Other fields, such as paleoceanography, paleoseismology, paleoclimatology, and paleomagnetism, apply geologic knowledge of ancient conditions to learn more about the earth. The Greek prefix paleo is used to identify ancient conditions or periods in time, and commonly means “the reconstruction of the past.”

B1

Stratigraphy

Stratigraphy is the study of the history of the earth's crust, particularly its stratified (layered) rocks. Stratigraphy is concerned with determining age relationships of rocks as well as their distribution in space and time. Rocks may be studied in an outcrop but commonly are studied from drilled cores (samples that have been collected by drilling into the earth). Most of the earth's surface is covered with sediment or layered rocks that record much of geologic history; this is what makes stratigraphy important. It is also important for many economic and environmental reasons. A large portion of the world's fossil fuels, such as oil, gas, and coal, are found in stratified rocks, and much of the world's groundwater is stored in sediments or stratified rocks.

Stratigraphy may be subdivided into a number of fields. Biostratigraphy is the use of fossils for age determination and correlation of rock layers; magnetostratigraphy is the use of magnetic properties in rocks for similar purposes. Newer fields in stratigraphy include chemostratigraphy, seismic stratigraphy, and sequence stratigraphy. Chemostratigraphy uses chemical properties of strata for age determination and correlation as well as for recognizing events in the geologic record. For example, oxygen isotopes (forms of oxygen that contain a different number of neutrons in the nuclei of atoms) may provide evidence of an ancient paleoclimate. Carbon isotopes may identify biologic events, such as extinctions. Rare chemical elements may be concentrated in a marker layer (a distinctive layer that can be correlated over long distances). Seismic stratigraphy is the subsurface study of stratified rocks using seismic reflection techniques. This field has revolutionized stratigraphic studies since the late 1970s and is now used extensively both on land and offshore. Seismic stratigraphy is used for economic reasons, such as finding oil, and for scientific studies. An offshoot of seismic stratigraphy is sequence stratigraphy, which helps geologists reconstruct sea level changes throughout time. The rocks used in sequence stratigraphy are bounded by, or surrounded by, surfaces of erosion called unconformities.

B2   Sedimentology

Sedimentology, or sedimentary geology, is the study of sediments and sedimentary rocks and the determination of their origin. Sedimentary geology is process oriented, focusing on how sediment was deposited. Sedimentologists are geologists who attempt to interpret past environments based on the observed characteristics, called facies, of sedimentary rocks. Facies analysis uses physical, chemical, and biological characteristics to reconstruct ancient environments. Facies analysis helps sedimentologists determine the features of the layers, such as their geometry, or layer shape; porosity, or how many pores the rocks in the layers have; and permeability, or how permeable the layers are to fluids. This type of analysis is important economically for understanding oil and gas reservoirs as well as groundwater supplies.

B3

Geochronology

The determination of the age of rocks is called geochronology. The fundamental tool of geochronology is radiometric dating (the use of radioactive decay processes as recorded in earth materials to determine the numerical age of rocks). Most radiometric dating techniques are useful in dating igneous and metamorphic rocks and minerals. One type of non-radiometric dating, called strontium isotope dating, measures different forms of the element strontium in sedimentary materials to date the layers. Geologists also have ways to determine the ages of surfaces that have been exposed to the sun and to cosmic rays. These methods are called thermoluminescence dating and cosmogenic isotope dating. Geologists can count the annual layers recorded in tree rings, ice cores, and certain sediments such as those found in lakes, for very precise geochronology. However, this method is only useful for time periods up to tens of thousands of years. Some geoscientists are now using Milankovitch cycles (the record of change in materials caused by variations in the earth's orbit) as a geologic time clock. See also Dating Methods: Radiometric Dating.

B4   Paleontology and Paleobiology

Paleontology is the study of ancient or fossil life. Paleobiology is the application of biological principles to the study of ancient life on earth. These fields are fundamental to stratigraphy and are used to reconstruct the history of organisms' evolution and extinction throughout earth history. The oldest fossils are older than 3 billion years, although fossils do not become abundant and diverse until about 500 million years ago. Different fossil organisms are characteristic of different times, and at certain times in earth history, there have been mass extinctions (times when a large proportion of life disappears). Other organisms then replace the extinct forms. The study of fossils is one of the most useful tools for reconstructing geologic history because plants and animals are sensitive to environmental changes, such as changes in the climate, temperature, food sources, or sunlight. Their fossil record reflects the world that existed while they were alive. Paleontology is commonly divided into vertebrate paleontology (the study of organisms with backbones), invertebrate paleontology (the study of organisms without backbones), and micropaleontology (the study of microscopic fossil organisms). Many other subfields of paleontology exist as well. Paleobotanists study fossil plants, and palynologists study fossil pollen. Ichnology is the study of trace fossils—tracks, trails, and burrows left by organisms. Paleoecology attempts to reconstruct the behavior and relationships of ancient organisms.

B5

Paleoceanography and Paleoclimatology

Paleoceanography (the study of ancient oceans) and paleoclimatology (the study of ancient climates) are two subfields that use fossils to help reconstruct ancient conditions. Scientists also study stable isotopes, or different forms, of oxygen to reconstruct ancient temperatures. They use carbon and other chemicals to reconstruct aspects of ancient oceanographic and climatic conditions. Detailed paleoclimatic studies have used cores from ice sheets in Antarctica and Greenland to reconstruct the last 200,000 years. Ocean cores, tree rings, and lake sediments are also useful in paleoclimatology. Geologists hope that by understanding past oceanographic and climatic changes, they can help predict future change.

VI

HISTORY OF GEOLOGY

Geology originated as a modern scientific discipline in the 18th century, but humans have been collecting systematic knowledge of the earth since at least the Stone Age. In the Stone Age, people made stone tools and pottery, and had to know which materials were useful for these tasks. Between the 4th century and 1st century bc, ancient Greek and Roman philosophers began the task of keeping written records relating to geology. Throughout the medieval and Renaissance periods, people began to study mineralogy and made detailed geologic observations. The 18th and 19th centuries brought widespread study of geology, including the publication of Charles Lyell’s book Principles of Geology, and the National Surveys (expeditions that focused on the collection of geologic and other scientific data). The concept of geologic time was further developed during the 19th century as well. At the end of the 19th century and into the 20th century, the field of geology expanded even more. During this time, geologists developed the theories of continental drift, plate tectonics, and seafloor spreading.

A

Ancient Greek and Roman Philosophers

In western science, the first written records of geological thought come from the Greeks and Romans. In the 1st century bc, for example, Roman architect Vitruvius wrote about building materials such as pozzolana, a volcanic ash that Romans used to make hydraulic cement, which hardened under water. Historian Pliny the Elder, in his encyclopedia, Naturalis Historia (Natural History), summarized Greek and Roman ideas about nature.

Science as an organized system of thought can trace its roots back to the Greek philosopher Aristotle. In the 4th century bc Aristotle developed a philosophical system that explained nature in a methodical way. His system proposed that the world is made of four elements (earth, air, fire, and water), with four qualities (cold, hot, dry, and wet), and four causes (material, efficient, formal, and final). According to Aristotle, elements could change into one another, and the earth was filled with water and air, which could rush about and cause earthquakes. Other philosophers of this era who wrote about earth materials and processes include Aristotle's student Theophrastus, the author of an essay on stones.

B

Chinese Civilizations

Chinese civilizations developed ideas about the earth and technologies for studying the earth. For example, in 132 AD the Chinese philosopher Chang Heng invented the earliest known seismoscope. This instrument had a circle of dragons holding balls in their mouths, surrounded by frogs at the base. The balls would drop into the mouths of frogs when an earthquake occurred. Depending on which ball was dropped, the direction of the earthquake could be determined.

C

Medieval and Renaissance Periods

The nature and origin of minerals and rocks interested many ancient writers, and mineralogy may have been the first systematic study to arise in the earth sciences. The Saxon chemist Georgius Agricola wrote De Re Metallica (On the Subject of Metals) following early work by both the Islam natural philosopher Avicenna and the German naturalist Albertus Magnus. De Re Metallica was published in 1556, a year after Agricola’s death. Many consider this book to be the foundation of mineralogy, mining, and metallurgy.

Medieval thought was strongly influenced by Aristotle, but science began to move in a new direction during the Renaissance Period. In the early 1600s, English natural philosopher Francis Bacon reasoned that detailed observations were required to make conclusions. Around this time French philosopher René Descartes argued for a new, rational system of thought. Most natural philosophers, or scientists, in this era studied many aspects of philosophy and science, not focusing on geology alone.

Studies of the earth during this time can be placed in three categories. The first, cosmology, proposed a structure of the earth and its place in the universe. As an example of a cosmology, in the early 1500s Polish astronomer Nicolaus Copernicus proposed that the earth was a satellite in a sun-centered system. The second category, cosmogony, concerned the origin of the earth and the solar system. The Saxon mathematician and natural philosopher Gottfried Wilhelm, Baron von Leibniz, in a cosmogony, described an initially molten earth, with a crust that cooled and broke up, forming mountains and valleys. The third category of study was in the tradition of Francis Bacon, and it involved detailed observations of rocks and related features. English scientist Robert Hooke and Danish anatomist and geologist Nicolaus Steno (Niels Stenson) both made observations in the 17th century of fossils and studied other geologic topics as well. In the 17th century, mineralogy also continued as an important field, both in theory and in practical matters, for example, with the work of German chemist J. J. Becher and Irish natural philosopher Robert Boyle.

D

Geology in the 18th and 19th Centuries

By the 18th century, geological study began to emerge as a separate field. Italian mining geologist Giovanni Arduino, Prussian chemist and mineralogist Johan Gottlob Lehmann, and Swedish chemist Torbern Bergman all developed ways to categorize the layers of rocks on the earth's surface. The German physician Georg Fuchsel defined the concept of a geologic formation—a distinctly mappable body of rocks. The German scientist Abraham Gottlob Werner called himself a geognost (a knower of the earth). He used these categorizations to develop a theory that the earth's layers had precipitated from a universal ocean. Werner's system was very influential, and his followers were known as Neptunists. This system suggested that even basalt and granite were precipitated from water. Others, such as English naturalists James Hutton and John Playfair, argued that basalt and granite were igneous rocks, solidified from molten materials, such as lava and magma. The group that held this belief became known as Volcanists or Plutonists.

By the early 19th century, many people were studying geologic topics, although the term geologist was not yet in general use. Scientists, such as Scottish geologist Charles Lyell, and French geologist Louis Constant Prevost, wanted to establish geology as a rational scientific field, like chemistry or physics. They found this goal to be a challenge in two important ways. First, some people wanted to reconcile geology with the account of creation in Genesis (a book of the Old Testament) or wanted to use supernatural explanations for geologic features. Second, others, such as French anatomist Georges Cuvier, used catastrophes to explain much of earth’s history. In response to these two challenges, Lyell proposed a strict form of uniformitarianism, which assumed not only uniformity of laws but also uniformity of rates and conditions. However, assuming the uniformity of rates and conditions was incorrect, because not all processes have had constant rates throughout time. Also, the earth has had different conditions throughout geologic time—that is, the earth as a rocky planet has evolved. Although Lyell was incorrect to assume uniformity of rates and conditions, his well reasoned and very influential three-volume book, Principles of Geology, was published and revised 11 times between 1830 and 1872. Many geologists consider this book to mark the beginning of geology as a professional field.

Although parts of their theories were rejected, Abraham Gottlob Werner and Georges Cuvier made important contributions to stratigraphy and historical geology. Werner's students and followers went about attempting to correlate rocks according to his system, developing the field of physical stratigraphy. Cuvier and his co-worker Alexandre Brongniart, along with English surveyor William Smith, established the principles of biostratigraphy, using fossils to establish the age of rocks and to correlate them from place to place. Later, with these established stratigraphies, geologists used fossils to reconstruct the history of life's evolution on earth.

E

Age of Geologic Exploration

In the late 18th and the 19th centuries, naturalists on voyages of exploration began to make important contributions to geology. Reports by German natural historian Alexander von Humboldt about his travels influenced the worlds of science and art. The English naturalist Charles Darwin, well known for his theory of evolution, began his scientific career on the voyage of the HMS Beagle, where he made many geological observations. American geologist James Dwight Dana sailed with the Wilkes Expedition throughout the Pacific and made observations of volcanic islands and coral reefs. In the 1870s, the HMS Challenger was launched as the first expedition specifically to study the oceans.

Expeditions on land also led to new geologic observations. Countries and states established geological surveys in order to collect information and map geologic resources. For example, in the 1860s and 1870s Clarence King, Ferdinand V. Hayden, John Wesley Powell, and George Wheeler conducted four surveys of the American West. These surveys led to several new concepts in geology. American geologist Grove Karl Gilbert described the Basin and Range Province and first recognized laccoliths (round igneous rock intrusions). Reports also came back of spectacular sites such as Yellowstone, Yosemite, and the Grand Canyon, which would later become national parks. Competition between these survey parties finally led the Congress of the United States to establish the U.S. Geological Survey in 1879.

F

Geologic Time

Determining the age of the earth became a renewed scholarly effort in the 19th century. Unlike the Greeks and most eastern philosophers, who considered the earth to be eternal, western philosophers believed that the planet had a definite beginning and must have a measurable age. One way to measure this age was to count generations in the Bible, as the Anglican Archbishop James Ussher did in the 1600s, coming up with a total of about 6000 years. In the 1700s, French natural scientist George Louis Leclerc (Comte de Buffon) tried to measure the age of the earth. He calculated the time it would take the planet to cool based on the cooling rates of iron balls and came up with 75,000 years. During the 18th century, James Hutton argued that processes such as erosion, occurring at observed rates, indicated an earth that was immeasurably old. By the early 19th century, geologists commonly spoke in terms of "millions of years." Even religious professors, such as English clergyman and geologist William Buckland, referred to this length of time.

Other means for calculating the age of the earth used in the 19th century included determining how long it would take the sea to become salty and calculating how long it would take for thick piles of sediment to accumulate. Irish physicist William Thomson (Lord Kelvin) returned to Buffon's method and calculated that the earth was no more than 100 million years old. Meanwhile, Charles Darwin and others argued that evolution proceeded slowly enough that it required at least hundreds of millions of years.

With the discovery of radioactivity in 1896 by French physicist Henri Becquerel, scientists, such as British physicist Ernest Rutherford and American radiochemist Bertram Boltwood, recognized that the ages of minerals and rocks could be determined by radiometric dating. By the early 20th century, Boltwood had dated some rocks to be more than 2 billion years old. During this time, English geologist Arthur Holmes began a long career of refining the dates on the geologic time scale, a practice that continues to this day.

G   Theory of Continental Drift

In 1910 American geologist Frank B. Taylor proposed that lateral (sideways) motion of continents caused mountain belts to form on their front edges. Building on this idea in 1912, German meteorologist Alfred Wegener proposed a theory that came to be known as Continental Drift: He proposed that the continents had moved and were once part of one, large supercontinent called Pangaea. Wegener was attempting to explain the origin of continents and oceans when he expanded upon Taylor’s idea. His evidence included the shapes of continents, the physics of ocean crust, the distribution of fossils, and paleoclimatology data.

Continental drift helped to explain a major geologic issue of the 19th century: the origin of mountains. Theories commonly called on the cooling and contracting of the earth to form mountain chains. The mountain-building theories of German geologist Leopold von Buch and French geologist Leonce Elie de Beaumont were catastrophic in nature. American geologists James Hall and James Dwight Dana proposed the geosynclinal theory of mountain building—a theory based on the downward bending of the earth’s crust (a geosyncline). Austrian geologist Eduard Suess developed a related theory. Hall, Dana, and Suess believed that continents and ocean basins were ancient, permanent features on earth and that mountain belts formed at their edges.

Most geologists did not accept the theory of continental drift in the 1920s and 1930s. British geologist Arthur Holmes supported continental drift and proposed that convection (a type of heat movement) inside the earth drove continental drift. Others who favored the idea included South African geologist Alex du Toit, who studied geologic evidence for the southern continents of Gondwanaland, part of the hypothetical supercontinent Pangaea. Other scientists, such as British geophysicist Harold Jeffreys, argued that continental drift was physically impossible. Paleontologists, such as American George Gaylord Simpson, said that the distribution of fossils could be explained by other means.

H   Theory of Seafloor Spreading

After World War II, geophysical evidence began to accumulate that confirmed the lateral motion of continents and indicated the young age of oceanic crust. This evidence led to the theories of seafloor spreading and plate tectonics in the 1960s. American marine geologists Robert S. Dietz and Harry H. Hess proposed the seafloor spreading hypothesis, the concept that the oceanic crust is created as the seafloor spreads apart along midocean ridges. American oceanographers Bruce C. Heezen, Marie Tharp, and others prepared detailed maps of the ocean floors and the mid-Atlantic ridge and rift system, a mountainous chain found throughout the ocean. These maps provided additional evidence that seemed to support the continental drift theory. Further evidence came from paleomagnetism, the record of the orientation of earth's magnetic field recorded in rocks. In the 1950s, British geophysicist S. Keith Runcorn determined that this evidence indicated that the continents had moved relative to the earth’s magnetic poles and to each other. British marine geophysicists Fred J. Vine and Drummond Matthews described the record of changes in the earth’s magnetic field when they discovered “magnetic stripes” formed at spreading centers of the mid-ocean ridges, leading to the Vine-Matthews hypothesis. Magnetic stripes were also independently described by Canadian geophysicist Lawrence Morley and confirmed by American marine geologist Walter Pitman and others. These stripes indicated reversals of the direction of the earth’s magnetic field recorded in rock as new ocean crust was created at mid-ocean ridges. Scientists used paleomagnetism and seafloor spreading to determine that the continents had moved relative to the magnetic poles and to each other.

I   Theory of Plate Tectonics

Canadian geophysicist J. Tuzlo Wilson and American geophysicist Jason Morgan, among others, proposed the outline of the theory of plate tectonics in the 1960s. This theory stated that the earth’s lithosphere is made up of several rigid plates. These plates slide and move over a less-rigid layer called the asthenosphere. A plate may be composed entirely of oceanic crust, like the Pacific Plate, or of part ocean crust and part continental crust, like the North American Plate. New ocean crust is generated at ocean ridges (underwater mountain chains formed by the young ocean crust). Older ocean crust sinks down, or subducts, into the earth’s mantle at subduction zones, which are found at the deepest parts of the ocean, called trenches. As the plates move, they collide and form mountains. The plates recycle crust, generate volcanoes, and move past each other along faults. Using satellites, scientists can now measure movement of the continental plates in centimeters per year. Plate boundaries are the sites of most of the earth's earthquakes and the majority of earth's volcanoes. The continents are made of remelted sediments and partially melted oceanic crust, forming a lower density layer that has collected through time. The mechanism that drives the earth’s crustal plates is still not known, but geologists can use plate tectonics to explain most geologic activity. See also Earth.

J   Earth as a Planetary Body

The full recognition by scientists of earth as a planetary body, combining the fields of solar-system astronomy and geology, is perhaps the latest revolution in the earth sciences. Although scientists have recognized earth as a planet for centuries, space exploration that began in the 1960s created a new view of the earth. Photographs of earth taken from space had a profound effect on how people saw the earth. The exploration of neighboring moons and planets has led to a new understanding of the earth as an evolving planet.

Contributed By:
Joanne Bourgeois

Igneous Rock

I   INTRODUCTION

Igneous Rock, rock formed when molten or partially molten material, called magma, cools and solidifies. Igneous rocks are one of the three main types of rocks; the other types are sedimentary rocks and metamorphic rocks. Of the three types of rocks, only igneous rocks are formed from melted material. The two most common types of igneous rocks are granite and basalt. Granite is light colored and is composed of large crystals of the minerals quartz, feldspar, and mica. Basalt is dark and contains minute crystals of the minerals olivine, pyroxene, and feldspar.

II   TYPES OF IGNEOUS ROCKS

Geologists classify igneous rocks according to the depth at which they formed in the earth’s crust. Using this principle, they divide igneous rocks into two broad categories: those that formed beneath the earth’s surface, and those that formed at the surface. Igneous rocks may also be classified according to the minerals they contain.

A

Classification by Depth of Formation

Rocks formed within the earth are called intrusive or plutonic rocks because the magma from which they form often intrudes into the neighboring rock. Rocks formed at the surface of the earth are called extrusive rocks. In extrusive rocks, the magma has extruded, or erupted, through a volcano or fissure.

Geologists can tell the difference between intrusive and extrusive rocks by the size of their crystals: crystals in intrusive rocks are larger than those in extrusive rocks. The crystals in intrusive rocks are larger because the magma that forms them is insulated by the surrounding rock and therefore cools slowly. This slow cooling gives the crystals time to grow larger. Extrusive rocks cool rapidly, so the crystals are very small. In some cases, the magma cools so rapidly that crystals have no time to form, and the magma hardens in an amorphous glass, such as obsidian.

One special type of rock, called porphyry, is partly intrusive and partly extrusive. Porphyry has large crystals embedded in a mass of much smaller crystals. The large crystals formed underground and only melt at extremely high temperatures. They were carried in lava when it erupted. The mass of much smaller crystals formed around the large crystals when the lava cooled quickly above ground.

B

Classification by Composition

Geologists also classify igneous rocks based on the minerals the rocks contain. If the mineral grains in the rocks are large enough, geologists can identify specific minerals by eye and easily classify the rocks by their mineral composition. However, extrusive rocks are generally too fine-grained to identify their minerals by eye. Geologists must classify these rocks by determining their chemical composition in the laboratory.

Most magmas are composed primarily of the same elements that make up the crust and the mantle of the earth: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), sodium (Na), and potassium (K). These elements make up the rock-forming minerals quartz, feldspar, mica, amphibole, pyroxene, and olivine. Rocks and minerals rich in silicon are called silica-rich or felsic (rich in feldspar and silica). Rocks and minerals low in silicon are rich in magnesium and iron. They are called mafic (rich in magnesium and ferrum, the Latin term for iron). Rocks very low in silicon are called ultramafic. Rocks with a composition between felsic and mafic are called intermediate.

B1

Felsic Rocks

The most felsic, or silicon-rich, mineral is quartz. It is pure silicon dioxide and contains no aluminum, iron, magnesium, calcium, sodium, or potassium. The other important felsic mineral is feldspar. In feldspar, a quarter or a half of the silicon has been replaced by aluminum. Feldspar also contains potassium, sodium, or calcium but no magnesium or iron.

Felsic intrusive rocks are classified as either granite or granodiorite, depending on how much potassium they contain. Both are light-colored rocks that have large crystals of quartz and feldspar. Extrusive rocks that have the same chemical composition as granite are called rhyolite and those with the same chemical composition as granodiorite are called dacite. Both rhyolite and dacite are fine-grained light-colored rocks.

B2

Intermediate Rocks

Rocks intermediate in composition between felsic and mafic rocks are termed syenite, monzonite, or monzodiorite if they are intrusive and trachyte, latite, and andesite if they are extrusive. Syenite and trachyte are rich in potassium while monzodiorite and andesite contain little potassium.

B3

Mafic Rocks

The mafic rock-forming minerals are olivine, pyroxene, and amphibole. All three contain silicon and a lot of either magnesium or iron or both. All three of these minerals are often dark colored.

Mafic intrusive rocks are termed diorite or gabbro. Both are dark rocks with large, dark, mafic crystals as well as crystals of light-colored feldspar. Neither contains quartz. Diorite contains amphibole and pyroxene, while gabbro contains pyroxene and olivine. The feldspar in diorite tends to be sodium-rich, while the feldspar in gabbro is calcium-rich. Extrusive rocks that have the same chemical composition as diorite or gabbro are called basalt. Basalt is a fine-grained dark rock.

Ultramafic rocks are composed almost exclusively of mafic minerals. Dunite is composed of more than 90 percent olivine; peridotites have between 90 and 40 percent olivine with pyroxene and amphibole as the other two principal minerals. Pyroxenite is composed primarily of pyroxene, and hornblendite is composed primarily of hornblende, which is a type of amphibole.

III

FORMATION OF IGNEOUS ROCKS

The magmas that form igneous rock are hot, chemical soups containing a complex mixture of many different elements. As they cool, many different minerals could form. Indeed, two magmas with identical composition could form quite distinct sets of minerals, depending on the conditions of crystallization.

As a magma cools, the first crystals to form will be of minerals that become solid at relatively high temperatures (usually olivine and a type of feldspar known as anorthite). The composition of these early-formed mineral crystals will be different from the initial composition of the magma. Consequently, as these growing crystals take certain elements out of the magma in certain proportions, the composition of the remaining liquid changes. This process is known as magmatic differentiation. Sometimes, the early-formed crystals are separated from the rest of the magma, either by settling to the floor of the magma chamber, or by compression that expels the liquid, leaving the crystals behind.

As the magma cools to temperatures below the point where other minerals begin to crystallize (such as pyroxene and another type of feldspar known as bytownite), their crystals will start to form as well. However, early-formed minerals often cannot coexist in magma with the later-formed mineral crystals. If the early-formed minerals are not separated from the magma, they will react with or dissolve back into the magma over time. This process repeats through several cycles as the temperature of the magma continues to cool to the point where the remaining minerals become solid. The final mix of minerals formed from a cooling magma depends on three factors: the initial composition of the magma, the degree to which already-formed crystals separate from the magma, and the speed of cooling.

IV

INTRUSIONS

When magma intrudes a region of the crust and cools, the resulting mass of igneous rock is called an intrusion. Geologists describe intrusions by their size, their shape, and whether they are concordant, meaning they run parallel to the structure of neighboring rocks, or discordant, meaning they cut across the structure of neighboring rocks. An example of a concordant intrusion is a horizontal bed formed when magma flows between horizontal beds of neighboring rock. A discordant intrusion would form when magma flows into cracks in neighboring rock, and the cracks lie at an angle to the neighboring beds of rock.

A batholith is an intrusion with a cross-sectional area of more than 100 sq km (39 sq mi), usually consisting of granite, granodiorite, and diorite. Deep batholiths are often concordant, while shallow batholiths are usually discordant. Deep batholiths can be extremely large; the Coast Range batholith of North America is 100 to 200 km (60 to 120 mi) wide and extends 600 km (370 mi) through Alaska and British Columbia, Canada.

Lopoliths are saucer-shaped concordant intrusions. They may be up to 100 km (60 mi) in diameter and 8 km (5 mi) thick. Lopoliths, which are usually basaltic in composition, are frequently called layered intrusions because they are strongly layered. Well-known examples are the Bushveld complex in South Africa and the Muskox intrusion in the Northwest Territories, Canada.

Laccoliths have a flat base and a domed ceiling, and are concordant with the neighboring rocks; they are usually small. The classic area from which they were first described is the Henry Mountains in the state of Utah.

Dikes and sills are sheetlike intrusions that are very thin relative to their length; sills are concordant and dikes are discordant. They are commonly fairly small features (a few meters thick) but can be larger. The Palisades Sill in the state of New York is 300 m (1000 ft) thick and 80 km (50 mi) long.

V

EXTRUSIVE BODIES

Many different types of extrusive bodies occur throughout the world. The physical characteristics of these bodies depend on their chemical composition and on how the magma from which they formed erupted. The chemical composition of the parent magma affects its viscosity, or its resistance to flow, which in turn affects how the magma erupts. Felsic magma tends to be thick and viscous, while mafic magma tends to be fluid. (See also Volcano)

Flood basalts are the most common type of extrusive rock. They form when highly fluid basaltic lava erupts from long fissures and many vents. The lava coalesces and floods large areas to considerable depths (up to 100 m/300 ft). Repeated eruptions can result in accumulated deposits up to 5 km (3 mi) thick. Typical examples are the Columbia River basalts in Washington and the Deccan trap of western India; the latter covers an area of more than 500,000 sq km (200,000 sq mi).

When basalt erupts underwater, the rapid cooling causes it to form a characteristic texture known as pillow basalt. Pillow basalts are lava flows made up of interconnected pillow-shaped and pillow-sized rocks. Much of the ocean floor is made up of pillow basalt.

Extrusive rocks that erupt from a main central vent form volcanoes, and these are classified according to their physical form and the type of volcanic activity. Mafic, or basaltic, lava is highly fluid and erupts nonexplosively. The fluid lava quickly spreads out, forming large volcanoes with shallow slopes called shield volcanoes. Mauna Loa (Hawaii) is the best-known example. Intermediate, or andesitic, magmas have a higher viscosity and so they erupt more explosively. They form steep-sided composite volcanoes. A composite volcano, or stratovolcano, is made up of layers of lava and volcanic ash. Well-known examples of composite volcanoes include Mount Rainier (Washington), Mount Vesuvius (Italy), and Mount Fuji (Japan).

Felsic (rhyolitic) magmas are so viscous that they do not flow very far at all; instead, they form a dome above their central vent. This dome can give rise to very explosive eruptions when pressure builds up in a blocked vent, as happened with Mount Saint Helens (Washington) in 1983, Krakatau (Indonesia) in 1883, and Vesuvius (Italy) in AD 79. This type of explosive behavior can eject enormous amounts of ash and rock fragments, referred to as pyroclastic material, which form pyroclastic deposits (See also Pyroclastic Flow)

VI

PLATE TECTONICS AND IGNEOUS ROCKS

The advent of the theory of plate tectonics in the 1960s provided a theoretical framework for understanding the worldwide distribution of different types of igneous rocks. According to the theory of plate tectonics, the surface of the earth is covered by about a dozen large plates. Some of these plates are composed primarily of basalt and are called oceanic plates, since most of the ocean floor is covered with basalt. Other plates, called continental plates because they contain the continents, are composed of a wide range of rocks, including sedimentary and metamorphic rocks, and large amounts of granite.

Where two plates diverge (move apart), such as along a mid-ocean ridge, magma rises from the mantle to fill the gap. This material is mafic in composition and forms basalt. Where this divergence occurs on land, such as in Iceland, flood basalts are formed.

When an oceanic plate collides with a continental plate, the heavier oceanic plate subducts, or slides, under the lighter continental plate. Some of the subducted material melts and rises. As it travels through the overriding continental plate, it melts and mixes with the continental material. Since continental material, on average, is more felsic than the mafic basalt of the oceanic plate, this mixing causes the composition of the magma to become more mafic. The magma may become intermediate in composition and form andesitic volcanoes. The Andes Mountains of South America are a long chain of andesitic volcanoes formed from the subduction of the Pacific Plate under the South American plate. If the magma becomes mafic, it may form rhyolitic volcanoes like Mount Saint Helens. Magma that is too viscous to rise to the surface may instead form granitic batholiths.

VII

ECONOMIC IMPORTANCE OF IGNEOUS ROCKS

Many types of igneous rocks are used as building stone, facing stone, and decorative material, such as that used for tabletops, cutting boards, and carved figures. For example, polished granite facing stone is exported all over the world from countries such as Italy, Brazil, and India.

Igneous rocks may also contain many important ores as accessory or trace minerals. Certain mafic intrusives are sources of chromium, titanium, platinum, and palladium. Some felsic rocks, called granitic pegmatites, contain a wealth of rare elements, such as lithium, tantalum, tin, and niobium, which are of economic importance. Kimberlites, formed from magmas from deep within the earth, are the primary source of diamonds. Many magmas release large amounts of metal-rich hot fluids that migrate through nearby rock, forming veins rich in metallic ores. Newly formed igneous rocks are also hot and can be an important source of geothermal energy.

Contributed By:
Frank Christopher Hawthorne

Plate Tectonics

I

INTRODUCTION

Plate Tectonics, theory that the outer shell of the earth is made up of thin, rigid plates that move relative to each other. The theory of plate tectonics was formulated during the early 1960s, and it revolutionized the field of geology. Scientists have successfully used it to explain many geological events, such as earthquakes and volcanic eruptions as well as mountain building and the formation of the oceans and continents.

Plate tectonics arose from an earlier theory proposed by German scientist Alfred Wegener in 1912. Looking at the shapes of the continents, Wegener found that they fit together like a jigsaw puzzle. Using this observation, along with geological evidence he found on different continents, he developed the theory of continental drift, which states that today’s continents were once joined together into one large landmass.

Geologists of the 1950s and 1960s found evidence supporting the idea of tectonic plates and their movement. They applied Wegener’s theory to various aspects of the changing earth and used this evidence to confirm continental drift. By 1968 scientists integrated most geologic activities into a theory called the New Global Tectonics, or more commonly, Plate Tectonics.

II

TECTONIC PLATES

Tectonic plates are made of either oceanic or continental crust and the very top part of the mantle, a layer of rock inside the earth. This crust and upper mantle form what is called the lithosphere. Under the lithosphere lies a fluid rock layer called the asthenosphere. The rocks in the asthenosphere move in a fluid manner because of the high temperatures and pressures found there. Tectonic plates are able to float upon the fluid asthenosphere because they are made of rigid lithosphere. See also Earth: Plate Tectonics.

A

Continental Crust

The earth’s solid surface is about 40 percent continental crust. Continental crust is much older, thicker and less dense than oceanic crust. The thinnest continental crust, between plates that are moving apart, is about 15 km (about 9 mi) thick. In other places, such as mountain ranges, the crust may be as much as 75 km (47 mi) thick. Near the surface, it is composed of rocks that are felsic (made up of minerals including feldspar and silica). Deeper in the continental crust, the composition is mafic (made of magnesium, iron, and other minerals).

B

Oceanic Crust

Oceanic crust makes up the other 60 percent of the earth’s solid surface. Oceanic crust is, in general, thin and dense. It is constantly being produced at the bottom of the oceans in places called mid-ocean ridges—undersea volcanic mountain chains formed at plate boundaries where there is a build-up of ocean crust. This production of crust does not increase the physical size of the earth, so the material produced at mid-ocean ridges must be recycled, or consumed, somewhere else. Geologists believe it is recycled back into the earth in areas called subduction zones, where one plate sinks underneath another and the crust of the sinking plate melts back down into the earth. Oceanic crust is continually recycled so that its age is generally not greater than 200 million years. Oceanic crust averages between 5 and 10 km (between 3 and 6 mi) thick. It is composed of a top layer of sediment, a middle layer of rock called basalt, and a bottom layer of rock called gabbro. Both basalt and gabbro are dark-colored igneous, or volcanic, rocks.

C

Plate Sizes

Currently, there are seven large and several small plates. The largest plates include the Pacific plate, the North American plate, the Eurasian plate, the Antarctic plate, and the African plate. Smaller plates include the Cocos plate, the Nazca plate, the Caribbean plate, and the Gorda plate. Plate sizes vary a great deal. The Cocos plate is 2000 km (1400 mi) wide, while the Pacific plate is the largest plate at nearly 14,000 km (nearly 9000 mi) wide.

III

PLATE MOVEMENT

Geologists study how tectonic plates move relative to a fixed spot in the earth’s mantle and how they move relative to each other. The first type of motion is called absolute motion, and it can lead to strings of volcanoes. The second kind of motion, called relative motion, leads to different types of boundaries between plates: plates moving apart from one another form a divergent boundary, plates moving toward one another form a convergent boundary, and plates that slide along one another form a transform plate boundary. In rare instances, three plates may meet in one place, forming a triple junction. Current plate movement is making the Pacific Ocean smaller, the Atlantic Ocean larger, and the Himalayan mountains taller.

A

Measuring Plate Movement

Geologists discovered absolute plate motion when they found chains of extinct submarine volcanoes. A chain of dead volcanoes forms as a plate moves over a plume, a source of magma, or molten rock, deep within the mantle. These plumes stay in one spot, and each one creates a hot spot in the plate above the plume. These hot spots can form into a volcano on the surface of the earth. An active volcano indicates a hot spot as well as the youngest region of a volcanic chain. As the plate moves, a new volcano forms in the plate over the place where the hot spot occurs. The volcanoes in the chain get progressively older and become extinct as they move away from the hot spot (see Hawaii: Formation of the Islands and Volcanoes). Scientists use hot spots to measure the speed of tectonic plates relative to a fixed point. To do this, they determine the age of extinct volcanoes and their distance from a hot spot. They then use these numbers to calculate how far the plate has moved in the time since each volcano formed. Today, the plates move at velocities up to 18.5 cm per year (7.3 in per year). On average, they move nearly 4 to 7 cm per year (2 to 3 in per year).

B

Divergent Plate Boundaries

Divergent plate boundaries occur where two plates are moving apart from each other. When plates break apart, the lithosphere thins and ruptures to form a divergent plate boundary. In the oceanic crust, this process is called seafloor spreading, because the splitting plates are spreading apart from each other. On land, divergent plate boundaries create rift valleys—deep valley depressions formed as the land slowly splits apart.

When seafloor spreading occurs, magma, or molten rock material, rises to the sea floor surface along the rupture. As the magma cools, it forms new oceanic crust and lithosphere. The new lithosphere is less dense, so it rises, or floats, higher above older lithosphere, producing long submarine mountain chains known as mid-ocean ridges. The Mid-Atlantic Ridge is an underwater mountain range created at a divergent plate boundary in the middle of the Atlantic Ocean. It is part of a worldwide system of ridges made by seafloor spreading. The Mid-Atlantic Ridge is currently spreading at a rate of 2.5 cm per year (1 in per year). The mid-ocean ridges today are 60,000 km (about 40,000 mi) long, forming the largest continuous mountain chain on earth. Earthquakes, faults, underwater volcanic eruptions, and vents, or openings, along the mountain crests produce rugged seafloor features, or topography.

Divergent boundaries on land cause rifting, in which broad areas of land are uplifted, or moved upward. These uplifts and faulting along the rift result in rift valleys. Examples of rift valleys are found at the Krafla Volcano rift area in Iceland as well as at the East African Rift Zone—part of the Great Rift Valley that extends from Syria to Mozambique and out to the Red Sea. In these areas, volcanic eruptions and shallow earthquakes are common.

C

Convergent Plate Boundaries

Convergent plate boundaries occur where plates are consumed, or recycled back into the earth’s mantle. There are three types of convergent plate boundaries: between two oceanic plates, between an oceanic plate and a continental plate, and between two continental plates. Subduction zones are convergent regions where oceanic crust is thrust below either oceanic crust or continental crust. Many earthquakes occur at subduction zones, and volcanic ridges and oceanic trenches form in these areas.

In the ocean, convergent plate boundaries occur where an oceanic plate descends beneath another oceanic plate. Chains of active volcanoes develop 100 to 150 km (60 to 90 mi) above the descending slab as magma rises from under the plate. Also, where the crust slides down into the earth, a trench forms. Together, the volcanoes and trench form an intra-oceanic island arc and trench system. A good example of such a system is the Mariana Trench system in the western Pacific Ocean, where the Pacific plate is descending under the Philippine plate. In these areas, earthquakes are frequent but not large. Stress in and behind the arc often causes the arc and trench system to move toward the incoming plate, which opens small ocean basins behind the arc. This process is called back-arc seafloor spreading.

Convergent boundaries that occur between the ocean and land create continental margin arc and trench systems near the margins, or edges, of continents. Volcanoes also form here. Stress can develop in these areas and cause the rock layers to fold, leading to earthquake faults, or breaks in the earth’s crust called thrust faults. The folding and thrust faulting thicken the continental crust, producing high mountains. Many of the world’s large destructive earthquakes and major mountain chains, such as the Andes Mountains of western South America, occur along these convergent plate boundaries.

When two continental plates converge, the incoming plate drives against and under the opposing continent. This often affects hundreds of miles of each continent and, at times, doubles the normal thickness of continental crust. Colliding continents cause earthquakes and form mountains and plateaus. The collision of India with Asia has produced the Himalayan Mountains and Tibetan Plateau.

D

Transform Plate Boundaries

A transform plate boundary, also known as a transform fault system, forms as plates slide past one another in opposite directions without converging or diverging. Early in the plate tectonic revolution, geologists proposed that transform faults were a new class of fault because they “transformed” plate motions from one plate boundary to another. Canadian geophysicist J. Tuzlo Wilson studied the direction of faulting along fracture zones that divide the mid-ocean ridge system and confirmed that transform plate boundaries were different than convergent and divergent boundaries. Within the ocean, transform faults are usually simple, straight fault lines that form at a right angle to ocean ridge spreading centers. As plates slide past each other, the transform faults can divide the centers of ocean ridge spreading. By cutting across the ridges of the undersea mountain chains, they create steep cliff slopes. Transform fault systems can also connect spreading centers to subduction zones or other transform fault systems within the continental crust. As a transform plate boundary cuts perpendicularly across the edges of the continental crust near the borders of the continental and oceanic crust, the result is a system such as the San Andreas transform fault system in California.

E

Triple Junctions

Rarely, a group of three plates, or a combination of plates, faults, and trenches, meet at a point called a triple junction. The East African Rift Zone is a good example of a triple plate junction. The African plate is splitting into two plates and moving away from the Arabian plate as the Red Sea meets the Gulf of Aden. Another example is the Mendocino Triple Junction, which occurs at the intersection of two transform faults (the San Andreas and Mendocino faults) and the plate boundary between the Pacific and Gorda plates.

F

Current Plate Movement

Plate movement is changing the sizes of our oceans and the shapes of our continents. The Pacific plate moves at an absolute motion rate of 9 cm per year (4 in per year) away from the East Pacific Rise spreading center, the undersea volcanic region in the eastern Pacific Ocean that runs parallel to the western coast of South America. On the other side of the Pacific Ocean, near Japan, the Pacific plate is being subducted, or consumed under, the oceanic arc systems found there. The Pacific Ocean is getting smaller as the North and South American plates move west. The Atlantic Ocean is getting larger as plate movement causes North and South America to move away from Europe and Africa. Since the Eurasian and Antarctic plates are nearly stationary, the Indian Ocean at present is not significantly expanding or shrinking. The plate that includes Australia is just beginning to collide with the plate that forms Southeast Asia, while India’s plate is still colliding with Asia. India moves north at 5 cm per year (2 in per year) as it crashes into Asia, while Australia moves slightly farther away from Antarctica each year.

IV

CAUSES OF PLATE MOTION

Although plate tectonics has explained most of the surface features of the earth, the driving force of plate tectonics is still unclear. According to geologists, a model that explains plate movement should include three forces. Those three forces are the pull of gravity; convection currents, or the circulating movement of fluid rocky material in the mantle; and thermal plumes, or vertical columns of molten rocky material in the mantle.

A

Plate Movement Caused by Gravity

Geologists believe that tectonic plates move primarily as a result of their own weight, or the force of gravity acting on them. Since the plates are slightly denser than the underlying asthenosphere, they tend to sink. Their weight causes them to slide down gentle gradients, such as those formed by the higher ocean ridge crests, to the lower subduction zones. Once the plate’s leading edge has entered a subduction zone and penetrated the mantle, the weight of the slab itself will tend to pull the rest of the plate toward the trench. This sinking action is known as slab-pull because the sinking plate edge pulls the remainder of the plate behind it. Another kind of action, called ridge-push, is the opposite of slab-pull, in that gravity also causes plates to slide away from mid-ocean ridges. Scientists believe that plates pushing against one another also causes plate movement.

B

Convection Currents

In 1929 British geologist Arthur Holmes proposed the concept of convection currents—the movement of molten material circulating deep within the earth—and the concept was modified to explain plate movement. A convection current occurs when hot, molten, rocky material floats up within the asthenosphere, then cools as it approaches the surface. As it cools, the material becomes denser and begins to sink again, moving in a circular pattern. Geologists once thought that convection currents were the primary driving force of plate movement. They now believe that convection currents are not the primary cause, but are an effect of sinking plates that contributes to the overall movement of the plates.

C

Thermal Plumes

Some scientists have proposed the concept of thermal plumes, vertical columns of molten material, as an additional force of plate movement. Thermal plumes do not circulate like convection currents. Rather, they are columns of material that rise up through the asthenosphere and appear on the surface of the earth as hot spots. Scientists estimate thermal plumes to be between 100 and 250 km (60 and 160 mi) in diameter. They may originate within the asthenosphere or even deeper within the earth at the boundary between the mantle and the core.

V

EXTRATERRESTRIAL PLATE TECTONICS

Scientists have also observed tectonic activity and fracturing on several moons of other planets in our solar system. Starting in 1985, images from the Voyager probes indicated that Saturn’s satellite Enceladus and Uranus’ moon Miranda also show signs of being tectonically active. In 1989 the Voyager probes sent photographs and data to Earth of volcanic activity on Neptune’s satellite Triton. In 1995 the Galileo probe began to send data and images of tectonic activity on three of Jupiter’s four Galilean satellites. The information that scientists gather from space missions such as these helps increase their understanding of the solar system and our planet. They can apply this knowledge to better understand the forces that created the earth and that continue to act upon it.

Scientists believe that Enceladus has a very tectonically active surface. It has several different terrain types, including craters, plains, and many faults that cross the surface. Miranda has fault canyons and terraced land formations that indicate a diverse tectonic environment. Scientists studying the Voyager 2 images of Triton found evidence of an active geologic past as well as ongoing eruptions of ice volcanoes.

Scientists are still gathering information from the Galileo probe of the Jupiter moon system. Three of Jupiter’s four Galilean satellites show signs of being tectonically active. Europa, Ganymede, and Io all exhibit various features that indicate tectonic motion or volcanism. Europa’s surface is broken apart into large plates similar to the plates found on Earth. The plate movement indicates that the crust is brittle and that the plates move over the top of a softer, more fluid layer. Ganymede probably has a metallic inner core and at least two outer layers that make up a crust and mantle. Io may also have a giant iron core interior that causes the active tectonics and volcanism. It is believed that Io has a partially molten rock mantle and crust. See also Planetary Science: Volcanism and Tectonic Activity.

VI

HISTORY OF TECTONIC THEORY

The theory of plate tectonics arose from several previous geologic theories and discoveries. As early as the 16th century, explorers began examining the coastlines of Africa and South America and proposed that these continents were once connected. In the 20th century, scientists proposed theories that the continents moved or drifted apart from each other. Additionally, in the 1950s scientists proposed that the earth’s magnetic poles wander, leading to more evidence, such as rocks with similar magnetic patterns around the world, that the continents had drifted. More recently, scientists examining the seafloor have discovered that it is spreading as new seafloor is created, and through this work they have discovered that the magnetic polarity of the earth has changed several times throughout the earth's history. The theory of plate tectonics revolutionized earth sciences by providing a framework that could explain these discoveries, as well as events such as earthquakes and volcanic eruptions, mountain building and the formation of the continents and oceans. See also Earthquake.

A

Continental Drift

Beginning in the late 16th century and early 17th century, many people, including Flemish cartographer Abraham Ortelius and English philosopher Sir Francis Bacon, were intrigued by the shapes of the South American and African coastlines and the possibility that these continents were once connected. In 1912, German scientist Alfred Wegener eventually developed the idea that the continents were at one time connected into the theory of continental drift. Scientists of the early 20th century found evidence of continental drift in the similarity of the coastlines and geologic features on both continents. Geologists found rocks of the same age and type on opposite sides of the ocean, fossils of similar animals and plants, and similar ancient climate indicators, such as glaciation patterns. British geologist Arthur Holmes proposed that convection currents drove the drifting movement of continents. Most earth scientists did not seriously consider the theory of continental drift until the 1960s when scientists began to discover other evidence, such as polar wandering, seafloor spreading, and reversals of the earth’s magnetic field. See also Continent.

B

Polar Wandering

In the 1950s, physicists in England became interested in the observation that certain kinds of rocks produced a magnetic field. They soon decided that the magnetic fields were remnant, or left over, magnetism acquired from the earth’s magnetic field as the rocks cooled and solidified from the hot magma that formed them. Scientists measured the orientation and direction of the acquired magnetic fields and, from these orientations, calculated the direction of the rock’s magnetism and the distance from the place the rock was found to the magnetic poles. As calculations from rocks of varying ages began to accumulate, scientists calculated the position of the earth’s magnetic poles over time. The position of the poles varied depending on where the rocks were collected, and the idea of a polar wander path began to form. When sample paths of polar wander from two continents, such as North America and Europe, were compared, they coincided as if the continents were once joined. This new science and methodology became known as the discipline of paleomagnetism. As a result, discussion of the theory of continental drift increased, but most earth scientists remained skeptical.

C

Seafloor Spreading

During the 1950s, as people began creating detailed maps of the world’s ocean floor, they discovered a mid-ocean ridge system of mountains nearly 60,000 km (nearly 40,000 mi) long. This ridge goes all the way around the globe. American geologist Harry H. Hess proposed that this mountain chain was the place where new ocean floor was created and that the continents moved as a result of the expansion of the ocean floors. This process was termed seafloor spreading by American geophysicist Robert S. Dietz in 1961. Hess also proposed that since the size of the earth seems to have remained constant, the seafloor must also be recycled back into the mantle beneath mountain chains and volcanic arcs along the deep trenches on the ocean floor.

These studies also found marine magnetic anomalies, or differences, on the sea floor. The anomalies are changes, or switches, in the north and south polarity of the magnetic rock of the seafloor. Scientists discovered that the switches make a striped pattern of the positive and negative magnetic anomalies: one segment, or stripe, is positive, and the segment next to it is negative. The stripes are parallel to the mid-ocean ridge crest, and the pattern is the same on both sides of that crest. Scientists could not explain the cause of these anomalies until they discovered that the earth’s magnetic field periodically reverses direction.

D

Magnetic Field Reversals

In 1963, British scientists Fred J. Vine and Drummond H. Matthews combined their observations of the marine magnetic anomalies with the concept of reversals of the earth’s magnetic field. They proposed that the marine magnetic anomalies were a “tape recording” of the spreading of the ocean floor as the earth’s magnetic field reversed its direction. At the same time, other geophysicists were studying lava flows from many parts of the world to see how these flows revealed the record of reversals of the direction of the earth’s magnetic field. These studies showed that nearly four reversals have occurred over the past 5 million years. The concept of magnetic field reversals was a breakthrough that explained the magnetic polarity switches seen in seafloor spreading as well as the concept of similar magnetic patterns in the rocks used to demonstrate continental drift.

E

Revolution in Geology

The theory of plate tectonics tied together the concepts of continental drift, polar wandering, seafloor spreading, and magnetic field reversals into a single theory that completely changed the science of geology. Geologists finally had one theory that could explain all the different evidence they had accumulated to support these previous theories and discoveries. Geologists now use the theory of plate tectonics to integrate geologic events, to explain the occurrence of earthquakes and volcanic eruptions, and to explain the formation of mountain ranges and oceans.

Contributed By:
Peter Coney

Crust (earth science)

Crust (earth science), outermost layer of Earth. The crust is solid and relatively thin, and it lies below both landmasses and oceans. The dry land of Earth’s surface is called the continental crust. It is about 15 to 75 km (9 to 47 mi) thick. The oceanic crust is thinner than the continental crust. Its average thickness is 5 to 10 km (3 to 6 mi). The crust is very thin in relation to the rest of Earth. If a trip to the center of Earth at 100 km/h (60 mph) were possible, it would take 64 hours, of which only the first 15 to 45 minutes would be in the crust. See Earth: Earth's Surface: Crust.

The crust has a definite boundary. This boundary, called the Mohorovičić discontinuity, or simply the Moho, after the Croatian geologist Andrija Mohorovičić, separates the crust from the underlying mantle. Mohorovičić discovered the boundary in 1909, when he observed that earthquake waves do not pass through Earth’s interior in a straight line but change course at a certain depth below the surface. He believed that the point at which these waves change course marked the boundary between the crust and the mantle. See Earthquake: Studying Earthquakes.

In comparison to the crust, the mantle is much thicker. The mantle extends for about 2,900 km (1,800 mi). It consists of an upper mantle and a lower mantle. The solid, outermost section of the mantle and the solid crust together form the lithosphere. The lithosphere is approximately 65 to 100 km (40 to 60 mi) thick and covers the asthenosphere (see Geology: Guiding Principles of Geology). The asthenosphere is approximately 100 to 350 km (60 to 220 mi) thick. It consists of rocky material that is softer and less rigid than that in the lithosphere. This softer, less rigid state results from higher pressures and temperatures, which cause the rocks partially to melt and become soft.

Oceanic crust and continental crust differ in composition. Oceanic crust consists of dark, dense rocks, such as basalt and gabbro. In contrast, continental crust consists of lighter colored, less dense rocks, such as granite and diorite. Continental crust also includes metamorphic rocks and sedimentary rocks, which the oceanic crust lacks. Metamorphic rocks, which include quartz, marble, and crystal, are formed when high temperatures and pressures at great depths inside Earth transform rock material. Sedimentary rocks form from accumulation and burial of fragments or particles of preexisting rocks. The rocks that make up continental crust possess an average density of 2.8 times the density of water. The rocks that make up oceanic crust are denser, with a density of 2.9 times the density of water.

Contributed By:
Parvinder Sethi

Metamorphic Rock

I

INTRODUCTION

Metamorphic Rock, type of rock formed when rocky material experiences intense heat and pressure in the crust of the earth. Metamorphic rocks are one of the three main groups of rocks. The other two groups are igneous rocks, which form when magma or molten lava solidifies, and sedimentary rocks, which form when wind or water deposit sediments and the sediments become compacted. Through the metamorphic process, both igneous rocks and sedimentary rocks can change into metamorphic rocks, and a metamorphic rock can change into another type of metamorphic rock. Heat and pressure do not change the chemical makeup of the parent rocks but they do change the mineral structure and physical properties of those rocks. By studying the composition and texture of metamorphic rocks, geologists can determine from what parent rocks the metamorphic rocks were formed.

II

FORMATION OF METAMORPHIC ROCKS

Forces within the earth create large amounts of heat and pressure, the factors that change igneous and sedimentary rocks into metamorphic rocks. Radioactive isotopes—forms of elements—generate heat within the earth as they decay. Magma (molten rock) moving from deep within the earth toward the surface also provides heat for metamorphism. Another source of heat within the earth that can lead to metamorphism is friction between rocks grinding past one another (along earthquake faults or at plate tectonic boundaries). In addition to heat, pressure within the earth contributes to the formation of metamorphic rocks by changing the texture and mineral density of rocks. See also Earthquake; Plate Tectonics.

A

Heat

Heat is the most important factor contributing to metamorphism. The temperature range over which metamorphic rocks form is approximately 150° C (300° F) to above 1,000° C (2,000° F), depending on composition of parent rock, pressure, and the presence of fluids such as water. At the upper range of temperature, metamorphic conditions stop as the rocks begin to melt, eventually forming igneous rocks. The melting temperature varies, from approximately 650° C (1,200° F) for rocks made of granite to well over 1,000° C (2,000° F) for rocks made of basalt.

Heat produced by radioactive decay may lead to the formation of metamorphic rocks. Radioactive isotopes within the earth emit heat as they decay, or disintegrate. Radioactivity is the process by which atoms of an element are transformed into new kinds of atoms, and heat is a by-product of this process. Some of the heat within the earth is produced by the radioactive decay of elements such as uranium, thorium, and potassium.

Another way for heat to form metamorphic rocks is through the introduction of underground magma into an area of preexisting solid rock. When underground magma flows through a crack (called a dike) into areas of surrounding solid rock (known as country rock), there is a significant difference between the temperature of the magma and the temperature of the surrounding rock. On cooling, the magma introduces great amounts of heat into the country rock, usually leading to recrystallization and mineral reactions in the rocks nearby. This process is known as contact (or thermal) metamorphism. The magma itself cools to form igneous rock, but the nearby surrounding rock will likely be metamorphic in nature. The envelope of contact-metamorphosed rocks around a magma intrusion is called an aureole. The size of an aureole depends on the amount of heat provided by the intrusion. A narrow dike may have an aureole a few millimeters wide, whereas the aureole surrounding a batholith (a large intrusion of igneous rock) may stretch for many hundreds of meters.

Another source of heat is friction between bodies of rock as they grind against each other. Along earthquake faults, two bodies of rock—one on either side of the fault—may slide against each other as the strain (caused by rocks pushing against each other over many years) within the bodies of rock builds up. At plate boundaries, bodies of rock produce friction and heat as they slide against each other, as one plate moves under another plate, or as the two plates push directly against each other.

B

Pressure

One unit that scientists use to measure pressure is the bar. One bar is equal to the amount of pressure applied by the atmosphere to the surface of the earth at sea level (1 bar = 1.02 kg/sq cm, or 14.7 lb/sq in). Metamorphic rocks form under pressures of many kilobars, or thousands of bars. Rocks that are buried deep beneath many layers of rock experience lithostatic (Greek lithos, “rock”; statikos, “in place”) pressure, which causes the rocks to compress into a smaller, denser form.

Regional metamorphism results from increases in both pressure and heat below Earth’s surface. These increases occur below the surface of the earth, as tectonic plates come into contact with each other. Rock formed below the surface is generally igneous rock, which is formed from cooling magma. However, later deposits of rocks may bury sedimentary and extrusive igneous rocks, which form on the earth’s surface. Such burial often happens through subsidence, the settling associated with the development of sedimentary basins. It may also happen through tectonic overthrusting, as continental and oceanic plates fold up or down because of stress from movement or from contact with each other. The increased temperature and pressure in these areas cause mineralogical and textural changes in the original rock. This type of metamorphism develops on a much larger scale than contact metamorphism, usually over an area of hundreds or thousands of square kilometers.

III

TEXTURES AND STRUCTURES

The heat and pressure that form metamorphic rocks often deform the rock, giving rise to a variety of textures and structures collectively referred to as fabric. Some common metamorphic rocks can be identified according to their fabric. Regional metamorphism often produces a fabric quality called foliation, while rocks formed by contact metamorphism are generally nonfoliated.

A

Foliated Rocks

Foliation is similar in appearance to the grain of wood. It occurs because certain minerals in a parent rock naturally form in parallel planes. Foliation may also occur when different minerals are sandwiched together and compressed, or when rock is fractured along parallel lines. Slate, phyllite, schist, and gneiss are examples of foliated rocks.

Slate is a fine-grained metamorphic rock formed from shale or clay sedimentary rock that has been exposed to low temperature and pressure. Slate is rich in silicates, which naturally form into planes. The low heat does not “overcook” the rock, so the foliation is very smooth in appearance. Greater pressure forms phyllite, which has a slightly coarser grain size than slate. The surface of a phyllite is visibly scaly and often has a silvery luster. More pressure, and subsequent heat, produces schist, a more coarsely foliated rock. Schist is usually foliated because of a planar mineral, but it may also be layered because of completely different mineral compositions. Foliation differs from layering, as the mineral grains in a foliated rock crystallize into parallel planes, whereas the mineral grains in a layered rock do not line up parallel with one another. More heat and pressure produce gneiss, a very coarse rock. The extreme foliation in gneiss is mainly due to the separation of different minerals that occurs at high pressure and temperature.

B

Nonfoliated Rocks

Nonfoliated rocks are produced mainly by contact metamorphism, or heat from cooling magma. Contact heat generally results in a finer recrystallization of the parent rock, so little foliation is visible. Quartzite is typically a tough, hard, light-colored rock in which all the sand grains of a sandstone or siltstone have recrystallized into a fabric of interlocking quartz grains. Marble is a softer, more brittle rock in which the dolomite or calcite of the limestone parent rock has recrystallized. Hornfels is a common metamorphic rock formed when basalt or shale is exposed to heat from magma.

IV

MINERAL REACTIONS

The mineral structure of metamorphic rocks depends both on the type of parent rock and on the amount of heat and pressure present when the rocks formed. To define the types of mineral changes that may occur, geologists organize metamorphic rocks into several metamorphic facies, or groups. This idea has two basic principles: for rocks formed under the same metamorphic conditions, different mineral assemblages represent different parent rock compositions. For a given parent rock composition, different mineral structures imply different physical conditions.

The possible range of metamorphic conditions is divided into several different facies. Each facies group is defined by a specific mineral assemblage in a known example that is constant over a given range of temperature and pressure. As temperature or pressure increases, the parent rock will generate different mineral assemblages. Finding rocks that belong to certain facies in an area helps geologists to determine the geologic history of that area.

A

Metamorphic Facies

Metamorphic facies are formed according to one of three processes: contact metamorphism, subduction-zone metamorphism, and regional metamorphism. Each of these processes occurs over a range of pressure and temperature to produce the different facies.

Low pressure (1 kilobar, or kb) and moderate to high temperatures of 300° to 850° C (600° to 1,560° F) produce hornfels facies during contact metamorphism. High pressures (5 to over 8 kb) and low to moderate temperatures of 250° to 600° C (480° to 1,100° F) form blueschist facies. Blueschist facies is typical of subduction-zone metamorphism as tectonic plates fold over one another. Five other groups, the greenstone, greenschist, amphibolite, granulite, and eclogite facies, are formed by regional metamorphism, such as the bending or buckling of continental plates into mountain ranges. Low pressure (1 kb) and low temperatures of 100° to 550° C (200° to 1,020° F) form greenstone and greenschist. Moderate pressures of 1 to 2 kb and medium temperatures ranging from 550° to 750° C (1020° to 1,380° F) form amphibolite. High pressures (over 10 kb) and extremely high temperatures of 700° to over 900° C (1,300° to 1,700° F) produce the granulite and eclogite facies.

B

How Minerals Change Through Facies

Metamorphism is not a single isolated event. Rather, it is the cumulative effect of all the continuous changes that have occurred to a rock. To get to its present state, a metamorphic rock follows a pressure-temperature-time (PTT) path. Both the PTT path a rock has followed and the chemical composition of the parent rock primarily determine the mineral assemblages in metamorphic rocks.

For instance, at moderate temperature and pressure, the mineral contents of a basaltic parent rock react to form new minerals, including actinolite, a greenschist. At increased heat and pressure, the feldspar becomes richer in anorthite and the ferromagnesian minerals (minerals that contain iron and magnesium) react to form amphibole, giving its name to the amphibolite facies. As the heat and pressure increase again, orthopyroxene forms and the granulite facies begins. At very high pressures, the basaltic parent rock passes through the blueschist facies, forming high-pressure, low-temperature minerals such as glaucophane and jadeite. At still higher pressures, garnet and pyroxene occur, characteristic of the eclogite facies.

Pelitic parent rocks—claylike sediments with a large amount of aluminum—may give rise to an extensive set of mineral structures, but most of them consist of quartz, muscovite (see Mica), and three or four other minerals. Because of the complexity of these mineral structures, pelitic rocks are very important in determining the PTT paths of metamorphic rocks. Sandstone parent rocks consist predominantly of quartz, which recrystallizes on metamorphism. The principal changes in quartzites are therefore textural rather than mineralogical. In limestone parent rocks, the carbonate minerals recrystallize extensively to form marble.

V

GEOLOGIC IMPORTANCE

The different facies series are strongly related to plate tectonic movement. Metamorphic areas give important information on convergent-plate margins, where downward motion of the plates induces metamorphism.

Convergent plates cause crustal thickening and an increase in pressure at low temperature. This often results in blueschist metamorphism, but additional heating may change the blueschist to greenschist- and amphibolite-facies metamorphism, depending on the mineral composition of the colliding plates.

The collision of oceanic and continental plates results in subduction, or burial, of cold oceanic crust. Such subduction produces high-pressure, low-temperature conditions, which result in the formation of blueschists. In contrast, the collision of continental plates produces thickening and extensive regional metamorphism of the greenschist-amphibole-granulite type. Continental collisions are typically involved in large-scale tectonic activity, and detailed variations in pressure and temperature are often the result of large-scale thrusting and nappe development (the thrusting of rocks over other rocks). Evidence of such conditions is clearly seen in certain mountainous regions, including the Adirondack Mountains of northern New York State, in the United States, and the Alps in Europe.

Contact-metamorphic rocks are much more limited in extent and relate to more local aspects of geology. Nevertheless, they provide important information concerning the heat content, heat flow, and cooling history of both large-scale and small-scale igneous intrusions.

VI

ECONOMIC IMPORTANCE

Metamorphic rocks are an important source of building materials. Slate and marble are commonly used as finishing stone in buildings. Low-grade metamorphism of ultramafic igneous rocks (dark igneous rocks composed mostly of magnesium and iron) produces serpentine, a group of sheet-silicate minerals (crysotile, lizardite, antigorite) that are the principal sources of asbestos.

Metamorphism of impure limestones produces talc, a very soft silicate mineral that is an important mineral filler in paints, rubber, paper, asphalt, and cosmetics. High-grade marbles and granulites are important sources of sapphires and rubies, particularly in Sri Lanka and Myanmar (known as Burma until 1989).

Metamorphism is also an important process in concentrating specific elements to form deposits of ore bodies. In this way, low-grade gold occurrences may be concentrated during metamorphism, forming economically important gold deposits. This is the case in some deposits located in the Abitibi region of Quebec, Canada.

Contributed By:
Frank Christopher Hawthorne

Sedimentary Rock

Sedimentary Rock, in geology, rock composed of geologically reworked materials, formed by the accumulation and consolidation of mineral and particulate matter deposited by the action of water or, less frequently, wind or glacial ice. Most sedimentary rocks are characterized by parallel or discordant bedding that reflects variations in either the rate of deposition of the material or the nature of the matter that is deposited.

Sedimentary rocks are classified according to their manner of origin into mechanical or chemical sedimentary rocks. Mechanical rocks, or fragmental rocks, are composed of mineral particles produced by the mechanical disintegration of other rocks and transported, without chemical deterioration, by flowing water. They are carried into larger bodies of water, where they are deposited in layers. Shale, sandstone, and conglomerate are common sedimentary rocks of mechanical origin.

The materials making up chemical sedimentary rocks may consist of the remains of microscopic marine organisms precipitated on the ocean floor, as in the case of limestone. They may also have been dissolved in water circulating through the parent rock formation and then deposited in a sea or lake by precipitation from the solution. Halite, gypsum, and anhydrite are formed by the evaporation of salt solutions and the consequent precipitation of the salts.

Shale

Shale, common name applied to fine-grained varieties of sedimentary rock formed by the consolidation of beds of clay or mud. Most shales exhibit fine laminations that are parallel to the bedding plane and along which the rock breaks in an irregular, curving fracture. Shales are usually composed of mica and clay minerals, but the grains are so fine that the rock seems to have a homogeneous appearance, and individual minerals cannot be identified without the aid of a microscope. Most varieties of shale are colored in various shades of gray, but other colors, such as red, pink, green, brown, and black, are often present. Shales are soft enough to be scratched with a knife and feel smooth and almost greasy to the touch. All gradations in consistency exist between shales and clay; true shales differ from clays in their lack of plasticity in water. Many shales yield oil when distilled by heat, and the sedimentary rocks containing larger quantities of oil are called oil shales. Widely distributed throughout the world, oil shales are a source of oil for countries lacking petroleum.

Sandstone

Sandstone, coarse-grained, sedimentary rock consisting of consolidated masses of sand deposited by moving water or by wind. The chemical constitution of sandstone is the same as that of sand; the rock is thus composed essentially of quartz. The cementing material that binds together the grains of sand is usually composed of silica, calcium carbonate, or iron oxide. The color of the rock is often determined largely by the cementing material, iron oxides causing a red or reddish-brown sandstone, and the other materials producing white, yellowish, or grayish sandstone. When sandstone breaks, the cement is fractured and the individual grains remain whole, thus giving the surfaces a granular appearance. Sandstones of various geologic ages and of commercial importance are widely distributed in the U.S. Besides serving as a natural reservoir for deposits of oil and gas, sandstone is used in building flagstone pavings and in the manufacture of whetstones and grindstones.

Limestone (mineral)

I

INTRODUCTION

Limestone (mineral), a common sedimentary rock composed primarily of the mineral calcite (CaCO₃). Limestone constitutes approximately 10 percent of the sedimentary rocks exposed on the earth’s surface. It forms either by direct crystallization from water (usually seawater) or by accumulation of shell and shell fragments. In the first case, it carries a record of the chemical composition of seawater and it provides evidence of how that composition has changed with time. In the second case, limestone provides a record of the evolution of many important fossils. Limestone usually forms in shallow water less than 20 m (70 ft) deep and thus also provides important geological information on the variation in sea level in the past. Limestone rocks are frequently riddled with caves. Limestone is an important building stone and is used to make cement and concrete.

When a drop of dilute hydrochloric acid is placed on a piece of limestone, the acid reacts with the calcite and forms bubbles of carbon dioxide. This “fizz” reaction is so characteristic of limestone than many geologists carry a small bottle of dilute hydrochloric acid into the field for a rapid and easy identification of limestone.

II

COMPOSITION AND ORIGIN

The principal component of limestone is the mineral calcite, but limestone frequently also contains the minerals dolomite (CaMg(CO₃)₂) and aragonite (CaCO₃). Pure calcite, dolomite, and aragonite are clear or white. However, with impurities, they can take on a variety of colors. Consequently, limestone is commonly light colored; usually it is tan or gray. However, limestone has been found in almost every color. The color of limestone is due to impurities such as sand, clay, iron oxides and hydroxides, and organic materials.

All limestone forms from the precipitation of calcium carbonate from water. Calcium carbonate leaves solutions in many ways and each way produces a different kind of limestone. All the different ways can be classified into two major groups: either with or without the aid of a living organism.

Most limestone is formed with the help of living organisms. Many marine organisms extract calcium carbonate from seawater to make shells or bones. Mussels, clams, oysters, and corals do this. So too do microscopic organisms such as foraminifera. When the organisms die their shells and bones settle to the seafloor and accumulate there. Wave action may break the shells and bones into smaller fragments, forming a carbonate sand or mud. Over millions of years, these sediments of shells, sand, and mud may harden into limestone. Coquina is a type of limestone containing large fragments of shell and coral. Chalk is a type of limestone formed of shells of microscopic animals.

Limestone can also be formed without the aid of living organisms. If water containing calcium carbonate is evaporated, the calcium carbonate is left behind and will crystalize out of solution. For example, at Mammoth Hot Springs in Yellowstone National Park, hot water containing calcium carbonate emerges from deep underground. As the hot water evaporates and cools, it can no longer hold all of the calcium carbonate dissolved in it and some of it crystallizes out, forming limestone terraces. Limestone formed from springs is called travertine. Calcium carbonate also precipitates in shallow tropical seas and lagoons where high temperatures cause seawater to evaporate. Such limestone is called oolite. Calcium carbonate that precipitates from water dripping through caves is responsible for the formation of beautiful cave features such as stalactites and stalagmites.

III

DIAGENESIS OF LIMESTONE

Diagenesis is the name for those processes that affect sediment after it is deposited and prior to any metamorphism. Two processes of diagenesis are important in the formation of limestone. One is cementation, in which calcium carbonate precipitates in the pore space between the loose grains of sediment and binds them together into a hard compact rock.

The other process involves the alteration of the minerals in the limestone. When calcium carbonate precipitates, it can form two different minerals—calcite and aragonite. Calcite and aragonite are polymorphs, meaning that they have the same chemical composition, but the atoms are stacked differently in the crystal. Fresh calcium carbonate sediments sometimes contain calcite, sometimes they contain aragonite, and often they contain a mixture of the two. This is because some animals make shells of calcite while others make shells of aragonite. Similarly, the direct precipitation of calcium carbonate without the aid of organisms sometimes produces calcite, sometimes produces aragonite, and often produces a mixture of the two, depending on factors such as temperature and pressure. However, calcite is more stable than aragonite, and so, through diagenesis, aragonite slowly changes to calcite. In addition, calcite slowly absorbs magnesium from surrounding water, slowly changing to dolomite.

IV

WEATHERING AND METAMORPHISM

Limestone is easily weathered and eroded. Water trickling through large limestone deposits often erodes extensive underground drainage systems of sinkholes and caves. The land surface above large limestone deposits is often irregular, marked with potholes (formed when the roofs of shallow caves collapse), sinkholes, and few streams, most of which are short in length and end in a sinkhole. The irregular surface is known as karst topography, named after a limestone plateau in the Dinaric Alps of northwest Slovenia and northeast Italy. Within the caverns, secondary precipitation of calcite by percolating waters produces stalactites and stalagmites, and spectacular underground ‘scenery’ can result.

When limestone undergoes metamorphism, it turns into marble. If the limestone contains other materials such as sand and clay, the calcite will react with them to produce minerals such as tremolite, epidote, diopside, and grossular garnet.

V

INDUSTRIAL IMPORTANCE

Limestone is an important building stone in many parts of the world. It is normally quarried from surface outcrops. Limestone is used as cut stone for building, and is common throughout Europe in cathedrals and palaces where the relatively soft nature of the stone allows decorative carving. Limestone is widely used as crushed stone, or aggregate, for general building purposes, roadbeds and railway lines. Finely crushed limestone is also used as filler in industrial products such as asphalt, rubber, plastic, and fertilizers. When heated, the calcium carbonate in limestone decomposes to lime, or calcium oxide, and is important as a flux in smelting copper and lead ores and in making iron and steel. Lime is a key ingredient in the manufacture of cement and concrete.

Limestone formations are sometimes associated with petroleum reserves or ore deposits, which accumulate in the eroded spaces of caves and caverns. The galena (lead sulfide) and sphalerite (zinc sulfide) ore deposits in the tri-state region of Missouri, Kansas, and Oklahoma in the United States are an example of such deposits.

Halite

Halite, mineral form of common salt, with the chemical composition sodium chloride, NaCl. Halite, also called rock salt, is a common mineral, formed by the drying of enclosed bodies of salt water; subsequently the beds so formed have often been buried by the rock strata formed from other sedimentary deposits. Beds of halite range in thickness from a few meters to 30 m (100 ft) and have been found at great depths beneath the surface of the earth. This mineral is often found associated with gypsum, sylvite, anhydrite, calcite, clay, and sand. Halite is widely disseminated over the world; in the United States notable deposits are found in New York, Michigan, Ohio, Kansas, New Mexico, and Utah.

Halite crystallizes in the isometric system (see Crystal), usually in the form of cubes, and shows perfect cubic cleavage. It is colorless and transparent when pure but is often tinted yellow, red, blue, or purple by impurities. It has a hardness of 2.5 and a specific gravity of 2.16.

Annual U.S. production of rock salt is about 36 million metric tons. See Salt.

Gypsum

Gypsum, common mineral consisting of hydrated calcium sulfate (CaSO₄·2H₂O). It is a widely distributed form of sedimentary rock, formed by the precipitation of calcium sulfate from seawater, and is frequently associated with other saline deposits, such as halite and anhydrite, as well as with limestone and shale. Gypsum is produced in volcanic regions by the action of sulfuric acid on calcium-containing minerals; it is also found in most clays as a product of the action of sulfuric acid on limestone. It occurs in all parts of the world; some of the best workable deposits are in France, Switzerland, and Mexico, as well as in California, Ohio, Michigan, and Utah in the United States. Alabaster, selenite, and satin spar are varieties of gypsum.

Artificial gypsum is obtained as a by-product in an old method for the manufacture of phosphoric acid. Phosphate rock, the essential constituent of which is tricalcium phosphate, is treated with sulfuric acid, producing phosphoric acid and gypsum. The gypsum is compacted into blocks and used for the construction of nonsupporting walls in buildings. By properly controlling the concentration and temperature of sulfuric acid added to phosphate rock, a mixture of monocalcium phosphate, dicalcium phosphate, and gypsum may be obtained. This mixture is a valuable fertilizer, superphosphate.

Gypsum crystallizes in the monoclinic system in white or colorless crystals, massive or foliated in formation. Many specimens are colored green, yellow, or black by impurities. With a hardness ranging from 1.5 to 2, it is soft enough to scratch with a fingernail and has a specific gravity of 2.3. When heated to 128° C (262.4° F), it loses part of its water of hydration and is converted into plaster of Paris, CaSO₄·yH₂O. Finely ground plaster of Paris, when moistened with water, sets in a short time into a hard mass of gypsum, the rehydrated crystals forming and interlocking in such a way as to cause expansion in volume.

Because of its property of swelling and filling all interstices upon drying, plaster of Paris is used extensively in making casts for statuary, ceramics, dental plates, fine metal parts for precision instruments, and surgical splints. Uncalcined gypsum is used as a fertilizer in the form of land plaster for arid, alkaline soil. It is also used as a bed for polishing plate glass and as a basis for paint pigments. Large amounts of gypsum are used as a retarder in portland cement.

Plaster

Plaster, a pasty composition that hardens on drying and is used for coating interior walls, ceilings, and partitions. Plaster is generally composed of sand, water, and a cementing agent such as gypsum, lime, or portland cement. When using plaster to coat a surface, hair or fiber is mixed with the first and second coats to strengthen the plaster. Wallboard, which is used as a substitute for plastering, is a large sheet of prefabricated material made of fiberboard, paper, or felt with a hardened gypsum plaster core. The term plaster is sometimes also applied to molded ornamental walls and ceilings (see Stucco) and to plaster of Paris.

Alabaster

Alabaster, varietal name applied to two different minerals. One, Oriental alabaster, was extensively used by the ancient Egyptians. It is a variety of calcite, with a hardness of 3; it is usually white and translucent, but is often banded with dark or colored streaks. The other mineral, true alabaster, is a variety of gypsum, usually snow-white in color with a uniform, fine grain. True alabaster is softer than Oriental alabaster; it has a hardness of 1.5 and is easily carved into intricate shapes. Deposits of fine gypsum alabaster are found in Italy, England, Iran, and Pakistan.

The materials making up chemical sedimentary rocks may consist of the remains of microscopic marine organisms precipitated on the ocean floor, as in the case of limestone. They may also have been dissolved in water circulating through the parent rock formation and then deposited in a sea or lake by precipitation from the solution. Halite, gypsum, and anhydrite are formed by the evaporation of salt solutions and the consequent precipitation of the salts.

Dolomite

Dolomite, common mineral with the formula CaMg (CO3)2, found chiefly in rock masses as dolomitic limestone, but occurring sometimes in veins. It has a hardness of 3.5 to 4 and a specific gravity of 2.85. Dolomite crystallizes in the hexagonal system (see Crystal). It is usually colorless, white, or pink, but may be brown, black, or green, depending on the impurities present. In the United States it is found in many localities in Vermont, Rhode Island, New York, and New Jersey. Good crystals of dolomite have been obtained from deposits at Joplin, Missouri. When treated with sulfuric acid, dolomite yields calcium sulfate (gypsum) and magnesium sulfate (Epsom salts). Calcined (heated) dolomite is extensively employed as a lining for Bessemer converters in the production of steel from pig iron

White Sands National Monument

White Sands National Monument, national monument in southern New Mexico, near Alamogordo. The monument is situated in the Tularosa Valley. The park, established in 1933, contains the largest gypsum dune field in the world. The dunes, some as high as 18 m (60 ft), consist of gypsum crystals deposited in an ancient lake bed. Holloman Air Force Base and the White Sands Missile Range, where the first explosion of an atomic bomb occurred in 1945, are located nearby. Area, 58,167 hectares (143,733 acres).

Fault (geology)

I

INTRODUCTION

Fault (geology), crack in the crust of the earth along which there has been movement of the rocks on either side of the crack. A crack without movement is called a joint. Faults occur on a wide scale, ranging in length from millimeters to thousands of kilometers. Large-scale faults result from the movement of tectonic plates, continent-sized slabs of the crust that move as coherent pieces (see Plate Tectonics). 

II

HOW FAULTS ARE CREATED

Faults are created by stress in the earth’s crust. Stress is a force, such as squeezing or stretching, that changes the shape of an object. When a material is stressed, the material may respond in three different ways. It can deform (stretch or compress) elastically, which means that when the stress is removed, the material goes back to its original shape. An elastic deformation is, therefore, reversible. Alternatively, a stressed material can deform inelastically, which means that when the stress is removed, the material stays in its new, deformed shape. An inelastic deformation is irreversible. Lastly, a stressed material can fracture, or break into pieces.

Most solid materials, including rocks, deform elastically under small stress. Ductile materials are materials that will deform inelastically under moderate stress and will fracture under higher stress. Brittle materials fracture with little or no inelastic deformation. Rocks tend to be brittle when they are cold and become more ductile when they are hot. Rocks also tend to become more ductile when they are under pressure. For these reasons, most rocks are brittle near the surface of the earth where there is less heat and pressure, and they become more ductile with depth. Most faults occur in the top 10 km (6 mi) of the crust. Below this depth, most rocks bend and fold in response to stress.

Rocks can experience three different kinds of stress: tension, compression, and shearing. Tension pulls rocks apart, pushing opposite sides away from each other. Compression squeezes rocks, pushing opposite sides toward each other. Shearing pushes opposite sides past each other in opposite but parallel directions. These different kinds of stress create different classes of faults, as discussed below. 

III

TERMINOLOGY AND CLASSIFICATION

The two sides of a fault are separated by a fault plane. Two different terms are used to describe a fault plane’s orientation, or position in the crust. These terms are strike and dip. The strike describes the orientation of a fault plane in terms of compass directions. The dip describes how steeply a fault plane dips into the ground. Dip varies between 0° for a horizontal fault and 90° for a vertical one.

Geologists are also interested in how far the two sides of a fault have moved along that fault. The total distance that the two sides have moved relative to each other is called the net slip. The net slip is made up of slip measured along the direction of strike and along the direction of dip of the fault plane. The strike-slip distance is the horizontal motion measured in the direction of the strike. The dip-slip distance is measured in the direction of the dip. The dip-slip distance is similar to the throw, which is the vertical movement along the fault.

Unless the dip is exactly 90°, one side of a fault will hang over the other. The side overhanging the fault plane is called the hanging wall and the side underlying the fault plane is called the footwall.

When the hanging wall has moved downward relative to the footwall, the fault is known as a normal fault. Such faults are associated with crustal tension and represent areas where the crust is being stretched. They are common at divergent-plate boundaries where two crustal plates move away from each other.

When the hanging wall has moved upward relative to the footwall the fault is called a reverse fault unless the dip is nearly horizontal, in which case it is called a thrust fault. Both of these kinds of faults are associated with crustal compression and represent areas where the crust is being shortened. They are common at convergent-plate boundaries where two crustal plates are colliding. Thrust faults can push old rocks over younger rocks, reversing the normal pattern of younger rocks lying on top of older rocks.

When the net slip is entirely horizontal (with no vertical component), the fault is known as a strike-slip fault because the net displacement is parallel to the strike. Such faults are associated with crustal shearing. They are common at transform-plate boundaries where two crustal plates are moving past each other. The San Andreas Fault in California is an example of a strike-slip fault that occurs at a transform boundary where the North America plate is sliding past the Pacific plate. If, when facing a strike-slip fault, the far block is displaced to the right, then the fault is known as a right-lateral fault. If the far block is displaced to the left, then the fault is known as a left-lateral fault. A fault that combines some motion along the strike and along the dip is known as an oblique-slip fault.

IV

EARTHQUAKES AND FAULTS

Earthquakes are sudden movements in the earth’s crust. They occur along faults when stress building up in the crust is suddenly released. Most earthquakes occur along plate boundaries. 

V

LANDFORMS ASSOCIATED WITH FAULTS

Faults create several unique landforms. One of the most common is a fault scarp, a cliff produced when the earth’s surface on one side of the fault plane rises relative to the surface on the other side of the fault. Another common landform produced by faults, especially normal faults, is a graben. A graben is a low block of rock surrounded on two sides by parallel fault scarps leading to higher land. The valley comprised of the graben and the fault scarps on either side is called a rift valley. The Great Rift Valley in Africa is an extensive example of a rift valley that stretches more than 4800 km (more than 3000 mi) from Syria to Mozambique. Sometimes, large sets of parallel normal faults produce an alternating pattern of raised and sunken blocks. The Basin and Range region in the southwestern United States consists of hundreds of sunken blocks (basins) and raised blocks (ranges).  

VI

EFFECTS ON ROCKS

Faults affect the rocks in which they occur. Movement on the fault surface can produce grooves or scratches, called striae, and the polished surface on which striae occur is termed a slickenside surface. Movement that is more extensive can result in the crushing of rock along the fault surface. This produces a zone of crushed rock called either a fault breccia (if the material is coarse grained) or a fault gouge (if the material is fine grained). Because breccia is porous, zones of breccia are extremely important as pathways for the flow of groundwater or hydrothermal fluids. The presence of fluids can lubricate the fault surface, promoting further movement. If the fluids that flow along fracture surfaces and brecciated zones carry dissolved metal, they can leave behind significant ore deposits along these zones.