All About Geology

Geology

I

INTRODUCTION

Cliffs of Normandy

Composed of the same form of limestone as the White Cliffs of Dover in England, the Cliffs of Normandy are a distinctive landmark on the French coastline. Geological evidence suggests that a land bridge connected the two land masses during the Cretaceous Era. The arches shown here are a result of the action of water on softer sections of the rock.

Alan Carr/Robert Harding Picture Library

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

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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

Geologic Time Scale

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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

Fossil-bearing Rocks

Sedimentary rocks, such as this fossil-bearing limestone, can help geologists determine geologic time. Because the bottom layers were deposited first, the oldest fossils are found in the bottom layers of sedimentary rocks. The accumulation of shells or shell fragments and other fossils in limestone provides geologists with a record of the evolution of the animals that used to live in the ancient oceans.

Carolina Biological Supply/Phototake NYC

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

Uluru

Uluru, also called Ayers Rock, is one of the largest monoliths, or rock masses, in the world. Located in Uluru-Kata Tjuta National Park in central Australia, the monolith measures about 3.6 km (2.2 mi) long and 348 m (1,142 ft) high. Rock paintings made thousands of years ago by Aboriginal artists cover the walls of many caves in Uluru.

Art Wolfe/Tony Stone Images

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

Eruption of Mount Saint Helens

The volcano known as Mount Saint Helens, in the southwestern portion of Washington state in the United States, began to erupt on March 27, 1980, after a long period of dormancy. It continued to burble until its first large-scale eruption on May 18, 1980. This violent blast sent clouds of ash and other volcanic debris into the atmosphere and killed 57 people. With the eruption, the mountain’s elevation dropped from 2950 m (9677 ft) to 2550 m (8365 ft).

Krafft-Explorer/Science Source/Photo Researchers, Inc.

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

Rock Crystals in a Lunar Sample

A petrographic microscope is used to make an image of a thin sample of lunar rock. The different colors represent different mineral compositions.

Jan Hinsch/Science Source/Photo Researchers, Inc.

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

Relief Map

Relief maps are three-dimensional models of the terrain in an area; on them, color and scale are used to indicate geographical features rather than simply to delineate political boundaries. Because of this feature, relief maps are extensively used in engineering and the military. This map shows portions of Alaska and northwestern Canada.

United States Geological Survey

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

Stratigraphic Column

Fossils preserved in rock strata provide scientists with clues to evolutionary history. This stratigraphic column is based on paleontological evidence and shows the order in which organisms appeared in the fossil-rich Paleozoic era. Each layer represents a particular time frame and shows a representative organism which flourished during that time. Although fossils are rarely found in the idealized and localized fashion shown here, they are often in more or less chronological order. Generally, the oldest fossils appear in lower layers and the most recent fossils at the top, so that placement may be used as an aid in dating the specimens.

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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

Folded Rocks of the Dynamic Earth

The theory of plate motions explains how mountains are built by forces that shape the earth’s crust. Large pieces of crust on the surface of the earth move laterally. This creates huge compressional forces that may bend or even break rocks. These sedimentary rock layers show an anticlinal fold, in which the layers bend downward from the crest.

V. Englebert/Photo Researchers, Inc.

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

Paleontologist with Seismosaurus Bone

Paleontologists often spend hours to uncover a single bone, painstakingly removing the dirt and rock that surrounds it. Here, a paleontologist reattaches a rib bone of a seismosaurus before excavation continues.

Ray Nelson/Phototake NYC

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 , , John Wesley Powell, and George Wheeler conducted four surveys of the American West. These surveys led to several new concepts in geology. American geologist 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. Tuzo 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

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