HISTORICAL GEOLOGY

CONCEPT

Geologists are concerned primarily with two subjects: Earth's physical features and the study of the planet's history. These two principal branches of geology are known, appropriately enough, as physical geology and historical geology. Today they are of equal importance, but in the early modern era, geologists were most focused on topics related to historical geology, in particular, Earth's age and the means by which Earth was formed. This debate pitted adherents of religion, which seemed to require a very young Earth, against adherents of science. A breakthrough came with the introduction of uniformitarianism, a still-influential principle based on the idea that the geologic processes at work today have always been at work. Opposing uniformitarianism was catastrophism, or the idea that Earth was formed in a short time by a series of cataclysmic events. Discredited at the time, catastrophism later gained acceptance, though this did not lead to support for the concept of a young Earth. In fact, the planet is very old—so old that all of human history is almost inconceivably short in comparison.

HOW IT WORKS

Explaining Origins

For thousands of years, humans were content to rely on religiously inspired stories, rather than scientific research, to provide an explanation regarding Earth's origins. This topic, along with the scientific challenge to those early accounts of Earth's formation, is discussed in considerable detail within the essay Earth, Science, and Nonscience. Other aspects of the subject, particularly the challenge to mythological explanations put forward by earth scientists in modern times, are examined here.

All religious explanations of the planet's origins can be called myths, which is not necessarily a pejorative term: a myth is simply a story to explain how something came into being. So pervasive are myths about geology that a term, geomythology, has been coined to identify such myths. Geomythology in particular and mythology in general stand in sharp contrast to scientific explanations derived by using the scientific method of observation, hypothesis formation, testing of hypotheses, and the development and testing of theories.

CONTRASTING SCIENCE AND RELIGIOUS GEOMYTHOLOGY.

Science and myth have in common the aim of explaining how things came to be, but the means by which they reach that explanation are quite different. So, too, are the reasons that drive science on the one hand and religion or myth on the other in seeking to develop such an explanation.

The most famous of all religious explanations of Earth's origins, of course, is that found in the biblical book of Genesis. Probably written in the latter part of the second millennium b.c., it offered a compelling story of creation that virtually defined the Western view of Earth's origins for more than 1,500 years, from about a.d. 300 to the beginning of the nineteenth century. Its purpose, of course, was not scientific, and it was not written as the result of research; rather, the Genesis account depicts nature as a vast stage on which a cosmic drama of love, sin, redemption, and salvation has been played out over the ages.

By contrast, the scientific search for Earth's origins is driven merely, or at least primarily, by curiosity to explain how things came to be as they are. Scientists certainly have their biases and are just as capable of error as anyone, yet at least they have a standard in the form of the scientific method. If a scientist's findings, and the resulting theory, with stand the rigorous testing required by the scientific method, the theory is rewarded with increasing acceptance and new research designed to test further its ability to explain the world. If the theory fails those tests, its adherents may hold on to it for a time, but eventually they die off, and the theory is discarded. On the other hand, adherence to the religious explanation of Earth's origins has proved more intractable, as we shall see.

The Religious War Concerning Earth's Origins

The great Italian artist and scientist Leonardo da Vinci (1452-1519) was among the first Western thinkers to speculate that fossils might have been made by the remains of long-dead animals. This was a daring supposition to make in the Renaissance, and it would become even more daring to uphold such an idea in the centuries that followed. The concept of fossils seemed to imply an Earth older than the biblical account suggested, and with the Catholic Church under attack by the forces of religious reformation (i.e., Protestantism) and other forms of "heresy," church leaders became less and less inclined to tolerate any deviation from orthodoxy.

The ecclesiastical view of Earth's history reached a sort of extreme in the seventeenth century, with the Irish bishop James Ussher (1581-1656). The New Testament contains a thorough accounting of Jesus' lineage, both through his mother and his earthly father, Joseph, all the way back to the time of Adam. Jesus was descended from David, whose lineage is provided in the Old Testament, complete with each ancestor's life span and the age at which he fathered a successor in the Davidic line of descent. From these figures, Ussher concluded that God finished making Earth at 9:00 a.m. on Sunday, October 23, 4004 b.c. Accepted by the Church, Ussher's calculation gave the idea of a very young Earth an aura of "scientific" justification.

Ironically, one of the first scientists to discover evidence that pointed toward an extremely old Earth was also a minister, the English astronomer Henry Gellibrand (1597-1636). While researching Earth's magnetic field, Gellibrand discovered that the field had changed over time (as indeed it has—for more on this subject, see Geomagnetism). This was one of the first indications that the planet's history can be studied scientifically, even though humans have no direct information regarding the origins of Earth.

Early Stratigraphic Studies

After Gellibrand came the Danish geologist Nicolaus Steno (1638-1687), who studied the age of rock beds. Thus was born the concept of stratigraphy, or the study of rock layers beneath Earth's surface, which revealed a great deal about the planet's age. (See Stratigraphy, which discusses many topics related to historical geology.) Along with the English physicist Robert Hooke (1635-1703) and others, Steno also became one of the first thinkers to confront the possibility that Earth must be much more than 6,000 years old.

During the eighteenth century, the German geologist Johann Gottlob Lehmann (1719-1767) built on ideas introduced by Steno concerning the formation of rock beds. Lehmann put forward the theory that certain groups of rocks tend to be associated with each other and that each layer of rock is a sort of chapter in the history of Earth. Aspects of Lehmann's theory were incorrect, but the general principle marked an advancement over previous ideas in geology and helped point the way toward a new view of the earth sciences.

Previously, geologic studies had tended to be qualitative and descriptive, meaning that earth scientists used very generalized terminology and failed to possess a grasp of larger issues. Thanks to Lehmann and others who followed him, the earth sciences became more truly quantitative and predictive, offering explanations of what had happened in the past, along with justifiable theories concerning what might happen in the future.

The German geologist Abraham Gottlob Werner (1750-1817) put forward a theory that was largely incorrect, yet one that nevertheless advanced the earth sciences. His "neptunist" theory was based on the idea that water had been the main force in shaping Earth's surface. Though this theory was not accurate, his idea was significant, because it constituted the first well-ordered geologic theory of Earth's origins and early history. At the same time, that history was turning out to be very long indeed.

Religion and Earth's Age

In 1774 the French mathematician Georges-Louis Leclerc, Comte de Buffon (1707-1788), applied new scientific ideas to the study of Earth and estimated its age at 75,000 years. Privately he admitted that he actually thought Earth was billions of years old but did not think that such a figure would be understood. At the time, after all, the concept of a "billion" was hardly a familiar one, as it is today. More important, the idea of Earth being that old was shocking and downright frightening to people who accepted a strict interpretation of the biblical account.

In an earlier century, the Italian astronomer Galileo Galilei (1564-1642) had been forced, on pain of death, to recant his support for the Polish astronomer Nicolaus Copernicus's (1473-1543) discovery that Earth is not the center of the universe. In Buffon's day, by contrast, few Europeans faced such dire threats for endorsing apparently unbiblical ideas. A scientist could still lose his job for supporting the wrong principles, and thus Buffon had to renounce his position on threat of losing his post at the University of Paris.

AN ATHEISTIC REACTION.

Other forces were at work in the sciences during the eighteenth century, and some were openly hostile to religious belief. An extreme example was the French physician and philosopher Julien de La Mettrie (1709-1751), a leading figure in the mechanist school of the biological sciences. La Mettrie maintained that humans are essentially a variety of monkey, to whom they were superior only by virtue of possessing the power of language. Moving far beyond the territory of science itself, he also taught that atheism is the only road to happiness and that the purpose of human life is to experience pleasure.

In the physical sciences, an interesting example of reaction to religious belief can be found in the case of the French mathematician Pierre Simon de Laplace (1749-1827). Like those of La Mettrie, Laplace's aims were not purely scientific; instead, he envisioned himself as a warrior against religious belief. Correctly enough, Laplace maintained that the origins of the universe as well as its workings could be explained fully without any reference to God. He also introduced a highly influential theory, widely accepted today, that the solar system originated from a cloud of gas. (See the entries Planetary Science and Sun, Moon, and Earth.)

Like La Mettrie, Laplace took his ideas far beyond their justifiable purview in the realm of science, however, wielding them as a sword in a religious war. Laplace maintained that because it was possible to discuss the origins of the cosmos without reference to God, there must be no God—which is far from a logically necessary conclusion. Misguided as La Mettrie's and Laplace's atheistic crusades may have been, they are historically understandable: in France, far more than anywhere in western Europe except perhaps Spain, the Church had come to be seen as a force of political oppression, allied as it was with the French royalty. It is no wonder, then, that the French Revolution of 1789 was directed as much against the Church as against the king.

REAL-LIFE APPLICATIONS

Uniformitarianism

Late in the eighteenth century, the Scottish geologist James Hutton (1726-1797) put forward an idea that transcended the debate over Earth's origins. Rather than speculate as to how Earth had come into being, Hutton analyzed the processes at work on the planet in his time and reasoned that they must be a key to understanding the means by which Earth was shaped. This was the principle of uniformitarianism, which is still a key concept in the study of Earth. Thanks to his introduction of this influential idea, Hutton today is regarded as the father of modern scientific geology.

Uniformitarianism, in general, is the idea that the geologic processes at work today provide a key to understanding the geologic past. This means that the laws of nature have always been the same. The uniformitarianism promoted by Hutton and his fellow Scottish geologist Charles Lyell (1797-1875), however, has undergone some modification, namely, by the addition of the qualifying statement that the speed and intensity of those processes may not always be the same at any juncture in geologic history. For instance, land does not erode today at the same rate that it did before plants existed to hold rocks and soil in place.

GOULD'S FOUR UNIFORMITIES.

In the late twentieth century, the American paleontologist Stephen Jay Gould (1941-2002) identified four different meanings of uniformity in science, not all of which are equally valid. Gould's listing and analysis of these four meanings is as follows:

• Uniformity of law: The assumption that natural laws do not change over time. This idea governs all sciences.
• Uniformity of process: The idea embodied in the most well-known definition of geologic uniformitarianism, "The present is key to the past."
• Uniformity of rate: The incorrect assumption that the rate at which processes occur presently is the same as the rate at which they occurred in the past.
• Uniformity of state: The incorrect assumption that the state of the universe always has been as it is today.

As noted, uniformity of law is essential to all sciences. For instance, there is every reason to believe that the conservation of energy (a law stating that the total amount of energy in the universe remains constant) always has been the case. If the contrary were true, the conservation of energy could no longer properly be called a law, because it might cease to be the case at some time in the future.

The statement "The present is key to the past" was formulated by yet another Scottish geologist, Sir Archibald Geikie (1835-1924). Geikie's statement often has been criticized as an oversimplification, because processes that occurred in the past may not necessarily be occurring now, or vice versa, even though they could occur again. This idea has required modification of uniformitarianism, as noted earlier, to take into account the fact that the speed and intensity of processes may not always be the same. Part of this modification has involved acceptance of a form of catastrophism, discussed later in this essay.

Variations in the speed and intensity of processes also were addressed by Gould, with his observation that "uniformity of rate" is a fallacy. So, too, is "uniformity of state," which is one of the few areas on which adherents of creationism (a strict interpretation of the Genesis account) would agree with their opponents. Even the Bible, after all, says "In the beginning … the earth was without form, and void."

Catastrophism

In Theory of the Earth (1795), Hutton suggested that the weathering effects of water produced the sedimentary layers of Earth. Based on observation of river flow and mud content, he realized that this process would require much longer than 6,000 years. So, too, did Lyell, author of the highly influential Principles of Geology, which appeared in 12 editions from 1830 to 1875 and which presented a strict version of uniformitarianism.

Aqueducts and other structures erected by the Romans had stood for a good one-fourth to one-third of the entire history of Earth, assuming that it was as young as Ussher's biblical interpretation implied. Yet these Roman constructions had experienced very little weathering and certainly much less than mountains would have had to experience to leave behind the sediments observed by geologists. Surely, then, Earth must be millions upon millions of years old, not just a few thousand.

CUVIER'S CATASTROPHIC THEORY.

Not so, countered adherents of a movement known as catastrophism, which arose in opposition to uniformitarianism during the late eighteenth and early nineteenth centuries. Catastrophism associates geologic phenomena with sudden, dramatic changes rather than ongoing and long-term processes, as in uniformitarianism. The leading proponent of catastrophism was the French geologist Baron Georges Cuvier (1769-1832), who used this theory to explain unconformities. These apparent gaps in the geologic record, revealed by observing rock layers, or strata, are discussed in the essay, Stratigraphy.

Whereas Cuvier's countryman (and fellow French noble) Buffon had asserted that Earth was 75,000 years old while actually believing that it was much older, Cuvier maintained that the planet is just 75,000 years old. The formation of mountains and other landforms, which should have taken millions of years, could be explained by sudden, violent changes, an example of which was Noah's Flood in the Book of Genesis. As the ocean waters receded, they moved rocks far from their sources, carved out valleys, and left behind lakes and other bodies of fresh water.

CATASTROPHISM TODAY.

As more and more evidence for a very old Earth began to accumulate during the nineteenth century, catastrophism fell into disfavor. Discoveries from the 1970s onward, however, influenced a new look at catastrophism, and, as a result, the idea has received new attention in later years.

This has not led to a wholesale endorsement of creationism; rather, scientists have come to understand that the generally steady pace of processes on Earth periodically is broken by catastrophic events. Most notable among types of catastrophe is the collision of a meteorite with Earth, a remarkable example of which apparently occurred some 65 million years ago. That dramatic event seems to have forced so much dust and gas into the atmosphere that it blocked out the Sun, leading to the ultimate extinction of the dinosaurs.

Understanding Geologic Time

So just how old is Earth? Modern earth scientists working in the realm of historical geology, and specifically geochronology, estimate its age at about 4.6 billion years. (The dating techniques used to determine the age of the planet are discussed in the essay Stratigraphy.) Such a vast span of time is more than a little difficult for humans to comprehend, given the fact that our lives last 70-80 years, on average, and the entire history of human civilization is only about 5,500 years long.

For this reason, it is helpful to use scales of comparison, such as that offered at the Web site listed under the title Comprehending Geologic Time. Suppose that the entire geologic history of Earth were likened to a single year of 365.25 days, starting with the formation of the planet from a cloud of dust and ending with the present. More than two months would have been required simply for the accretion of Earth from a gas cloud to a planetesimal to something like its present form, but by about March 5 this evolution would have been accomplished.

The entire spring would be analogous to a long, long period of time in which Earth was pounded by meteor showers and the oceans began to form. Not even the oldest known rocks date back this far, and many of our ideas about this phase in Earth's history are based on conjecture. Much more is known about the second half of geologic history, beginning with the origins of the first single-cell life-forms on June 16.

FROM SINGLE CELLS TO DINOSAURS.

We are now almost halfway through the year and still a long, long way from any sort of complex living beings. This is not surprising, given the fact that the formation of the continental plates and the development of oxygen in the atmosphere would have occurred only by about August 26. Even in the week after Thanksgiving, the most complex organisms would have been snails. Finally, a few days before the beginning of December, creatures would have begun to invade the land.

We tend to associate the dinosaurs with the early phases of Earth's history, but this only illustrates our distorted view of geologic time. In fact the Jurassic period, when dinosaurs roamed Earth, would be parallel to a period of about five days, from December 15 to 20. By Christmas Day, the meteorite referred to earlier would have hit Earth, and the dinosaurs would be headed toward extinction, their dead bodies eventually forming the fossil fuels that have powered much of human civilization.

THE SHORT SPAN OF HUMANITY'S EXISTENCE.

By this point, we are within a few days of the year's end, and yet nothing remotely resembling a human has appeared. Our own species, Homo sapiens, would not have come on the scene until the last 0.16 days of the year—that is, at a few minutes after 8:00 p.m. on December 31. The New Year's Eve countdown would be nearing by the time human civilization began, at about 42 seconds before midnight.

Now we have come to a period about 6,000 years ago, or the point at which, according to Bishop Ussher, Earth was created. No wonder many people wanted to believe in a young Earth, and some even hold on to that belief today: when viewed against the backdrop of the planet's true age, humanity seems very insignificant indeed. Christ's birth would have occurred at about 14 seconds before midnight, and the final 10-second countdown would begin about the time the Roman Empire fell. The life span of the average person would correspond to about half a second or less.

How Do We Know Earth's Age?

What we know about Earth's age comes, of course, not from direct observation but from the study of materials. One of the most important techniques for determining the age of samples taken from the earth is radiometric dating, discussed in more detail in Geologic Time. Radio-metric dating involves ratios between two different kinds of atoms for a given element: stable and radioactive isotopes. Because chemists know how long it takes for half the isotopes in a given sample to stabilize (a half-life), they can judge the age of the sample by examining the ratio of stable to radioactive isotopes. In the case of uranium, one isotopic form, uranium-238, has a half-life of 4,470 million years, which is very close to the age of Earth itself. Use of uranium dating has detected rocks of an age between 3.8 and 3.9 billion years old, as well as even older crystal formations that suggest the earth had solid ground as early as 4.2 billion years ago.

A rock discovered in the Australian desert during the early 1980s appears to be the oldest rock sample in the world, according to data originally reported in Nature and included on the Scientific American Web site in early 2001. This zircon crystal, according to Simon Wilde of Curtin University in Western Australia, is 4.4 billion years old. Wilde and associates reported that extensive study of the sample suggested that at the time of its formation, Earth was already covered in water—something that had supposedly happened many millions of years later. If this was the case, it could suggest the possibility that life appeared much earlier than has previously been supposed, and perhaps even that life disappeared and reappeared several times before finally taking hold.

WHERE TO LEARN MORE

Bishop, A. C., A. Woolley, and A. Hamilton. Cambridge Guide to Minerals, Rocks, and Fossils. New York: Cambridge University Press, 1992.

Boggy's Links to Stratigraphy and Geochronology (Web site). <http://geologylinks.freeyellow.com/stratigraphy.html>.

Comprehending Geologic Time (Web site). <http://www.athro.com/geo/hgfr1.html>.

Harris, Nicholas, Alessandro Rabatti, and Andrea Ricciardi. The Incredible Journey to the Beginning of Time. New York: Peter Bedrick Books, 1998.

"Historical Geology." Georgia Perimeter College (Web site). <http://www.dc.peachnet.edu/~pgore/geology/geo102.htm>.

Lamb, Simon, and David Sington. Earth Story: The Shaping of Our World. Princeton, NJ: Princeton University Press, 1998.

MacRae, Andrew. Radiometric Dating and the Geological Time Scale (Web site). <http://www.talkorigins.org/faqs/dating.html>.

Reeves, Hubert. Origins: Cosmos, Earth, and Mankind. New York: Arcade, 1998.

Spickert, Diane Nelson, and Marianne D. Wallace. Earthsteps: A Rock's Journey Through Time. Golden, CO: Fulcrum Kids, 2000.

UCMP (University of California, Berkeley, Museum of Paleontology) Web Time Machine (Web site). <http://www.ucmp.berkeley.edu/help/timeform.html>.

KEY TERMS

CATASTROPHISM:

The idea that geologic phenomena are brought about by sudden dramatic changes rather than ongoing and long-term processes, as inuniformitarianism. Although it was once used to promote the idea of a very young Earth, catastrophism today is accepted, in a very modified form, by many earth scientists.

GEOCHRONOLOGY:

The study of Earth's age and the dating of specific formations in terms of geologic time.

GEOLOGY:

The study of the solid earth, in particular, its rocks, minerals, fossils, and land formations.

GEOMYTHOLOGY:

Folklore inspired by geologic phenomena.

HISTORICAL GEOLOGY:

The study of Earth's physical history. Historical geology is one of two principal branches of geology, the other being physical geology.

QUALITATIVE:

Involving a comparison between qualities that are not definedprecisely, such as "fast" and "slow" or "warm" and "cold."

QUANTITATIVE:

Involving a comparison between precise quantities—for instance, 10 lb. versus 100 lb. or 50 mi. per hour versus 120 mi. per hour.

SCIENTIFIC METHOD:

A set of principles and procedures for systematic study that includes observation; the formation of hypotheses, theories, and laws; and continual testing and reexamination.

SEDIMENT:

Material deposited at or near Earth's surface from a number of sources, most notably preexisting rock.

STRATIGRAPHY:

The study of rock layers, or strata, beneath Earth's surface.

UNCONFORMITY:

An apparent gap in the geologic record, as revealed by observing rock layers, or strata.

UNIFORMITARIANISM:

The idea that the geologic processes at work today provide a key to understanding the geologicpast. The speed and intensity of those processes, however, may not always be the same at any juncture in geologic history. Uniformitarianism usually is contrasted with catastrophism.

WEATHERING:

The breakdown of rocks and minerals at or near the surface of Earth due to physical or chemical processes or both.

Historical Geology

Historical geology is the study of changes in Earth and its life forms over time. It includes sub-disciplines such as paleontology, paleoclimatology, and paleoseismology. In addition to providing a scientific framework for understanding the evolution of Earth over time, historical geology provides important information about ancient climate changes, volcanic eruptions, and earthquakes that can be used to anticipate the sizes and frequencies of future events.

Scientific interpretation of Earth’s history requires an understanding of currently operating geologic processes. According to the doctrine of actualism, geologists understand that geologic processes operating today are similar to those that operated in the past. The rates at which the processes occur, however, may be different. By studying modern geologic processes and their products, geologists can interpret rocks that are the products of past geologic processes and events. For example, the layering and distribution of different grain sizes within a sandstone layer may be similar to those in a modern beach, leading geologists to infer that the sandstone was deposited in an ancient beach environment. There have been some past geologic events, however, that are beyond the range of human experience. Evidence of catastrophic events such asteroid impacts on Earth has led geologists to abandon the strict doctrine of uniformitarianism, which holds that all of the geologic past could be explained in terms of currently observable processes, in favor of actualism.

Rocks preserve evidence of the events that formed them and the environments in which they were formed. Fossils are an especially useful type of biological evidence preserved in sedimentary rocks (they generally do not occur in igneous or metamorphic rocks). Organisms thrive only in those conditions to which they have become adapted over time. Therefore, the presence of particular fossils in a rock provides paleontologists with insights into the environment in which the fossilized organisms lived. Sediments and sedimentary rocks also preserve a variety of tracks, trails, burrows, and footprints known as trace fossils. Information about tree ring widths and changes in the isotopic composition of some sedimentary rocks and glacial ice over time have been used to reconstruct patterns of past climate changes over millennial time scales. These patterns, in turn, provide important information about the magnitude and frequency of future climate changes.

Any study of Earth’s history involves the element of time. Relative geologic time considers only the sequence in which geologic events occurred. For example, rock A is older than rock B, but younger than rock C. Relative geologic time is based largely on the presence or absence of index fossils that are known to have existed over limited ranges of geologic time. Using the concept of relative geologic time, geologists in the nineteenth century correlated rocks around the world and developed an elaborate time scale consisting of eons, eras, periods, and epochs. The development of radiometric dating techniques during the second half of the twentieth century allowed geologists to determine the absolute ages of rocks in terms of years and assign specific dates to the relative time boundaries, which had previously been defined on the basis of changes in fossil content.

See also Fossil and fossilization; Geochemical analysis; Geochemistry; Geophysics; Stratigraphy.

Historical geology

Historical geology is the study of changes in Earth and its life forms over time . It includes sub-disciplines such as paleontology , paleoclimatology, and paleoseismology. In addition to providing a scientific basis for understanding the evolution of Earth over time, historical geology provides important information about ancient climate changes, volcanic eruptions, and earthquakes that can be used to anticipate the sizes and frequencies of future events.

Scientific interpretation of Earth's history requires an understanding of currently operating geologic processes. According to the doctrine of actualism, most geologic processes operating today are similar to those that operated in the past. The rates at which the processes occur, however, may be different. By studying modern geologic processes and their products, geologists can interpret rocks that are the products of past geologic processes and events. For example, the layering and distribution of different grain sizes within a sandstone layer may be similar to those in a modern beach, leading geologists to infer that the sandstone was deposited in an ancient beach environment. There have been some past geologic events, however, that are beyond the range of human experience. Evidence of catastrophic events such asteroid impacts on Earth has led geologists to abandon the doctrine of uniformitarianism , which holds that all of the geologic past could be explained in terms of currently observable processes, in favor of actualism.

Rocks preserve evidence of the events that formed them and the environments in which they were formed. Fossils are an especially useful type of biological evidence preserved in sedimentary rocks (they generally do not occur in igneous or metamorphic rocks). Organisms thrive only in those conditions to which they have become adapted over time. Therefore, the presence of particular fossils in a rock provides paleontologists with insights into the environment in which the fossilized organisms lived. Sediments and sedimentary rocks also preserve a variety of tracks, trails, burrows, and footprints known as trace fossils. Information about tree ring widths and changes in the isotopic composition of some sedimentary rocks and glacial ice over time have been used to reconstruct patterns of past climate changes over millennial time scales. These patterns, in turn, provide important information about the magnitude and frequency of future climate changes.

Any study of Earth's history involves the element of time. Relative geologic time considers only the sequence in which geologic events occurred. For example, rock A is older than rock B, but younger than rock C. Relative geologic time is based largely on the presence or absence of index fossils that are known to have existed over limited ranges of geologic time. Using the concept of relative geologic time, geologists in the nineteenth century correlated rocks around the world and developed an elaborate time scale consisting of eons, eras, periods, and epochs. The development of radiometric dating techniques during the second half of the twentieth century allowed geologists to determine the absolute ages of rocks in terms of years and assign specific dates to the relative time boundaries, which had previously been defined on the basis of changes in fossil content.

See also Fossil and fossilization; Geochemical analysis; Geochemistry; Geophysics; Stratigraphy.

Historical geology

All areas of geologic study are subdisciplines of either historical geology , which focuses on the chemical, physical, and biological history of Earth, or physical geology, which is the study of Earth materials and processes. Historical geology uses theory, observation, and facts derived from studying rocks and fossils to learn about the evolution of Earth and its inhabitants.

According to the principle of uniformitarianism , most physical and chemical processes occurring today are very similar to those that operated in the geologic past, although their rates may be different. Therefore, by studying modern geologic activities and their products, geologists can understand how these activities produced the ancient rock record. In other words, the present is the key to the past. The principle of uniformitarianism has been very useful in deciphering much of the rock record.

Studies in historical geology rely on the rock record for factual information about Earth's past. As geologists collect data, they develop hypotheses to explain phenomena they observe. Geologists test hypotheses by making further observations of rocks and the fossils they contain. If this and other research supports a hypothesis, eventually it will be accepted as a theory explaining how Earth, and the life on it, evolved through time.

Rocks preserve a record of the events that formed them. The trained observer can examine the physical, chemical, and biological characteristics of a rock and interpret its origin. Fossils are an especially useful type of biological evidence preserved in sedimentary rocks (they do not occur in igneous or metamorphic rocks). Organisms thrive only in those conditions to which they have become adapted over time. Therefore, the presence of particular fossils in a rock provides paleontologists with very specific insights into the environment that formed that rock.

In addition to body fossils, sediments also preserve a variety of tracks and trails (for example, footprints, burrows, etc.). These biological impressions preserve traces of the daily activities of organisms, rather than their bodies, and so are called trace fossils. These too provide important clues to certain aspects of Earth history.

Through studies of rocks and fossils, geologists have produced what is called the geologic time scale. This is a convenient way of representing the vast amounts of time and the numerous details of historical geology in a way that is easily expressed and understood. The geologic time scale consists of the dates of major events in Earth's history, placed in chronological order. These events, primarily major extinctions and episodes of organic evolution, separate the scale into distinct time units. From largest to smallest, these units are the geologic eon, era, period, and epoch. The age of each boundary event is determined by radiometric dating of rocks associated with the time unit boundary. Radiometric dating uses the rates of atomic decay for radioactive elements to determine the age of geologic materials.

See also Big Bang theory; Dating methods; Earth science; Fossil record; Evolution, evidence of; Evolutionary mechanisms; Stratigraphy