GEOLOGIC TIME

CONCEPT

The expression geologic time refers to the vast span from Earth's beginnings to the present, about 4.6 billion years. To examine the history of Earth, one must discard most familiar ideas about time. Instead of thinking in terms of years, centuries, or even millennia, the most basic unit is a million years, and even that is rather small when compared with the four eons into which geologic time is divided. Earth scientists' knowledge of the first three eons is fairly limited. What they do know comes from a combination of absolute dating, mostly by the study of radioactive decay, and relative dating through the stratigraphic record of rock layers.

HOW IT WORKS

Historical Geology

The study of geologic time is encompassed within the larger subject of historical geology. The latter, the study of Earth's physical history, is one of the two principal branches of geology, the other being physical geology, or the study of Earth's physical components and the forces that have shaped them.

The background of historical geology is discussed in some detail within the Historical Geology essay. Its principal subdisciplines include stratigraphy, the study of rock layers, or strata, beneath Earth's surface; geochronology, the study of Earth's age and the dating of specific formations in terms of geologic time; sedimentology, the study and interpretation of sediments, including sedimentary processes and formations; paleontology, the study of fossilized plants and animals; and paleoecology, the study of the relationship between prehistoric plants and animals and their environments. Several of these subjects are examined in essays within this book.

Divisions of Geologic Time

Geologic time is divided according to two scales. The more well-known of these is the geologic scale, which divides time into named groupings according to six basic units: eon, era, period, epoch, age, and chron. In addition, the chronostratigraphic scale identifies successive layers of rock with specific units of time.

As noted earlier, stratigraphy is the study of rock layers, or strata, beneath Earth's surface, while chronostratigraphy is a subdiscipline devoted to studying the ages of rocks and what they reveal about geologic time. The chronostratigraphic scale likewise has six time units, analogous to those of the geologic scale: eonothem, era them, system, series, stage, and chronozone. For the most part, we will not be concerned with the chronostratigraphic terms in the present context.

RELATIVE AND ABSOLUTE TIME.

To discuss the divisions of geologic time, it is necessary first to discuss the concepts of relative and absolute time. The term relative refers to a quality or quantity that is comparative, or dependent on something else. Its opposite is absolute, a term designating a quality or quantity that is independent and not defined in relation to another quality or quantity.

If we say that Abraham Lincoln was born in 1809, it is an absolute designation of his birth year, whereas if we say that he was born 10 years after the death of George Washington (which occurred in 1799), that is an example of a relative time measurement. In actuality, of course, there is no truly absolute measure of time. For example, the reference to 1809 as Lincoln's birth year is based on the system of time measurement developed in the West, which, in turn, is based on early ideas regarding the date of Christ's birth. (As it turns out, Christ likely was born in about 6 b.c.)

Since the b.c./a.d. system of dating is widely accepted and used, or at least recognized, by most of the non-Western world, a date rendered according to this system constitutes the closest possible approximation to an absolute measure of time. In any case, one knows the difference between absolute and relative when one sees it: thus, to say that Lincoln was born ten years after Washington died is obviously and unmistakably a relative statement.

In terms of geology, the absolute age of a geologic phenomenon is its age in Earth years. On the other hand, its relative age is its age in comparison with other geologic phenomena, particularly the stratigraphic record of rock layers. Thus, references to relative age are given in terms of chronostratigraphic time divisions rather than millions of years.

Relative Dating

Given the meaning of relative age, it is easy enough to guess what relative dating would be, once one knows that dating, in a scientific context, usually refers to any effort directed toward finding the age of a particular item or phenomenon. Relative dating, then, assigns an age relative to that of other items, whereas absolute dating determines the age in actual years or millions of years.

One of the principal means of relative dating is through stratigraphy, which is based on the assumption that the deeper a layer of rock lies beneath Earth's surface, the earlier it was deposited. This holds true, however, for only one of the three major types of rock: sedimentary rock, which is formed by compression and deposition (i.e., formation of deposits) on the part of rock and mineral particles. (The other types of rock are igneous and metamorphic.)

Aside from stratigraphy, discussed in a separate essay, other relative dating techniques include seriation, faunal dating, and pollen dating, or palynology. Used, for instance, in archaeological studies, seriation analyzes the abundance of a particular item (for instance, pieces of pottery) and assigns relative dates based on this abundance. The term faunal dating refers to fauna, or animal life, and faunal dating is the use of animal bones to determine age. Finally, pollen dating, or palynology, involves analysis of pollen deposits.

Absolute Dating

As dating technology has progressed, it has become increasingly possible for scientists to provide absolute dates for specimens. One such method, introduced in the 1960s, is amino-acid racimization. Amino acids exist in two forms, designated L -forms and D -forms, which are stereoisomers, or mirror images of each other. Virtually all living organisms (except some microbes) incorporate only the L-forms, but once the organism dies the L-amino acids gradually convert to D-amino acids. Several factors influence the rate of conversion, and though amino-acid racimization was popular in the 1970s, these uncertainties have led scientists to treat it with increasing disfavor.

The principles that undergird amino-acid racimization, however, are essential to most forms of absolute dating. Generally, absolute dating uses ratios between the quantities of a particular substance (let us call it Substance A ) and the quantities of a mirror substance (Substance B ) to which it is converted over a period of time. The greater the ratio of Substance B to Substance A, the longer the time that has elapsed. The scale of time for various substances, however, differs greatly. Carbon-14 decay, for instance, takes place over a few thousand years, making it useful for measuring the age of human artifacts. On the other hand, uranium decay takes billions of years, and thus it is used for dating rocks.

Cation-ratio dating, for instance, measures the amount of cations, or positively charged ions, that have formed on an exposed rock surface. (An ion is an atom or group of atoms that have lost or gained electrons, thus acquiring a net electric charge. Electron loss creates a cation, as opposed to a negatively charged anion, created when an atom or atoms gain electrons.) Cation-ratio dating is based on the idea that the ratio of potassium and calcium cations to titanium cations decreases with age. It is applicable only to rocks in desert areas, where the dry air stabilizes the cation "varnish."

RADIOACTIVE DECAY.

Various forms of radiometric dating employ ratios as well. Every element has a particular number of protons, or positively charged particles, in its nucleus, but it may have varying numbers of neutrons, particles with a neutral electric charge but relatively great mass. (Neutrons and protons have approximately the same mass, which is more than 1,800 times greater than that of an electron.) When two or more atoms of the same element have a differing number of neutrons, they are called isotopes.

Some types of isotopes "fit" better with a particular element and tend to be most abundant. For instance, carbon has six protons, and it so happens that the most abundant carbon isotope has six neutrons. Because there are six protons and six neutrons, totaling 12, this carbon isotope is designated carbon-12, which accounts for 98.9% of the carbon in nature. Generally speaking, the most abundant isotope is also the most stable one, or the one least likely to release particles and thus change into something else.

This release of particles is known as radioactive decay. In the context of radioactivity, "to decay" does not mean "to rot" rather, the isotope expels alpha particles (positively charged helium nuclei), beta particles (either electrons or subatomic particles called positrons), or gamma rays, which occupy the highest energy level in the electromagnetic spectrum. In so doing, it eventually will become another isotope, either of the same element or of a different element, and will stabilize. The amount of time it takes for half the isotopes in a sample to stabilize is called its half-life. This half-life varies greatly between isotopes, some of which have a half-life that runs into the billions of years.

Determining Absolute Age

When an organism is alive, it incorporates a certain ratio of carbon-12 in proportion to the amount of the radioisotope (that is, radioactive isotope) carbon-14 that it receives from the atmosphere. As soon as the organism dies, however, it stops incorporating new carbon, and the ratio between carbon-12 and carbon-14 will begin to change as the carbon-14 decays to form nitrogen-14. A scientist can use the ratios of carbon-12, carbon-14, and nitrogen-14 to ascertain the age of an organic sample.

Carbon-14, known as radiocarbon, has a half-life of 5,730 years, meaning that it takes that long for half the isotopes in a sample to decay to nitrogen-14. Note that half-life is not half the amount of time it takes for the entire sample to decay, especially because the first half of the sample usually decays faster than the second half. Imagine, for instance, that you had 100 units and wanted to reduce it to zero units by continually halving it. At first, the results would be dramatic, as 100 became 50, then 25, then 12.5, and so on. Eventually you would be down to smaller and smaller fractions of 1, and each division by 2 would yield a smaller number—but never zero.

Radioactive decay works that way as well, and, thus, while carbon-14 has a half-life of less than 6,000 years, it takes much longer than 6,000 years for the other half of the isotopes in a carbon-14 sample to decay. For this reason, the use of proper instrumentation makes it possible to judge the age of charcoal, wood, and other biological materials over a span of as long as 70,000 years. While this may be useful for archaeologists, it is not very helpful for measuring the vast spans of time encompassed in the earth sciences. Furthermore, there is a good likelihood that the sample will become contaminated by additional carbon from the soil. Moreover, it cannot be said with certainty that the ratio of carbon-12 to carbon-14 in the atmosphere has been constant throughout time.

POTASSIUM-ARGON DATING.

Much more useful, from the standpoint of geology, is potassium-argon dating. When volcanic rocks are subjected to extremely high temperatures, they release the element argon, a noble gas. As the rocks cool, the stable isotope argon-40 accumulates. Because argon-40 is formed by the radioactive decay of a potassium isotope, potassium-40, the amount of argon-40 that forms is proportional to the rate of decay for potassium-40.

Potassium-40 has a half-life of 1.3 billion years, and with the help of argon-40, geologists have been able to estimate the age of volcanic layers above and below fossil and artifact remains in eastern Africa. Potassium-argon dating is most effective for rocks that are at least three million years old, because it takes about that long to accumulate enough argon-40 to make accurate measurements possible.

This brings up a notable aspect of radiometric dating techniques. No one technique is most effective; rather, each technique is suited to a particular span of time. Thus, potassium-argon dating would be virtually useless for measuring the relatively short time scales for which radiocarbon dating is ideally suited. The converse is also true: as we have noted, radiocarbon dating simply does not cover a wide enough span of time to be useful in most geologic studies.

URANIUM-SERIES DATING.

We now come to the element most useful for dating the age of material samples over a broad chronological spectrum: uranium, which has an atomic number of 92. This means that it has 92 protons in its nucleus, making uranium atoms typically the heaviest atoms that occur in nature. (There are about 20 elements with atomic numbers higher than 92, but all of them have been created artificially, either in laboratories or as the result of nuclear testing.)

Both uranium and thorium, with an atomic number of 90, have unstable "parent" isotopes that decay into even more unstable "daughter" isotopes before eventually stabilizing as isotopes of lead. These daughter isotopes have half-lives that range from just a few years to a few hundred thousand years, whereas the half-lives of the parent isotopes are much longer. That of uranium-235, for instance, is 7.038 × 10⁸ years, or more than 700 million years. On the other hand, the daughter isotope protactinium-231 has a half-life of 32,760 years.

When uranium-235 is deposited in an area, over time it will decay to form daughter isotopes. Assuming that the sample has been left undisturbed (isotopes have neither entered nor exited the deposit since its initial formation), the age of certain types of sample may thus be determined. For mollusks and corals, for instance, the amount of protactinium-231, a daughter isotope that begins to accumulate only after the organism dies, makes it possible to date a sample. In some cases, large amounts of a daughter isotope may be deposited initially alongside samples of a parent, and if these are present in water, the quantities of each can be judged according to the amount that has dissolved. For example, the daughter isotope uranium-234 dissolves more readily in water than the parent, uranium-238.

REAL-LIFE APPLICATIONS

How Do We Know Earth's Age?

We can now begin to answer a question almost inevitably raised when discussing geologic time: just how do we know that Earth is about 4.6 billion years old? A clue lies in the half-life of uranium-238, which is 4.47 × 10⁹ years, or 4,470 million years. Geologists typically would abbreviate this as 4.47 Ga, the latter referring to "gigayears," a unit of a billion years.

As uranium atoms undergo fission, or splitting, this process releases energy that causes marks, called tracks, to form on the surface of volcanic minerals. In splitting, two daughter atoms shoot away from each other, forming tracks, and thus the rate of track formation is proportional to the rate of decay on the part of the parent isotope.

Incidentally, fission-track dating with uranium-238 defies the statement made earlier that certain types of dating are more suited to long periods of time, while others are best for shorter periods. When heated, the tracks disappear from a sample containing uranium-238, thus resetting the dating clock. As a result, if an object was heated just a few decades ago, it can be dated; so, too, can meteorites billions of years old.

Most meteorites found in the solar system tend to be about 4.56 Ga; hence, the rough figure of 4.6 Ga is used for the point at which the solar system, including Earth, began to form. The oldest known materials on Earth are zircon crystals from western Australia, dated about 4.3 Ga. Small samples of gneiss in Canada's Northwest Territories have been dated to about 4.0 Ga, but the oldest large-scale sample is a belt of 3.8 Ga gneiss in western Greenland.

A Question of Scale

Having discussed at least the rudiments of the dating system used by geologists, it is possible to examine geologic time itself. This requires a mental adjustment of monumental proportions, because one must discard all notions used in studying the history of human civilization. Concepts such as medieval, ancient, and prehistoric are practically useless when discussing geologic time, which dwarfs the scale of human events.

Human civilization has existed for about 5,500 years, the blink of an eye in geologic terms. Even the span of time that the human species, Homo sapiens, has existed—about two million years—is negligible in the grand scheme of Earth's history. The latter stretches back some 4,600 million years, meaning that human beings have existed on this Earth for just 0.043% of the planet's history. As discussed in the essay Historical Geology, if the entire history of Earth were likened to a single year, humans would have appeared on the scene at a few minutes after 8:00 p.m. on December 31. Human civilization would date only from about 42 seconds before midnight, and the age of machinery and industrialization would not fill up even the final two seconds of the year.

ANOTHER ANALOGY: LOS ANGELES TO NEW YORK.

When discussing distances in space, astronomers dispense with miles, because they would be useless, given the vastness of the scale involved. The same is true of geologic time, in which the concept of years is hardly relevant. Instead, geologists speak in terms of millions of years, or megayears, abbreviated "Ma." (Geologists also use the much larger unit of a gigayear, to which we have already referred.) To discuss the age of Earth in terms of years, in fact, would be rather like measuring the distance from Los Angeles to New York in feet; instead, of course, we use miles. Now let us consider geologic time in terms of the 2,462 mi. between Los Angeles and New York, with 1 mi. equal to 1.8684 Ma, or 1,868,400 years.

Suppose we have left Los Angeles and driven a good deal of the distance to New York—46% of the way, in fact, to western Nebraska, a spot analogous to the beginning of the Proterozoic era. In the preceding miles, a duration equivalent to about 1,133 Ma, Earth was formed from a cloud of gas, pounded by meteors, and gradually became the home to oceans—but no atmosphere resembling the one we know now. The end of the Proterozoic era (about 545 Ma, or 545 million years ago) would be at about 88% of the distance from Los Angeles to New York—somewhere around Pittsburgh, Pennsylvania. By this point, the continental plates have been formed, oxygen has entered the atmosphere, and soft-bodied organisms have appeared.

We are a long way from Los Angeles, and yet almost the entire history of life on Earth, at least in terms of relatively complex organisms, lies ahead of us. If we skip ahead by about 339 Ma (a huge leap in terms of biological development), we come to the time when the dinosaurs appeared. We are now 95% of the way from the beginning of Earth's history to the present, and if measured against the distance from Los Angeles to New York, this would put us at a longitude equivalent to that of Baltimore, Maryland. Another 89 mi. would put us at about 65 million years ago, or the point when the dinosaurs became extinct.

We would then have only 33.7 mi. to drive to reach the point where humans appeared, by which time we would be in the middle of Manhattan. Compared with the distance from Los Angeles to New York, the span of human existence would be much smaller than the cab ride from Central Park to the Empire State Building. The entire sweep of written human history, from about a thousand years before the building of the pyramids to the beginning of the third millennium a.d., would be much smaller than a city block. In fact, it would be about the width of a modest storefront, or 15.54 ft.

The Very, Very Distant Past

So what happened for all those hundreds of millions of years before humans appeared on the scene? We will attempt to answer that question in an extremely cursory, abbreviated fashion, but for further clarification, the reader is strongly encouraged to consult a chart of geologic time. Such a chart can be found in virtually any earth sciences textbook; indeed, several versions (including a chronostratigraphic chart) may appear in a single book.

In addition to showing geologic time in both absolute and relative terms, these charts typically provide information about the magnetic polarity over a given span, since that has changed many times since Earth came into existence. In other words, what is today the magnetic North Pole was once the magnetic South Pole, and vice versa. (For more on this subject, see the discussion of paleomagnetism in the entries Plate Tectonics and Geomagnetism.)

As one might expect, disagreement between earth scientists is greatest with regard to the most distant phases of Earth's geologic history. This encompasses nearly 90% of all geologic time, dating back to about 545 Ma, thus showing how little geologists know, even today, about the geologic events of the very distant past. For this reason, when discussing Precambrian time, it is usually necessary to consider only the three eons that composed it. Discussion of era and period, on the other hand, is reserved for the three eras, and 11 periods, of the Phanerozoic eon. The smaller division of epoch is generally only of concern with regard to the most recent era, the Cenozoic. As for divisions smaller than an epoch, these will not concern us here.

THE PRECAMBRIAN EONS.

The last paragraph of the preceding section encompasses a number of ideas, which now need to be explained, in at least general terms. The term Precambrian encompasses about four billion years of Earth's history, including three of the four eons (Hadean or Priscoan, Archaean, and Proterozoic) of the planet's existence. The names of these eons are derived from Greek, with the first being taken from the name of the deity who ruled over the Underworld. The latter two are derived, respectively, from the Greek words for beginning and new life.

The Hadean eon (sometimes called the Priscoan) lasted from about 4,560 Ma to 4,000 Ma ago, when the planet was being formed, or accreted, as pieces of solid matter floating around in the young solar system began to join one another. Meteorites showered the planet, bringing both solid matter and water, and thus forming the basis of the oceans. There was no atmosphere as such, but by the end of the eon, volcanic activity had ejected enough carbon dioxide and other substances into the air to form the beginnings of one. The oceans began to cool, making possible the beginnings of life—that is, molecules of carbon-based matter that were capable of replicating themselves. These appeared at the end of the Hadean eon, perhaps arriving from space in a meteorite.

The boundaries of the Precambrian eons are far from certain, so it is possible only to say that the Archaean eon lasted from about 4,000 Ma to 2,500 Ma ago. The earliest known datable materials, described earlier, all come from this time; in fact, outcrops of Archaean rock have been found on all seven continents. The rocks of this eon contain the first clear evidence of life, in the form of microorganisms. Over the course of the Archaean eon, prokaryotes, or cells without a nucleus, made their appearance, and later they were followed by eukaryotes, or cells with a nucleus.

During this great span of time, more than 20% of Earth's history, the atmosphere and hydrosphere developed considerably, even as the biosphere had its true beginnings. As for the geosphere, it also matured enormously in the course of the Archaean eon. During the Hadean eon, Earth's interior had begun to differentiate into core, mantle, and crust, and cooling in the two upper layers influenced the beginnings of the earliest plate-tectonic activity (see Plate Tectonics).

Even longer was the Proterozoic eon, which appears to have lasted from about 2,500 Ma to 545 Ma. This phase saw the beginnings of very basic forms of plant life, such that photosynthesis (the biological conversion of electromagnetic energy from the Sun into chemical energy in plants) began to take place. Plate-tectonic processes accelerated as well, with continents moving about over Earth's surface and smashing against one another. Oxygen in the atmosphere assumed about 4% of its present levels, but animal life still consisted primarily of eukaryotes.

THE PHANEROZOIC ERAS AND PERIODS.

The end of the Proterozoic eon, once again, is not sharply defined in the stratigraphic record, such that there is considerable dispute as to the time periods involved. In any case, it is clear that the pace of development in the biosphere increased dramatically in the Phanerozoic, the eon in which we are now living. During the beginning of the Phanerozoic eon, algae appeared, and there followed an acceleration in the development of living organisms that ultimately produced the varied biosphere we know today.

As noted earlier, the only eras and periods that need concern most students of the earth sciences are those of the Phanerozoic eon. The three eras are as follows:

Eras of the Phanerozoic Eon

• Paleozoic (about 545 to 248.2 Ma)
• Mesozoic (about 248.2 to 65 Ma)
• Cenozoic (about 65 Ma to the present)

Within these eras are the following periods:

Periods of the Paleozoic Era

• Cambrian (about 545 to 495 Ma)
• Ordovician (about 495 to 443 Ma)
• Silurian (about 443 to 417 Ma)
• Devonian (about 417 to 354 Ma)
• Carboniferous (about 354 to 290 Ma)
• Permian (about 290 to 248.2 Ma)

Periods of the Mesozoic Era

• Triassic (about 248.2 to 205.7 Ma)
• Jurassic (about 205.7 to 142 Ma)
• Cretaceous (about 142 to 65 Ma)

Periods of the Cenozoic Era

• Palaeogene (about 65 to 23.8 Ma)
• Neogene (about 23.8 to 1.8 Ma)
• Quaternary (about 1.8 Ma to present)

These divisions, as well as the two most recent epochs of the Quaternary period (Pleistocene and Holocene), are discussed elsewhere in this book. It should be noted that there are variations for many of the eon, era, and period names given here; also, the Palaeogene and Neogene are often grouped together as a subera called the Tertiary. The latter nomenclature fits with a mnemonic device used by geology students memorizing the names of the 11 Phanerozoic periods: "Camels Ordinarily Sit Down Carefully; Perhaps Their Joints Creak Tremendously Quietly."

WHERE TO LEARN MORE

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>.

Hancock, Paul L., and Brian J. Skinner. The Oxford Companion to the Earth. New York: Oxford University Press, 2000.

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. Earth-steps: A Rock's Journey Through Time. Golden, Colo.: 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

ABSOLUTE AGE:

The absolute age of a geologic phenomenon is its age in Earthyears. Compare with relative age.

ATMOSPHERE:

In general, an atmosphere is a blanket of gases surrounding a planet. Unless otherwise identified, however, the term refers to the atmosphere of Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%.

ATOM:

The smallest particle of an element, consisting of protons, neutrons, and electrons. An atom can exist either alone or in combination with other atoms in a molecule.

BIOSPHERE:

A combination of all living things on Earth—plants, animals, birds, marine life, insects, viruses, single-cell organisms, and so on—as well as all formerly living things that have not yet decomposed.

CHRONOSTRATIGRAPHY:

A subdiscipline of stratigraphy devoted to studying the ages of rocks and what they reveal about geologic time.

DATING:

Any effort directed toward finding the age of a particular item or phenomenon. Methods of geologic dating are either relative (i.e., comparative and usually based on rock strata) or absolute. The latter, based on such methods as the study of radioactive isotopes, usually is given in terms of actual years or millions of years.

ELECTRON:

A negatively charged particle in an atom, which spins around the nucleus.

ELEMENT:

A substance made up of only one kind of atom. Unlike compounds, elements cannot be chemically broken into other substances.

EON:

The longest phase of geologic time. Earth's history has consisted of four eons, the Hadean or Priscoan, Archaean, Proterozoic, and Phanerozoic. The next-smallest subdivision of geologic time is the era.

EPOCH:

The fourth-longest phase of geologic time, shorter than an era and longer than an age and a chron. The current epoch is the Holocene, which began about 0.01 Ma (10,000 years) ago.

ERA:

The second-longest phase of geologic time, after an eon. The current eon, the Phanerozoic, has had three eras, the Paleozoic, Mesozoic, and Cenozoic, which is the current era. The next-smallest subdivision of geologic time is the period.

GA:

An abbreviation meaning "giga-years," or "billion years." The age of Earth is about 4.6 Ga.

GEOCHRONOLOGY:

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

GEOLOGIC TIME:

The vast stretch of time over which Earth's geologic development has occurred. This span (about 4.6 billion years) dwarfs the history of human existence, which is only about two million years. Much smaller still is the span of human civilization, only about 5,500 years.

GEOSPHERE:

The upper part of Earth's continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources.

HISTORICAL GEOLOGY:

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

HYDROSPHERE:

The entirety of Earth's water, excluding water vapor in the atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice.

ISOTOPES:

Atoms that have an equal number of protons and hence are of the same element but differ in their number of neutrons. This results in a difference ofmass. An isotope may be either stable or radioactive.

MA:

An abbreviation used by earth scientists, meaning "million years" or "megayears." When an event is dated to, for instance, 160 Ma, it usually means that it took place 160 million years ago.

NEUTRON:

A subatomic particle that has no electric charge. Neutrons are found at the nucleus of an atom, alongside protons.

NUCLEUS:

The center of an atom, a region where protons and neutrons are located and around which electrons spin.

PERIOD:

The third-longest phase of geologic time, after an era. The current eon, the Phanerozoic, has had 11 periods, and the current era, the Cenozoic, has consisted of three periods, of which the most recent is the Quaternary. The next-smallest subdivision of geologic time is the epoch.

PRECAMBRIAN TIME:

A term that refers to the first three of four eons in Earth's history, which lasted from about4, Ma to about 545 Ma ago.

PROTON:

A positively charged particle in an atom.

RADIOACTIVITY:

A term describing a phenomenon whereby certain materials are subject to a form of decay brought about by the emission of high-energy particles or radiation. Forms of particles or energy include alpha particles (positively charged helium nuclei), beta particles (either electrons or subatomic particles called positrons, or gamma rays, which occupy the highest energy level in the electromagnetic spectrum.

RADIOMETRIC DATING:

A method of absolute dating using ratios between "parent" isotopes and "daughter" isotopes, which are formed by the radioactive decay of parent isotopes.

RELATIVE AGE:

The relative age of a geologic phenomenon is its age in comparison with other geologic phenomena, particularly the stratigraphic record of rock layers. Compare with absolute age.

SEDIMENT:

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

SEDIMENTARY ROCK:

Rock formed by compression and deposition (i.e., formation of deposits) on the part of other rock and mineral particles.

SEDIMENTOLOGY:

The study and interpretation of sediments, including sedimentary processes and formations.

STRATIGRAPHY:

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

Geologic Time

History of the concept of geologic time

Relative age determination

Radiometric age determination

Resources

Although historical time covers centuries and archeological times covers millennia, geologic time describes the span of time—billions of years—revealed in the fossil and rock record. Geochronology is the science of determining the age of rocks and minerals. Absolute time and relative time are terms used by geologists used to describe the age of rocks and events. Radiometric age determination is a method used by geologists to determine the absolute age, in years, of rocks and minerals. Knowledge of stratigraphy, the branch of geology that catalogues Earth’s successions of rock layers, is essential to establish the relative ages of rock units.

History of the concept of geologic time

Before scientific methods were used to investigate geologic time, ideas about time and Earth history came from religious theories. The Hindu and Mayan religions taught about endlessly repeating cycles of time, each lasting for billions of years. Ideas in western culture about the age of Earth were also imprecise. In the 1650s, the Irish clergyman and scholar James Ussher (1581–1656) used apparent genealogy within the Bible’s Book of Genesis to determine that Earth was created in 4004 BC. Isaac Newton also speculated on the age of Earth. As early as the eighteenth century, scientists knew that Earth’s age must be great. But geologists were not able to measure the dimensions of Earth’s history until mass spectrometers became available in the 1950s. The mass spectrometer is an instrument used to separate different varieties of atoms from each other. Before that time, inferences were made by comparing the rock record from different parts of the world and estimating how long it would take natural processes to form all the rocks on Earth.

Georges Louis Leclerc de Buffon (1707–1788), for example, calculated Earth to be 74,832 years old by figuring how long it would take the planet to cool down to the present temperature. Writing around 1770, he was among the first to suggest that Earth’s history can be known about by observing its current state.

James Hutton (1726–1797) did not propose a date for the formation of Earth, but is famous for the statement that Earth contains “no vestige of a beginning— no prospect of an end.” The German geologist Abraham Werner (1750–1817) was the first scientist to make use of a stratigraphic column, which is a diagram showing order of sedimentary layers. The French zoologist and paleontologist Georges Cuvier (1769–1832) observed that specific fossil animals occurred in specific rock layers, forming recognizable groups, or assemblages. William Smith (1769–1768) combined Werner’s and Cuvier’s approaches, using fossil assemblages to identify identical sequences of layers distant from each other, linking or correlating rocks which were once part of the same rock layer but had been separated by faulting or erosion.

In 1897, the British physicist Lord Kelvin (1824-1907) developed a model that assumed that Earth has been cooling steadily since its formation. Because he did not know that heat is transported by convection currents beneath Earth’s surface or that Earth generates its own heat from the decay of radioactive minerals buried inside it, Kelvin proposed that Earth was formed from 20 to 40 million years ago.

In the late eighteenth century, geologists began to name periods of geologic time. In the nineteenth century, geologists such as William Buckland (1784-1856), Adam Sedgwick (1785–1873), Henry de la Beche (1796–1855), and Roderick Murchison (1792–1871) identified widespread rock layers beneath continental Europe, the British isles, Russia, and America. They named periods of time after the places in which these rocks were first described. For instance, Cambrian Period was named for Cambria (the Roman name for Wales), and Permian, for the Perm province in Czarist Russia. Pennsylvanian and Mississippian periods widely used by American geologists were named for a U.S. state and a region around the upper reaches of a large river, respectively. By the mid-nineteenth century, most of the modern names of the periods of geologic time had been proposed. All of them are still in use.

Relative age determination

A rock layer may or may not contain evidence that reveals its age. Rock layers for which ages are defined by relationships with the dated rock units around it are examples of relative age determination. That relationship is found by observing the unknown rock layer’s stratigraphic relationship with the rock layers for which ages are known. If the known rock layer is on top of the unknown layer, then the lower layer is probably the older of the two. That inference is based on the principle of superposition, which states that when two rock layers are stacked one above the other, the lower one was formed before the overlying one, unless the layers have been overturned.

Radiometric age determination

Every rock and mineral exists in the world as a mixture of elements, and every element exists as a population of atoms. One element’s population of atoms will not all have the same number of neutrons, and so two or more kinds of the same element will have different atomic masses or atomic numbers. These different kinds of the same chemical element are called nuclides of that element. A nuclide of a radioactive element is known as a radionuclide.

The nucleus of every radioactive element spontaneously disintegrates over time. This process results in radiation, and is called radioactive decay. Losing highenergy particles from their nuclei transforms the atoms of a radioactive nuclide into the daughter product of that nuclide. A daughter product is either a different element altogether, or is a different nuclide of the same parent element. A daughter product may or may not be radioactive. If it is, it also decays to form its own daughter product. The last radioactive element in a series of these transformations will decay into a stable element, such as lead.

While there is no way to discern whether an individual atom will decay today or two billion years from today, the behavior of large numbers of the same kind of atom is so predictable that certain nuclides are known as radioactive clocks. The use of these radioactive clocks to calculate the age of a rock is referred to as radiometric age determination. First, an appropriate radioactive clock must be chosen. The sample must contain measurable quantities of the element to be tested for, and its radioactive clock must tell time for the appropriate interval of geologic time. Then, the amount of each nuclide present in the rock sample must be measured.

Each radioactive clock consists of a radioactive nuclide and its daughter product, which accumulate within the atomic framework of a mineral. These radioactive clocks decay at various rates, which govern their usefulness in particular cases. A three-billion year old rock needs to have its age determined by a radioactive clock that still has a measurable amount of the parent nuclide decaying into its daughter product. The same radioactive clock would reveal nothing about a two million year old rock, because the rock would not yet have accumulated enough of the daughter product to measure.

The time it takes for half of the parent nuclide to decay into the daughter product is called one half-life. The remaining population of the parent nuclide is halved again, and the population of daughter product doubled, with the passing of every succeeding half-life. The amount of parent nuclide measured in the sample is plotted on a graph of that radioactive clock’s known half-life. The absolute age of the rock, within its margin of error, can then be read directly from the time axis of the graph.

When a rock is tested to determine its age, different minerals within the rock are tested using the same radioactive clock. Ages may be determined on the same sample by using different radioactive clocks. When the age of a rock is measured in two different ways, and the results are the same, the results are said to be concordant.

KEY TERMS

Blocking temperature— Temperature below which a mineral’s atomic framework is rigid enough to trap the daughter products of radioactive decay, thus starting the radioactive clock.

Concordant— Said of a rock’s age when determination of the absolute age of two or more rocks or minerals by radiometric methods yields the same result.

Daughter product— The element made when a radioactive element’s nucleus spontaneously falls apart (or decays). The daughter product may or may not be radioactive.

Discordant— Said of a rock’s age when different radiometric methods produce different estimates of the age.

Fission-track dating— An age determination technique in which the number of trails torn by alpha particles through a zircon crystal’s crystal framework.

Geochronology— The study of determining how old rocks and minerals are, in order to sort out the events of Earth history.

Half-life— The time it takes for half of the original atoms of a radioactive element to be transformed into the daughter product.

Nuclides— Different versions of the same chemical element that have slightly different numbers of subatomic particles in their nuclei, and therefore different atomic masses or in some cases different atomic numbers.

Radioactive clock— The ratio of a radionuclide and its daughter product, which accumulate within the atomic framework of a mineral. Also used to mean the radionuclide itself.

Radioactive decay— The predictable manner in which a population of atoms of a radioactive element spontaneously fall apart, and lose subatomic particles from their nuclei, becoming their daughter products.

Radiometric date— A measurement of time derived by measuring how much of a short-lived radioisotope (carbon-14) is left in a formerly living object, or how much of a long-lived radioisotope is left in a rock or mineral compared to how much of the daughter product is there into which the long-lived radioisotope decayed.

Radionuclide— Radioactive or unstable nuclide.

Stratigraphy— The study of layers of rock or soil, based on the assumption that the oldest material will usually be found at the bottom of a sequence.

Discordant ages means the radioactive clock showed different absolute ages for a rock sample, or different ages for different minerals within the rock. A discordant age result means that at some time after the rock was formed, something happened to it which reset one of the radioactive clocks.

For example, if a potassium-argon test produces a discordant result, the rock may have been heated to a blocking temperature above which a mineral’s atomic framework becomes active enough to allow trapped gaseous argon-40 to escape.

Concordant ages mean that no complex sequence of events-deep burial, metamorphism, and mountainbuilding, for example has happened that can be detected by the two methods of age determination that were used.

A form of radiometric dating is used to determine the ages of organic matter. A short-lived radioisotope, carbon-14, is accumulated by all living things on Earth. At death, the amount of carbon-14 is fixed and then begins to decay into carbon-12 at a known rate (its half-life is 5,730 years). By measuring how much of the carbon-14 is left in the remains, and plotting that amount on a graph showing how fast the carbon-14 decays, the approximate date of the organism’s death can be known.

When uranium atoms decay, they emit fast, heavy alpha particles. Inside a zircon crystal, these subatomic particles leave trails of destruction in the zircon’s crystal framework. The age of a zircon crystal can be estimated by counting the number of these trails. The rate at which the trails form has been found by determining the age of rocks containing zircon crystals, and noting how torn-up the zircon crystals become over time. This age determination technique is called fission-track dating. This technique has detected the world’s oldest rocks, between 3.8 billion and 3.9 billion years old, and yet older crystals, which suggest that Earth had some solid ground on it 4.2 billion years ago.

The age of Earth is deduced from the ages of other materials in the solar system, namely, meteorites.

Meteorites are pieces formed from the cloud of dust and debris left behind by a supernova, the explosive death of a star. Through this cloud the infant Earth spun, attracting more and more pieces of matter. The meteorites that fall to Earth today have orbited the sun since that time, unchanged and undisturbed by the processes that have destroyed Earth’s first rocks. Radiometric ages for meteorites fall between 4.45 billion and 4.55 billion years.

The radionuclide iodine-129 is formed in nature only inside stars. A piece of solid iodine-129 will almost entirely decay into the gas xenon-129 within a hundred million years. If this decay happens in open space, the xenon-129 gas will float off into space, blown by the solar wind. Alternatively, if the iodine-129 were in a rock within a hundred million years of being formed in a star, then some very old rocks should contain xenon-129 gas. Both meteorites and Earth’s oldest rocks contain xenon-129. That means the star that provided the material for the solar system died less than 4.65 billion (4,650,000,000) years ago.

See also Dating techniques; Fossil and fossiliza-tion; Spectroscopy; Strata.

Resources

BOOKS

Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, New Jersey: Prentice Hall.

Walker, M.J.C. Quaternary Dating Methods. Chichester, United Kingdom: Wiley, 2005.

Clinton Crowley

Geologic time

Although historical time covers centuries, and archeological times covers millennia, geologic time describes the immense span of time—billions of years—revealed in the fossil and rock record of Earth . Geochronology is the science of finding out how old rocks and minerals are. Absolute time and relative time are terms used to describe the age of rocks and events used by geologists. Radiometric age determination is a method used by geologists to determine the absolute age, in years, of rocks and minerals. Knowledge of stratigraphy , the branch of geology that catalogues Earth's successions of rock layers, is essential to establish the relative ages of rock units. By finding which rock unit formed first, the order of the events in Earth history can be sorted out.

History of the concept of geologic time

Before scientific methods were used to investigate geologic time, ideas about time and Earth history came from religious theories. The Hindu and Mayan religions taught about endlessly repeating cycles of time, each lasting for billions of years. Ideas in western culture about the age of Earth were just as imprecise, and just as incorrect. In the 1650s, the Irish clergyman and scholar James Ussher (1581–1656) used apparent genealogy within the Bible's Book of Genesis to determine that the Earth was created in 4004 B.C. Ussher based his results on the only information he had available to study the question of Earth's age. Ussher's estimates have been shown to be flawed, but unfortunately have been repeated by more modern writers who oppose modern views of Earth's age. Isaac Newton also speculated on the age of Earth, using the investigative techniques of the time that could be considered archaic today. As early as the eighteenth century, scientists knew that Earth's lifetime must have been immense. But geologists were not able to measure the dimensions of Earth's history until mass spectrometers became available in the 1950s. The mass spectrometer is an instrument used to separate different varieties of atoms from each other. Before that time, educated guesses had been made by comparing the rock record from different parts of the world and estimating how long it would take natural processes to form all the rocks on Earth.

Georges Louis Leclerc de Buffon (1707–1788), for example, calculated Earth to be 74,832 years old by figuring how long it would take the planet to cool down to the present temperature . Writing around 1770, he was among the first to suggest that Earth's history can be known about by observing the planet's current state.

James Hutton (1726–1797) did not propose a date for the formation of Earth, but is famous for the statement that Earth contains "no vestige of a beginning—no prospect of an end." The German geologist Abraham Werner (1750–1817), the first scientist to make use of a stratigraphic column, a diagram of order of sedimentary layers. An original approach to geological history was suggested by the French zoologist and paleontologist Georges Cuvier (1769–1832), who observed that specific fossil animals occurred in specific rock layers, forming recognizable groups, or assemblages. William Smith (1769–1768) combined Werner's and Cuvier's approaches, using fossil assemblages to identify identical sequences of layers distant from each other, linking or correlating rocks which were once part of the same rock layer but had been separated by faulting or erosion .

In 1897, the physicist Lord Kelvin (1824–1907) developed a model for Earth history, which assumed that Earth has been cooling steadily since its formation. Because he did not know that heat moves around in currents in the earth (convection ), or that Earth generates its own heat from the decay of radioactive minerals buried inside it, Kelvin proposed that the earth was formed from 20 to 40 million years ago.

In the late eighteenth century, geologists began to name periods of geologic time. In the nineteenth century, geologists such as William Buckland (1784–1856), Adam Sedgwick (1785–1873), Henry de la Beche (1796–1855), and Roderick Murchison (1792–1871) identified widespread rock layers beneath continental Europe , the British isles, Russia, and America. They named periods of time after the places in which these rocks were first described. For instance, Cambrian Period was named for Cambria (the Roman name for Wales), and Permian, for the Perm province in Czarist Russia. Mississippian and Pennsylvanian Periods widely used by American geologists were named for a U.S. state and a region around the upper reaches of a large river, respectively. By the mid-nineteenth century, most of the modern names of the periods of geologic time had been proposed; all of them are still in use.

Relative age determination

A rock layer may or may not contain evidence that reveals its age. Rock layers whose ages are defined by relationships with the dated rock units around it are examples of relative age determination. That relationship is found by observing the unknown rock layer's stratigraphic relationship with the rock layers whose ages are known. If the known rock layer is on top of the unknown layer, then the lower layer is probably the older of the two. That is based on the principle of superposition, which states that when two rock layers are stacked one above the other, the lower one was formed before the overlying one, unless the layers have been overturned.

Radiometric age determination

Every rock and mineral exists in the world as a mixture of elements, and every element exists as a population of atoms. One element's population of atoms will not all have the same number of neutrons, and so two or more kinds of the same element will have different atomic masses or atomic numbers. These different kinds of the same chemical element are called nuclides of that element. A nuclide of a radioactive element is known as a radionuclide.

The nucleus of every radioactive element spontaneously disintegrates over time. This process results in radiation , and is called radioactive decay . Losing high-energy particles from their nuclei turns the atoms of a radioactive nuclide into the daughter product of that nuclide. A daughter product is either a different element altogether, or is a different nuclide of the same parent element. A daughter product may or may not be radioactive. If it is, it also decays to form its own daughter product. The last radioactive element in a series of these transformations will decay into a stable element, such as lead .

While there is no way to discern whether an individual atom will decay today or two billion years from today, the behavior of large numbers of the same kind of atom is so predictable that certain nuclides of elements are called radioactive clocks. The use of these radioactive clocks to calculate the age of a rock is referred to as radiometric age determination. First, an appropriate radioactive clock must be chosen. The sample must contain measurable quantities of the element to be tested for, and its radioactive clock must tell time for the appropriate interval of geologic time. Then, the amount of each nuclide present in the rock sample must be measured.

Each radioactive clock consists of a radioactive nuclide and its daughter product, which accumulate within the atomic framework of a mineral. These radioactive clocks decay at various rates, which govern their usefulness in particular cases. A three-billion year old rock needs to have its age determined by a radioactive clock that still has a measurable amount of the parent nuclide decaying into its daughter product after that long. The same radioactive clock would reveal nothing about a two million year old rock, for the rock would not yet have accumulated enough of the daughter product to measure.

The time it takes for half of the parent nuclide to decay into the daughter product is called one half-life . The remaining population of the parent nuclide is halved again, and the population of daughter product doubled, with the passing of every succeeding half-life. The amount of parent nuclide measured in the sample is plotted on a graph of that radioactive clock's known half-life. The absolute age of the rock, within its margin of error , can then be read directly from the time axis of the graph.

When a rock is tested to determine its age, different minerals within the rock are tested using the same radioactive clock—similar to questioning different witnesses at a crime scene to determine if they saw the same event happen in the same way. Ages may be determined on the same sample by using different radioactive clocks. When the age of a rock is measured in two different ways, and the results are the same, the results are said to be concordant.

Discordant ages means the radioactive clock showed different absolute ages for a rock sample, or different ages for different minerals within the rock. A discordant age result means that at some time after the rock was formed, something happened to it which reset one of the radioactive clocks back to zero .

For example, if a discordant result happens in the potassium-argon test, the rock may have been heated to a blocking temperature above which a mineral's atomic framework becomes active and wiggly enough to allow trapped gaseous argon-40 to escape.

Concordant ages mean that no complex sequence of events-deep burial, metamorphism , and mountain-building, for example has happened that can be detected by the two methods of age determination that were used.

A form of radiometric dating is used to determine the ages of organic matter . A short-lived radioisotope, carbon-14, is accumulated by all living things on Earth. Upon the organism's death, the carbon-14 is fixed and then begins to decay into carbon-12 at a known rate (its half-life is 5,730 years). By measuring how much of the carbon-14 is left in the remains, and plotting that amount on a graph showing how fast the carbon-14 decays, the approximate date of the organism's death can be known.

When uranium atoms decay, they emit fast, heavy alpha particles. Inside a zircon crystal , these subatomic particles tear long trails of destruction through the zircon's crystal framework. The age of a zircon crystal can be estimated by counting the number of these trails. The rate at which the trails form has been found by determining the age of rocks containing zircon crystals, and noting how torn-up the zircon crystals become over time. This age determination technique is called fission-track dating. This technique has detected the world's oldest rocks, between 3.8 billion and 3.9 billion years old, and yet older crystals, which suggest that Earth had some solid ground on it 4.2 billion years ago.

The age of Earth is deduced from the ages of other materials in the solar system , namely, meteorites. Meteorites are pieces formed from the cloud of dust and debris left behind by a supernova , the explosive death of a star . Through this cloud the infant Earth spun, attracting more and more pieces of matter. The meteorites that fall to Earth today have orbited the Sun since that time, unchanged and undisturbed by the processes that have destroyed Earth's first rocks. Radiometric ages for meteorites fall between 4.45 billion and 4.55 billion years.

The radionuclide iodine-129 is formed in nature only inside stars. A piece of solid iodine-129 will almost entirely decay into the gas xenon-129 within a hundred million years. If this decay happens in open space , the xenon-129 gas will float off into space, blown by the solar wind . Alternatively, if the iodine-129 was stuck in a rock within a hundred million years of being formed in a star, then some very old rocks should contain xenon-129 gas. Both meteorites and Earth's oldest rocks contain xenon-129. That means the star that provided the material for the solar system died its cataclysmic death less than 4.65 billion (4,650,000,000) years ago.

See also Dating techniques; Fossil and fossilization; Spectroscopy; Strata.

Resources

books

Hartman, William, and Ron Miller. The History of the Earth. New York: Workman Publishing, 1991.

Press, Frank, and Raymond Sevier. Understanding Earth. San Francisco: Freeman, 2000.

Tarbuck, Edward. J., Frederick K. Lutgens, and Dennis Tassa, eds. Earth: An Introduction to Physical Geology, 7th ed. Upper Saddle River, New Jersey: Prentice Hall, 2002.

other

Newman, William L. "Geologic Time." United States Geological Survey, July 28, 1997 [cited January 5, 2003]. <http://pubs.usgs.gov/gip/geotime/contents.html>.

Clinton Crowley

KEY TERMS

Blocking temperature

—Temperature below which a mineral's atomic framework is rigid enough to trap the daughter products of radioactive decay, thus starting the radioactive clock.

Concordant

—Said of a rock's age when determining the absolute age of two or more rocks or minerals by radiometric methods yields the same result.

Daughter product

—The element made when a radioactive element's nucleus spontaneously falls apart (or decays). The daughter product may or may not be radioactive.

Discordant

—Said of a rock's age when different results come from determining the absolute age of two or more rocks or minerals by radiometric methods.

Fission-track dating

—An age determination technique in which the number of trails torn by alpha particles through a zircon crystal's crystal framework.

Geochronology

—The study of determining how old rocks and minerals are, in order to sort out the events of Earth history.

Half-life

—The time it takes for half of the original atoms of a radioactive element to be transformed into the daughter product.

Nuclides

—Different versions of the same chemical element that have slightly different numbers of subatomic particles in their nuclei, and therefore different atomic masses or in some cases different atomic numbers.

Radioactive clock

—The ratio of a radionuclide and its daughter product, which accumulate within the atomic framework of a mineral. Also used to mean the radionuclide itself.

Radioactive decay

—The predictable manner in which a population of atoms of a radioactive element spontaneously fall apart, and lose subatomic particles from their nuclei, becoming their daughter products.

Radiometric date

—A measurement of time derived by measuring how much of a short-lived radioisotope (carbon-14) is left in a formerly living object, or how much of a long-lived radioisotope is left in a rock or mineral compared to how much of the daughter product is there into which the long-lived radioisotope decayed.

Radionuclide

—Radioactive or unstable nuclide.

Stratigraphy

—The study of layers of rock or soil, based on the assumption that the oldest material will usually be found at the bottom of a sequence.

Earth Science

Earth, air, and water

Resources

As befits a dynamic Earth, the study of earth science embraces a multitude of subdisciplines. At the heart of earth science is the study of geology. Literally meaning “to study the Earth,” traditional geological studies of rocks, minerals, and local formations have within the last century, especially in the light of the development of plate tectonic theory, broadened to include studies of geophysics and geo-chemistry that offer sweeping and powerful explanations of how continents move, to explanations of the geochemical mechanisms by which magma cools and hardens into a multitude of igneous rocks.

Earth’s formation and the evolution of life upon its fragile outer crust was dependent upon the conditions established during the formation of the solar system. The sun provides the energy for life and drives the turbulent atmosphere. A study of earth science must, therefore, not ignore a treatment of Earth as an astronomical body in space.

At the opposite extreme, deep within Earth’s interior, radioactive decay adds to the heat left over from the condensation of Earth from cosmic dust. This heat drives the forces of plate tectonics and results in the tremendous variety of features that distinguish Earth. To understand Earth’s interior structure and dynamics, seismologists probe the interior structure with seismic shock waves.

It does not require the spectacular hurricane, tornado, landslide, or volcanic eruption to prove that Earth’s atmosphere and seas are dynamic entities. Forces that change and shape the Earth appear on a daily basis in the form of wind and tides. What earth scientists, including meteorologists and oceanographers seek to explain—and ultimately to quantify—are the physical mechanisms of change and the consequences of those changes. Only by understanding the mechanisms of change can predictions of weather or climatic change hope to achieve greater accuracy.

The fusion of disciplines under the umbrella of earth science allows a multidisciplinary approach to solving complex problems or multi-faceted issues of resource management. In a addition to hydrogeologists and cartographers, a study of ground water resources could, for example, draw upon a wide diversity of earth science specialists.

Although modern earth science is a vibrant field with research in a number of important and topical areas (e.g., identification of energy resources, waste disposal sites, etc.), the span of geological process and the enormous expanse of geologic time make critical the study of ancient processes (e.g., paleogeo-logical studies). Only by understanding how processes have shaped Earth in the past—and through a detailed examination of the geological record—can modern science construct meaningful predictions of the potential changes and challenges that lie ahead.

Earth, air, and water

Earth science is the study of the physical components of Earth—its water, land, and air—and the processes that influence them. Earth science can also be thought of as the study of the five physical spheres of Earth: atmosphere (gases), lithosphere (rock), pedosphere (soil and sediment), hydrosphere (liquid water), and cryosphere (ice). As a result, earth scientists must consider interactions between all three states of matter—solid, liquid, and gas—when performing investigations. The subdisciplines of earth science are many, and include the geosciences, oceanography, and the atmospheric sciences.

The geosciences involve studies of the solid part of Earth and include geology, geochemistry, and geophysics. Geology is the study of Earth materials and processes. Geochemistry examines the composition and interaction of Earth’s chemical components. Geophysicists study the dynamics of Earth and the nature of interactions between its physical components.

Oceanography involves the study of all aspects of the oceans: chemistry, water movements, depth, topography, etc. Considerable overlap exists between oceanography and the geosciences. However, due to the special tools and techniques required for studying the oceans, oceanography and the geosciences continue to be thought of as separate disciplines.

The atmospheric sciences, meteorology and climatology, involve the study of the atmosphere. Meteorology is the study of the physics and chemistry of the atmosphere. One of the primary goals of meteorology is the analysis and prediction of short-term weather patterns. Climatology is the study of long-term weather patterns, including their causes, variation, and distribution.

Due to the interactions between the different spheres of Earth, scientists from these different subdisciplines often must work together. Together, earth scientists can better understand the highly involved and interrelated systems of Earth and find better answers to the difficult questions posed by many natural phenomena. In addition, due to the interwoven nature of the biotic (living) and abiotic (nonliving) parts of Earth’s environment, Earth scientists sometimes work with life scientists (i.e., biologists, ecologists, agronomists, etc.) who study the Earth’s biosphere.

Earth science research focuses on solving the many problems posed by increasing human populations, decreasing natural resources, and inevitable natural hazards. Computer and satellite technologies are increasingly utilized in the search for and development of Earth’s resources for present and future use.

See also Astronomy; Atmosphere, composition and structure; Atmospheric circulation; Atmospheric optical phenomena; Atmospheric pressure; Atmospheric temperature; Biochemistry; Earth’s magnetic field; Earth’s rotation; Fossil and fossilization; Fossil fuels; Gravity and gravitation; Latitude and longitude; Mineralogy; Sediment and sedimentation.

Resources

BOOKS

Lutgens, Frederick K., et al. Foundations of Earth Science. Upper Saddle River, NJ: Prentice Hall, 2004.

Plummer, C., et al. Physical Geology. New York: McGraw-Hill, 2004.

Tarbuck, Edward J., et al. Earth: An Introduction to Physical Geology. 8th ed. Upper Saddle River, NJ: Prentice Hall, 2004.

PERIODICALS

Garnero, Edward J. “A New Paradigm for Earth’s Core-Mantle Boundary.” Science. 304 (2004): 834-836.

van der Hilst, Robert D. “Changing Views on Earth’s Deep Mantle.” Science. 306 (2004): 817-818.

OTHER

United States Geological Survey. “Science Topics.” 2006. <http://www.usgs.gov/science/> (accessed October 25, 2006).

K. Lee Lerner

Jack Vine

Geologic time

Geologic time describes the immense span of time—billions of years—revealed in the complex rock surface of Earth. Geologists have devised a geologic time scale that divides Earth's history into units of time. These units are eras, periods, and epochs. A unit is defined in terms of the fossils or rock types found in it that makes it different from the other units. Eras, the four largest time blocks in the scale, are named to indicate the fossils they contain: Precambrian (before ancient life), Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (recent life). The last three eras are then subdivided into 11 periods. The two most recent periods are further subdivided into seven epochs.

History of the concept of geologic time

Before the eighteenth century, ideas about time and the history of Earth came mostly from religious theories. Many people believed Earth was only a few thousand years old. They also believed that all the physical features of Earth—mountains, valleys, oceans, rivers, continents—were the same as they had always been. Everything that existed on Earth was the same as it had been in the beginning.

In the eighteenth century, geologists began to theorize that Earth's lifetime was immense. However, since they lacked sophisticated scientific measuring devices, they could only offer educated guesses. They compared the rock record from different parts of the world and estimated how long it would take natural processes to form all the rocks on Earth.

Relative dating

Since that time, geologists have learned to study the strata, or the thousands of layers of sedimentary rock that make up the Earth's crust. Over the course of history, the natural weathering of Earth's surface created sediments (rock debris) that settled in layers on the surface. As the layers built up, the underlying layers were compressed together to form sedimentary rock. A basic geological assumption is that lower layers of rock almost always formed before higher layers.

The fossils of changing lifeforms found in the different strata also help geologists determine the long history of Earth. Determining the age of strata by looking at the fossils, position, grain size, minerals, color, and other physical properties contained within them is known as relative dating.

Geologists easily identify rocks containing fossils of primitive life-forms as being older than rocks containing fossils that are more evolved or advanced. However, the method of relative dating is exactly that, relative. It is not absolute. In addition, complex lifeforms have existed on Earth for only the last 600 million years. Therefore, their fossils represent less than 15 percent of Earth's history.

Absolute dating

A more specific and far-reaching method to assign dates to layers of rocks and events that have occurred in Earth's history is absolute dating. This method employs the natural process of radioactive decay.

Every rock and mineral exists in the world as a mixture of elements. Every element exists as a population of atoms. At the center of an atom is the positively charged nucleus made up of protons and neutrons. Over time, the nucleus of every radioactive element (such as radium and uranium) spontaneously disintegrates, transforming itself into the nucleus of an atom of a different element. In the process of disintegration, the atom gives off radiation (energy emitted in the form of waves). Hence the term radioactive decay.

The nucleus of an atom will continue to decay if it is radioactive. The last radioactive element in a series of these transformations will decay into a stable element, such as lead.

Words to Know

Half-life: The time it takes for one-half of the population of a radioactive element to decay.

Radioactive decay: The predictable manner in which a population of atoms of a radioactive element spontaneously fall apart.

Radiometric dating: Use of naturally occurring radioactive elements and their decay products to determine the absolute age of the rocks containing those elements.

Stratigraphy: The branch of geology that catalogues Earth's successions of rock layers.

Half-life is a measurement of the time it takes for one-half of a radioactive element to decay. An important feature of the radioactive decay process is that each element decays at its own rate. The half-life of a particular element, therefore, is constant and is not affected by any physical conditions (temperature, pressure, etc.) that occur around it. Because of this stable process, scientists are able to estimate when a particular element was formed by measuring the amount of original and transformed atoms in that element. This is known as radiometric or isotope dating.

Geologic Time Scale

Determining the age of Earth

The age of the whole Earth is deduced from the ages of other materials in the solar system, namely, meteorites. Meteorites are pieces formed from the cloud of dust and debris left behind after the beginning of the solar system. The meteorites that fall to Earth today have orbited the Sun since that time, unchanged and undisturbed by the processes that have destroyed Earth's first rocks. Radiometric ages for these meteorites fall between 4.45 and 4.55 billion years old.

[See also Dating techniques; Fossil and fossilization; Paleontology; Radioactivity; Rocks ]