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.