Geologic Time Scale

Introduction

In climate change studies many reference are made to the geologic record or geologic time. The geologic time scale is an internationally developed and agreed scheme of subdividing the passage of time since the origin of Earth. This time scale is universally used among geologists, paleontologists, and other natural scientists who deal with Earth history and Earth antiquity issues. The original structure of the geologic time scale was developed during the nineteenth and twentieth centuries, mainly as a result of studies of stratigraphy (the study of Earth's rock layers). The finer details of the geologic time scale continue to evolve today. At first, the geologic time scale was a relative time scale, which means that the absolute ages of various intervals within the time scale were not known. With the advent of radiometric age-dating techniques, the ages of the boundaries of these intervals has become established in most instances and the modern geologic time scale became essentially an absolute time scale. Ages of boundaries between subdivisions are usually given in millions of years (abbreviated Ma) before present.

Historical Background and Scientific Foundations

Geologists were unable to accurately measure the dimensions of Earth's history until mass spectrometers (a device that allows the determination of the elements and compounds in a sample) became available in the 1950s. 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 create formations.

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.

WORDS TO KNOW

INTERNATIONAL UNION OF GEOLOGICAL SCIENCES (IUGS): Non-governmental group of geologists founded in 1961 and headquartered in Trondheim, Norway. The group fosters international cooperation on geological research of a transnational or global nature.

RADIOMETRIC AGE: The age of an object as determined by the levels of certain radioactive substances in its substance. Radioactive substances break down into other substances (some radioactive and some not) over time in a regular way: measurements of their relative quantities can allow an estimate of an object's age (e.g., rock, piece of wood). Many techniques for determining radiometric age are used in the field of radiometric dating.

STRATIGRAPHY: The branch of geology that deals with the layers of rocks or the objects embedded within those layers.

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 the 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, the 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.

Today's Geologic Time Scale

Today, the geologic time scale is hierarchically divided (from largest to smallest) into Eons, Eras, Periods, Epochs, and Ages. The corresponding rocks that represent these subdivisions are referred to as Eonothems, Erathems, Systems, Series, and Stages. At this time, the geologic time scale is administered worldwide by the International Union of Geological Sciences (IUGS) and its International Commission on Stratigraphy. IUGS geological maps display standard colors for the geologic time scale. In the United States, slightly different colors are used on geological maps, which follow the color scheme of the U.S. Geological Survey.

As the precision and accuracy of radiometric agedating techniques have improved over time, so too are the age dates of geologic time scale boundaries more precise and accurate. Over the past few decades, international working groups have been established under the auspices of the IUGS to refine the definition and ages of geologic time scale boundaries. These working groups have established many reference sites on Earth, called GSSPs (Global Stratotype and Point), where a geological boundary has been established and age-dated. These GSSPs help establish firmly the age of Stage (and thus Age) boundaries in the geologic time scale. These boundaries in turn establish the ages of the other hierarchical groups within the geologic time scale.

The development of the geologic time scale was critically important to the development of the science of geology because it gave a universally accepted framework into which geological events of the past could be fit. This framework permitted correlation of events in Earth history, a key aspect in the evolving science of Earth history (Historical Geology). In addition, the history of life as revealed in the fossil record could be described within this framework. As a communication device, the geologic time scale is essential because the named subdivisions of the geologic time scale are the same in all places over the Earth where they are exposed.

Eons, Eras, Periods, Epochs, and Ages

According to the modern geologic time scale, the Archean was the first eon, which spanned the time from Earth's formation (about 4,570 million years ago) to an arbitrary point in time about 2,500 million years ago. From 2,500 to 542 million years ago is the Proterozoic Eon. All remaining geologic time, up to present, is contained within the Phanerozoic Eon. The term Phanerozoic means visible life, which refers to the fact that fossils are usually quite evident in sedimentary rocks deposited during the Phanerozoic. Within the Archean and Proterozoic eons, Epochs and Stages are not delineated, so the Period is the level of subdivision used by geologists at this time. The ages of these Period boundaries are called GSSAs (Global Standard Stratigraphic Ages), a concept similar to the GSSPs noted earlier.

Within Phanerozoic rocks, there are three Eras. In age order (oldest first), they are Paleozoic, Mesozoic, and Cenozoic. Paleozoic is divided into six Periods. From oldest to youngest, they are: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. Mesozoic is similarly divided into three Periods: Triassic, Jurassic, and Cretaceous. Cenozoic is similarly divided into Paleogene, Neogene, and Quaternary Periods.

All Phanerozoic Periods are divided into two or more Epochs. Some Epochs are named, for example, the Miocene and Pliocene Epochs within Neogene, and some are given position names, such as the Lower and Upper Cretaceous. Ages are named subdivisions of Epochs. Some Ages are not formally named within the Cambrian, Ordovician, and Silurian Periods. Stage names, by international agreement, are based mainly on U.K. and European reference locations and thus are named for the British and European sites where they are well known.

There are two Sub-Periods in the geologic time scale, Mississippian and Pennsylvanian. These are vestiges of an interval in the development of the geologic time scale when geologists in the United States advocated splitting the Carboniferous into two Periods. This was never agreed internationally, and the Sub-Periods are retained because of this controversy.

Because of the close association of some dominant fossil groups with some geologic time scale subdivisions, there are informal fossil names for some of the subdivisions. For example, the Mesozoic Era is informally called “the age of the dinosaurs.” Likewise, the Cenozoic Era is informally called “the age of mammals.”

As radiometric age dates and suitable reference sites become available and are agreed upon, additional GSSPs will be established until the whole of the Phanerozoic has been delineated with GSSPs. Likewise, in older rocks, work will continue to refine GSSAs within Archean and Proterozoic rocks. The status of Quaternary as a Period within the Cenozoic is being examined as is the status of the Holocene as an Epoch. There are many unsettled questions on the organization of Earth's youngest time scale subdivisions and many science groups are interested in this issue, including archeologists, glacial geologists, and others.

Other Geologic Time Scales

As noted, there is one internationally recognized geologic time scale (or International Stratigraphic Chart), which is administered and updated by the IUGS. There are other geologic time scales, which are not as widely embraced as the IUGS effort. For example, the Geological Society of America (GSA) has published a geologic time scale that is widely used, especially in publications of that society. The GSA geologic time scale is not vastly different from the IUGS time scale, but some of the Age names within the older Paleozoic Periods are different as are some of the radiometric age dates at geologic time scale boundaries. The differences in ages may be attributed to the fact that the GSA geologic time scale is a few years older than the IUGS time scale and has not been recently updated. The GSA geologic time scale uses the Tertiary Period for the pre-Quaternary Cenozoic and considers the Paleogene and Neogene (Periods in the IUGS time scale) as Sub-Periods.

Age Determinations

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.

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 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.

Although 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.

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 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 (matter from once living things that contain carbon centered molecules and compounds). 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 years ago.

Impacts and Issues

The geologic time scale provides a temporal frame of reference for scientists to communicate about Earth's past. Such communication may involve, among other things, fossil evolution, Earth's tectonic changes, and climatic change. For example, the warm, equitable Cretaceous (144-65 million years ago) climates that supported a tremendous diversity of animal (including dinosaurs) and plant life eventually changed to globally cooler climates during the Neogene period (23 million years ago to the near-present). During the Cretaceous, there was little or no polar ice, and oceans that were much warmer than today covered more of Earth's land area. Climatic changes near the end of the Cretaceous resulting from volcanism, the asteroid impact credited for the Cretaceous-Tertiary mass extinction, and effects of continental drift resulted in more distinct seasons, dropping sea levels, and greater extremes between temperatures at the equator and the poles.

See Also Ice Ages.

BIBLIOGRAPHY

Books

Gradstein, F. M., et al. A Geologic Time Scale. Cambridge: Cambridge University Press, 2004.

Tarbuck, E. J., F. K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, NJ: Prentice Hall, 2005.

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

Web Sites

“Geologic Time: Online Edition.” U.S. Geological Survey, 2007. < http://pubs.usgs.gov/gip/geotime> (accessed December 4, 2007).

“Tour of Geologic Time.” University of California Museum of Paleontology, 2007.< http://www.ucmp.berkeley.edu/exhibits/geologictime.php> (accessed December 4, 2007).