Geologic Time

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Geologic Time

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.