Geologic Time | Research & Encyclopedia Articles

Geologic Time

Geologic time describes the immense span of time—billions of years-revealed in the complex rock surface of the 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 actual age, in years, of rocks and minerals. Knowledge of stratigraphy, the branch of geology that catalogues the earth's successions of rock layers, is essential to establish the relative ages of rock units. By finding which rock unit came first, the order of the events in earth history can be sorted out.

Before scientific methods were used to find out about geologic time, ideas about time and earth history came from religious theories. The Hindu and Mayan religions believed in endlessly repeating cycles of time, each lasting for billions of years. Ideas in western culture about the age of the earth were just as precise--and just as wrong. In the 1650s, the Irish clergyman and scholar James Ussher (1581-1656) used the Book of Genesis in the Bible to determine that the Earth was created in 4004 B.C. For its time, Ussher's exercise was perfectly good science. He was basing his results on the only information that was considered relevant to the question of the Earth's age. Within those assumptions, Ussher engaged in rational philosophical inquiry, and certainly did not intend to suppress earth science. Isaac Newton also speculated on the age of the earth, using the investigative techniques of the time--which would be considered archaic today.

As early as the eighteenth century scientists knew that the Earth's lifetime was immense. But geologists were not able to measure the Earth's history was 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 the 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 the 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 the earth, but is famous for the statement that the earth contains "no vestige of a beginning--no prospect of an end." The German geologist Abraham Werner (1750-1817) invented the stratigraphic column, a diagram of layers within the earth. 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 has been cooling steadily since its formation. Because he did not know that heat moves around in currents in the earth (convection), or that the Earth generates its own heat from the decay of radioactive minerals buried inside it, he proposed that the earth was formed from 20-40 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 in Wales, and the Permian, for the Perm basin in Russia. The Mississippian and Pennsylvanian periods widely used by American geologists were named for the American states. By the mid-nineteenth century, most of the modern names of the periods of geologic time had been proposed; many of them are still in use.

A rock layer may or may not contain evidence which 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, derived from the writings of Nicolaus Steno (1638-1686), which states that when two rock layers are stacked one above the other, the lower one was almost always formed before the higher one. The undated rock can thus be integrated into a frame of reference.

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 tell 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 begins to change 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 leaves the body, 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 3,800,000,000 and 3,900,000,000 years old, and yet older crystals, which suggest that the earth had some solid ground on it 4,200,000,000 years ago.

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 by a supernova, the explosive death of a star. Through this cloud the infant earth spun, attracting more and more pieces of matter like a ball of wet mud rolled through a sandpile. The meteorites which fall to Earth today have orbited the sun since that time, unchanged and undisturbed by the processes which have destroyed the earth's first rocks. Radiometric ages for meteorites fall between 4,450,000,000 and 4,550,000,000 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 the 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 4,650,000,000 years ago.

This is the complete article, containing 1,916 words (approx. 6 pages at 300 words per page).