The most precise method of obtaining estimates of the absolute ages of geological deposits is

Geologists generally know the age of a rock by determining the age of the group of rocks, or formation, that it is found in. The age of formations is marked on a geologic calendar known as the geologic time scale. Development of the geologic time scale and dating of formations and rocks relies upon two fundamentally different ways of telling time: relative and absolute.

Relative dating places events or rocks in their chronologic sequence or order of occurrence. Absolute dating places events or rocks at a specific time. If a geologist claims to be younger than his or her co-worker, that is a relative age. If a geologist claims to be 45 years old, that is an absolute age.

Relative Dating

Superposition: The most basic concept used in relative dating is the law of superposition. Simply stated, each bed in a sequence of sedimentary rocks (or layered volcanic rocks) is younger than the bed below it and older than the bed above it. This law follows two basic assumptions: (1) the beds were originally deposited near horizontal, and (2) the beds were not overturned after their deposition.

Faunal Succession: Similar to the law of superposition is the law of faunal succession, which states that groups of fossil animals and plants occur throughout the geologic record in a distinct and identifiable order. Following this law, sedimentary rocks can be “dated” by their characteristic fossil content. Particularly useful are index fossils, geographically widespread fossils that evolved rapidly through time.

Crosscutting Relationships: Relative ages of rocks and events may also be determined using the law of crosscutting relationships, which states that geologic features such as igneous intrusions or faults are younger than the units they cut across.

Inclusions: Inclusions, which are fragments of older rock within a younger igneous rock or coarse-grained sedimentary rock, also facilitate relative dating. Inclusions are useful at contacts with igneous rock bodies where magma moving upward through the crust has dislodged and engulfed pieces of the older surrounding rock.

Gaps in the geologic record, called unconformities, are common where deposition stopped and erosion removed the previously deposited material. Fortunately, distinctive features such as index fossils can aid in matching, or correlating, rocks and formations from several incomplete areas to create a more complete geologic record for relative dating. Relative dating techniques provide geologists abundant evidence of the incredible vastness of geologic time and ancient age of many rocks and formations. However, in order to place absolute dates on the relative time scale, other dating methods must be considered.

Absolute Dating

The nuclear decay of radioactive isotopes is a process that behaves in a clock-like fashion and is thus a useful tool for determining the absolute age of rocks. Radioactive decay is the process by which a “parent” isotope changes into a “daughter” isotope. Rates of radioactive decay are constant and measured in terms of half-life, the time it takes half of a parent isotope to decay into a stable daughter isotope.

Some rock-forming minerals contain naturally occurring radioactive isotopes with very long half-lives unaffected by chemical or physical conditions that exist after the rock is formed. Half-lives of these isotopes and the parent-to-daughter ratio in a given rock sample can be measured, then a relatively simple calculation yields the absolute (radiometric) date at which the parent began to decay, i.e., the age of the rock.

Of the three basic rock types, igneous rocks are most suited for radiometric dating. Metamorphic rocks may also be radiometrically dated. However, radiometric dating generally yields the age of metamorphism, not the age of the original rock. Most ancient sedimentary rocks cannot be dated radiometrically, but the laws of superposition and crosscutting relationships can be used to place absolute time limits on layers of sedimentary rocks crosscut or bounded by radiometrically dated igneous rocks.

Sediments less than about 50,000 years old that contain organic material can be dated based on the radioactive decay of the isotope Carbon 14. For example, shells, wood, and other material found in the shoreline deposits of Utah’s prehistoric Lake Bonneville have yielded absolute dates using this method. These distinct shorelines also make excellent relative dating tools. Many sections of the Wasatch fault disturb or crosscut the Provo shoreline, showing that faulting occurred after the lake dropped below this shoreline which formed about 13,500 years ago. As this example illustrates determining the age of a geologic feature or rock requires the use of both absolute and relative dating techniques.

The physical process of radioactive decay has provided Earth scientists, anthropologists, and evolutionary biologists with their most important method for determining the absolute age of rocks and other materials (Dalrymple 1991; Dickin 2005). This remarkable technique, which depends on measurements of the distinctive properties of radioactive materials, is called radioisotope geochronology, or simply “radiometric dating.”

Trace amounts of isotopes of radioactive elements, including carbon-14, uranium-238, and dozens of others, are all around us—in rocks, in water, and in the air (Table 1). These isotopes are unstable, so they gradually break apart or “decay.” Radiometric dating works because radioactive elements decay in predictable fashion, like the regular ticking of a clock. Here’s how it works. If you have a collection of one million atoms of a radioactive isotope, half of them will decay over a span of time called the “half life.” Uranium-238, for example, has a half life of 4.468 billion years, so if you start with a million atoms and come back in 4.468 billion years, you’ll find only about 500,000 atoms of uranium-238 remaining. The rest of the uranium will have decayed to 500,000 atoms of other elements, ultimately to stable (i.e., nonradioactive) atoms of lead-206. Wait another 4.468 billion years and only about 250,000 atoms of uranium will remain (Fig. 8).

Radiometric dating relies on the clock-like characteristics of radioactive decay. In one half-life, approximately half of a collection of radioactive atoms will decay. By knowing how many atoms a material started with, and then measuring what’s left, you can measure ages of old objects. Source: NCSE

The best-known radiometric dating method involves the isotope carbon-14, with a half life of 5,730 years. Every living organism takes in carbon during its lifetime. At this moment, your body is taking the carbon in your food and converting it to tissue, and the same is true of all other animals. Plants are taking in carbon dioxide from the air and turning it into roots, stems, and leaves. Most of this carbon (about 99%) is in the form of stable (non-radioactive) carbon-12, while perhaps 1% is the slightly heavier stable carbon-13. But a certain small percentage of the carbon in your body and every other living thing—no more than one carbon atom in every trillion—is in the form of radioactive carbon-14.

As long as an organism is alive, the carbon-14 in its tissues is constantly renewed in the same small, part-per-trillion proportion that is found in the general environment. All of the isotopes of carbon behave the same way chemically, so the proportions of carbon isotopes in the living tissue will be nearly the same everywhere, for all living things. When an organism dies, however, it stops taking in carbon of any form. From the time of death, therefore, the carbon-14 in the tissues is no longer replenished. Like a ticking clock, carbon-14 atoms transmute by radioactive decay to nitrogen-14, atom-by-atom, to form an ever-smaller percentage of the total carbon. Scientists can thus determine the approximate age of a piece of wood, hair, bone, or other object by carefully measuring the fraction of carbon-14 that remains and comparing it to the amount of carbon-14 that we assume was in that material when it was alive. If the material happens to be a piece of wood taken out of an Egyptian tomb, for example, we have a pretty good estimate of how old the artifact is and, by inference, when the tomb was built. What’s more, scientists have conducted meticulous year-by-year comparisons of carbon-14 dates with those of tree ring chronologies (Reimer et al. 2004). The result: the two independent techniques yield exactly the same dates for ancient fossil wood.

Carbon-14 dating often appears in the news in reports of ancient human artifacts. In a highly publicized discovery in 1991, an ancient hunter was found frozen in the ice pack of the Italian Alps (Fig. 9). “Ötzi the iceman,” as he was called, was shown by carbon-14 techniques to date from about 5,300 years ago. The technique provided similar age determinations for the tissues of the iceman, his clothing, and his implements (Fowler 2000).

Ötzi the iceman was discovered in 1991 frozen in the Italian Alps. Carbon-14 dating revealed that he died about 5,300 years ago. Photo courtesy South Tyrol Museum of Archaeology, www.iceman.it

Carbon-14 dating has been instrumental in mapping human history over the last several tens of thousands of years. When an object is more than about 50,000 years old, however, the amount of carbon-14 left in it is so small that this dating method cannot be used. To date rocks and minerals that are millions of years old, scientists must rely on similar techniques that use radioactive isotopes of much greater half-life (Table 1). Among the most widely used radiometric clocks in geology are those based on the decay of potassium-40 (half-life of 1.248 billion years), uranium-238 (half-life of 4.468 billion years), and rubidium-87 (half-life of 47 billion years). In these cases, geologists measure the total number of atoms of the radioactive parent and stable daughter elements to determine how many radioactive nuclei were present at the beginning. Thus, for example, if a rock originally formed a long time ago with a small amount of uranium atoms but no lead atoms, then the ratio of uranium-to-lead atoms today can provide an accurate geologic stop watch.

When you see geologic age estimates reported in scientific publications or in the news, chances are those values are derived from radiometric dating techniques. In the case of the early settlement of North America, for example, carbon-rich campfire remains and associated artifacts point to a human presence by about 13,000 years ago. Much older events in the history of life, some stretching back billions of years, are often based on potassium-40 dating. This technique works well because fossils are almost always preserved in layers of sediments, which also record periodic volcanic ash falls as thin horizons. Volcanic ash is rich in potassium-bearing minerals, so each ash fall provides a unique time marker in a sedimentary sequence. The rise of humans about 2.5 million years ago, the extinction of the dinosaurs 65 million years ago, the appearance of animals with hard shells starting about 540 million years ago, and other key transitions in life on Earth are usually dated in this way (Fig. 10).

Paleontologists rely on radiometric dating to determine the ages of fossils, such as this 310-million-year-old trilobite, Ameura major, from near Kansas City, Kansas. Photo courtesy Hazen Collection, Smithsonian Institution

The oldest known rocks, including basalt and other igneous formations, solidified from incandescent red-hot melts. These durable samples from the moon and meteorites are typically poor in potassium, but fortunately, they incorporate small amounts of uranium-238 and other radioactive isotopes. As soon as these molten rocks cool and harden, their radioactive elements are locked into place and begin to decay. The most ancient of these samples are several types of meteorites, in which slightly more than half of the original uranium has decayed to lead. These primordial space rocks, the leftovers from the formation of Earth and other planets, yield an age of about 4.56 billion years for the nascent solar system. The oldest known moon rocks, at about 4.46 billion years, also record these earliest formative events (Norman et al. 2003).

Earth must have formed at about the same time, but our restless planet’s original surface has now eroded away. Only a few uranium-rich, sand-sized grains of the hardy mineral zircon, some as old as 4.4 billion years, survive (Wilde et al. 2001). Nevertheless, uranium-bearing rocks, on every continent provide a detailed chronology of the early Earth (Hazen et al. 2008, 2009). The oldest Earth rocks, at about four billion years, point to the early origins of continents. Rocks from almost 3.5 billion years ago host the oldest unambiguous fossils—primitive microbes and dome-like structures called stromatolites, which formed their rocky homes (Fig. 11). Distinctive uranium-rich sedimentary formations and layered deposits of iron oxides from about 2.5 to 2.0 billion years document the gradual rise of atmospheric oxygen through photosynthesis (Hazen et al. 2008, 2009). Indeed, every stage of Earth history has been dated with exquisite accuracy and precision thanks to radiometric techniques.

Stromatolites, such as this 2.45-billion-year-old example from the Tervola Region of northwest Finland, form by microbial action. Radiometric methods provide an accurate approach to dating such ancient sediments. Photo courtesy of Dominic Papineau

Page 2

Dating by tree rings, or dendrochronology, provides a continuous record back more than 20,000 years. Tree rings also preserve data on forest fires, climate change, and other environmental conditions. Source: Wikicommons; photograph by Mark A. Wilson