Geologic Timescale

The geologic timescale divides the 4.5-billion-year history of Earth into unequal divisions of time. Originally, these divisions were based on sedimentary rocks and their corresponding fossils. Each time interval has its own unique fossil assemblage, and the boundaries between many intervals of geologic time correspond to mass extinction events. The most famous of these was at the boundary between the Mesozoic and the Cenozoic eras. At that time, land-dwelling dinosaurs became extinct, likely due in part to the impact of a large asteroid near the present-day Yucatan Peninsula.

The intervals of the geologic timescale are hierarchic—eons include eras, eras include periods,

Table 1 Geologic timescale

Source: National Park Service.

and then periods are divided into epochs. Epochs can be further subdivided into stages. In developing the geologic timescale, geologists frequently relied on the law of superposition, which states that younger sediments were deposited on top of older sediments. Thus, by applying this principle to a single exposure, one can determine the relative sequence of events visible in the geologic record. Unfortunately, only a small part of the entire expanse of geologic time is recorded in one place, either because the older parts of the record are still buried or because much of the record has been destroyed by erosion. Thus, the geologic timescale was created by combining records from many different places using the process of correlation, where the relative ages of rocks in different areas are determined, often by matching the fossil species they have in common. Most of the periods in the currently accepted geologic timescale were named by the 19th-century European geologists, and they were frequently named after the places where the rocks are exposed. For example, the Devonian Period was named by Adam Sedgwick and Sir Roderick Impey Murchison after Devonshire, England. In discussing geologic time, geologists distinguish between the time intervals themselves (geochronologic units) and the rocks deposited and formed during those times (chronostratigraphic units). So the Devonian Period, a geochronologic unit, is now represented on Earth by the rocks of that age, referred to as the Devonian System, a chronostratigraphic unit.

One unfortunate outcome of the patchy record of Earth's history is that geologists working in different areas have at times developed separate and incompatible versions of the geologic timescale. Such disputes are resolved by the International Commission of Stratigraphy (ICS). In one recent case, many Russian scientists used the term Vendian for the interval of time between 632 and 542 mya (million years ago), while many Australian scientists used the term Ediacaran instead. Arguments were made for both names, and in 2004, the ICS decided on the Ediacaran Period, which is now a part of the internationally recognized geologic timescale.

Although the law of superposition and the use of fossils in correlation allowed for a relative geologic timescale to be developed, throughout the 19th century, the durations of geologic time intervals, the age of their boundaries, and the age of the Earth were speculative. This changed shortly after the discovery of radioactivity by Henri Becquerel in 1906, and by 1913, Arthur Holmes published the first geologic timescale with absolute ages determined by isotopic abundances as a result of radioactive decay. In addition to the well-known method of carbon-14 dating applied to organic material, many rocks include radioactive isotopes whose rates of decay are exceedingly slow (e.g., 238U [uranium], 147Sm [samarium], 187Re [rhenium]). By using radioactive isotopes with low rates of decay, scientists have determined that some meteorites are 4.5 billion yrs. (years) old and record the composition of the early solar system.

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