Geological Timescale

Geological Timescale, a table showing the divisions of Earth history from its origins to the present. The history of the Earth is recorded in its rocks and these provide the means of identifying the multitude of events that have occurred during its lifetime of more than four billion years.

The first steps in the scientific determination of the age of rocks were concerned with relative rather than absolute ages. As early as 1669, Nicholas Steno, observing that sedimentary rocks tended to be laid down horizontally, or nearly horizontally, proposed the Principle of Superposition—which stated simply that in a succession of strata the upper layers are younger than those lying below them. This has formed the basis of the branch of geology known as stratigraphy—the study of rocks as strata and their chronological connections. In addition to superposition, stratigraphical relations have been determined by the use of other simple principles. In sedimentary rocks, for example, a bed containing pebbles produced by erosion from a neighbouring rock is the younger layer; however, among igneous rocks the younger intrusion cuts across the older intrusion and has a chilled (fine-grained) margin against the older. During the 19th century these stratigraphical studies were enhanced by the recognition that sedimentary rocks tended to contain distinctive collections of fossils, and that these fossils had evolved over vast stretches of time.

At any one place, the rocks exposed and available for study represent a small fraction of Earth's history. The Grand Canyon of the Colorado River is one of the most spectacular examples of visible rock layering on Earth—yet the record even here is incomplete. In order to build up a complete picture, it is necessary to take partial successions from many regions and determine how these pieces fit together (see Geology: Stratigraphy).

In the process of correlation from one area to another, fossils have provided an invaluable tool. Since flora and fauna tend to be widespread, it has been possible to correlate rocks of similar ages over the entire world and build up a comprehensive table of worldwide events from the very old to the very recent.

Although no dates could be assigned with any certainty to the timescale, one of the great achievements of the 19th century was, nevertheless, the creation of an elaborate table of the Earth's history. The major divisions of time were all based on fossil content; therefore the prime divisions, Precambrian (or Cryptozoic, meaning “life hidden”) and Phanerozoic (meaning “life appearing”), separate out periods when fossils are rare or absent from those periods with abundant organic remains. Within the Phanerozoic eon, ancient life-forms appear in the Palaeozoic era, and successively younger forms characterize the Mesozoic (middle life) era and the Cenozoic (recent life) era. These very broad divisions, often comprising sediments many kilometres in thickness, were further divided into Systems (referring to the rocks) and Periods (referring to the time during which the rocks of a System were deposited).

The naming of the Systems was somewhat haphazard. Many were based on location: for example, the Cambrian System was named after rocks found in Wales and the Jurassic System, from the Jura Mountains. Others were named after distinctive rock types of the System, such as Cretaceous, from the Latin, creta, meaning “chalk”, and Carboniferous, from the abundant coals of that System. Within the Systems, finer divisions have been created on the basis of rock types and their fossils.

Along with increasing refinement of relative ages, a variety of unconvincing attempts were made to estimate absolute ages in years. By and large, geologists and biologists, impressed with the slowness of geological and biological change, thought in terms of almost limitless time. Some therefore took issue with the eminent physicist, Lord Kelvin, who calculated in the late 1890's (on the basis of likely cooling rates of the Earth) that the Earth was between 20 and 400 million years old. A resolution to this disagreement was forthcoming with the discovery of a source of heat which Kelvin had not envisaged: radioactivity. This nullified his calculations, as did the further discovery that naturally occurring minerals contained elements which decayed spontaneously at constant rates, providing a means of determining the actual ages of rocks (see Dating Methods).

The first determinations at the beginning of this century were restricted to rocks containing uranium minerals, but the method has developed over the years to include many isotopes of other elements. It seems that the longer-lived elements, such as uranium and potassium, with decay rates of some 4 billion years or more (given in terms of the time taken for half of the isotope to decay), are appropriate for the measurement of geological materials. However, the isotope of carbon (carbon-14), which has a half-life of 5,715 years, is more appropriate for dating archaeological materials a few thousand years old.

Radiometric dating emphasizes the enormous length of time involved in the Precambrian (more than 4,000 million years), as compared to the Phanerozoic (570 million years). The divisions within the fossiliferous rocks show Systems spanning Periods of the order of around 50 million years each and the finer divisions—the epochs—each a few million years. With an increasing number of dates being fixed from the Precambrian as a result of geological research, it has been possible to add some divisions—but these are still very much broader than those of the younger rocks.

A new stratigraphy has developed since the 1960s, when it was discovered that the direction of the Earth's magnetic field has reversed at different times and these alternations have been correlated with the radiometric dating of rocks. Magnetostratigraphy allows the age of a rock containing magnetic particles to be determined from the nature of its magnetism.

See also Geophysics.