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Geologic Time

When one mentions "geologic time" to a geoscientist, that person immediately starts to think in terms of millions to billions of years. The Earth organized as a large accreted body between 4.5 and 4.6 billion years ago. The unit of time measurement applied to geologic history remains the year, which probably has had only limited lengthening (from slowdown in Earth rotation) since the beginning. For practical purposes, we continue to work with the standard year of 365.25 days (although the days were once shorter as the Earth rotated faster in the early days of Earth history).

The rocks at the Earth's surface or accessible (e.g., by drilling) in its shallow interior have a variety of ages as one moves from place to place. The question becomes: How does one know at least the approximate age, i.e., how much earlier in the past did this rock or rock unit (e.g., a layer) become initially emplaced (it may be moved by erosion or crustal deformation, etc.)? Two solutions to the question are: 1) determine its relative age and estimate its time of formation by approximation techniques; and 2) use some sort of absolute time dating method.

Relative age determination involves some important rules: 1) the Law of Uniformity (James Hutton):The Present is the Key to the Past; this means that processes we observe in today's world likely operated throughout the past, back even billions of years ago; 2) the Law of Superposition: Under most circumstances when sedimentary layers are deposited in a sequence, the lowest in that sequence was deposited first and is the oldest whereas the highest was last deposited and is the youngest (this may become spatially "untrue" when rocks are displayed by faulting or severe overfolding); 3) the Law of Cross-cutting: any geologic feature (igneous intrusion; fault, etc) that cuts into/across rock units must be younger than these units; 4) the Law of Faunal Succession: in a sequence of rocks of significant thicknesses, there may likely be remains of animals/plants (fossils) that show some systematic evolutionary progression; most commonly the change is towards complexity and diversity and follows morphological and taxonomic modifications.

Until the 18th and 19th Centuries, the age of the Earth was estimated more from biblical interpretations than from scientific determinations. Then, following Hutton, Lyell, Lamarck, Darwin and others two things were realized: 1) it takes thousands to millions of years to deposit layered sedimentary rock units of thicknesses in the 10s to 100s of meters realm; and 2) progressive evolutionary changes also take millions of years for significant advances in the development of genera and species to become recognizable. By the end of the 19th Century, Earth had "aged" from the 4004 BC estimate by Bishop Usher to millions of years. In the 20th Century the age passed two billion years and with more precise methods (radiometric; see below) zeroed in on the present value near 4.6 billion years.

The most indicative method for reaching the billion year age category was simply to measure the thicknesses of units in one place and estimate the time to deposit them (millions of years), then go elsewhere and find at least some of the same units that had different units below or above them, which had to be older or younger. This involves correlation of units (see second Grand Canyon diagram below) based on specifying in the sequence something in common from place to place, as for example certain animal/plant fossils that are the same or similar, or distinctive rock types in the same sequences. By roaming over a continent and between continents, sequences were recognized, described, and matched with sequences elsewhere that had new member units, within, up, or below. This leads to a general "worldwide" Geologic Column that tries to account for all the deposits laid down during given time intervals (spans) in various locations that can be matched and then expanded by overlapping correlation (see below).

Around the turn of the 18th Century into the 19th, various geologists working in different regions who were adept at stratigraphic analysis (recognition of layers that had characteristic time markers such as fossils) began to publish their descriptions of the sequences they studied. Others found different sequences and/or some of the same sequences in the descriptions. Combining the geographically separated observations, using correlation to tie units of the same age to the sequences, a composite sequence (which included filling gaps where rocks of an age in one region were missing) known as the Geologic Column was built up. As the Geologic Column grew, estimates of their ages were made using mainly deposition rates. Individual sequences within the column were assigned times in the past which resulted from the column estimates.
It became conventional to give a name to a sequence that seemed to represent a long span of time but had certain diagnostic properties (e.g., a collection of life forms that, while evolving, possessed traceable similarities). These sequences became Periods in a temporal-stratigraphic nomenclature and all rocks contained within the sequence made up a System. Subdivision of Periods into smaller time spans yielded Epochs, with their Rocks being Series. Broader divisions of time made up of a some number of Periods (each younger one overlying an older Period - Law of Superposition) were called Eras. Names given to Periods either had some geographic significance (the Cambrian Period was first described and measured in Cambria within the British Isles) or at the Era Level were defined by an appraisal of the dominant life forms and their stages of evolution (the Mesozoic means "Middle" "Life" [zo- is part of a Greek word pertaining to life).

From this approach, a system for expressing geologic time and naming subdivisions has emerged, as shown in this diagram: