Geologic Time

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There are several different ways that Earth scientists consider time. Geologic time is generally thought of as the period of time that begins with the initial formative processes of Earth and ends with the onset of recorded human history. The human era begins with the end of geologic time and continues today. Archeologic time, which begins with early hominid evolution and also continues through today, overlaps with geologic time. Encompassing all other measures of time is cosmologic time, which begins with the formation of the Universe and will continue until it ends.

In the early days of the Enlightenment era, geologic and cosmologic time were viewed as one, because there was no perceived difference between the age of the earth and the Universe as a whole. James Hutton (1726–1795), a founding father of geology during this era, stated his view of geologic/cosmologic time thusly: "We find no vestige of a beginning—no prospect of an end." One of Hutton's contemporaries, John Playfair (1748–1819), is noted for having said of the well-exposed stratigraphic relations at Siccar Point, Scotland: "The mind seemed to grow giddy by looking so far into the abyss of time." Then, as today, vast spans of pre-historic time seemed unimaginable. Now we know that there is a difference between geologic and cosmologic time, and appreciate that there was a beginning for Earth—and for the Universe—and that there is a theoretical end for both.

Notions of archaeologic, geologic, and cosmologic time have been rejected and in some instances suppressed by persons who possessed strong beliefs in the concept of Biblical time. Biblical time was the prevailing view of time espoused as a generally unquestioned belief by Christian people for nearly 2,000 years. In 1650–1654, Archbishop James Ussher (1581–1656) of England attempted to quantify Biblical time by studying Biblical genealogies. Based on his analysis, he reckoned that the Earth was formed on October 23, 4004 B.C. This date was printed after 1658 as a footnote in the English Bible, and was accepted as a part of scripture. Until the Enlightenment era (c. 1750–1850), any estimate of Earth antiquity that conflicted with Ussher's age was dismissed as inconsistent with scripture. During and after the Enlightenment (and in some quarters still today), resistance persisted to notions of ages that predate the approximate 6,000-year figure of Ussher.

Enlightenment era notions that geologic time was infinite, indefinite, or inconceivably long, would be replaced in time by finite estimates of ages of geological materials. This occurred not long after the nature of radioactive decay was understood and analytical equipment was developed to a level of precision and accuracy in order to measure minute amounts of radioactive isotopes. The advent of this analytic process lead to radiometric dating of minerals within rocks using isotopic ratios. For instance, the American chemist Bertram Boltwood (1870–1927) was obtaining rock ages, based on lead isotopes, of 410 to 2.2 billion years in the early years of the twentieth century. In 1931, a carefully prepared and highly reviewed report to the United States National Research Council established the validity of radiometric dating of rocks in a robust way that has not been debated since that time. Not long after radiometric dating of rocks began, the geological time scale, which had been only a relative time scale up to that time, became a geochronometric scale as well (i.e., ages of the geologic eras and periods, and eventually their finer subdivisions, were added and refined). For the first time, it was known that, for example, Cretaceous spanned 146.5 to 65 million years ago.

Geologic time for the geologist is the numerical aspect of change in the history of the earth (i.e., the rock record). This differs from other scientific views of time. For example, for the classical physicist, time is more the numerical aspect of motion, because motion is measured by time. Non-scientists normally do not think of time in either of these ways. To the average person, time is the numerical aspect that allows one to order events within a day, a year, or a lifetime or a way to know when to begin or end a task. Thus, the geologic concept of time is rather foreign to most people. Because of its vast dimensions, geologic (and cosmologic) time has the potential to broaden the perspective of anyone who is willing to consider its implications and compare it with human or historic time. Among the many implications of geologic time is the fact that an unlikely or very rare event will become common. Geologic time is vast enough to accommodate, for example, all stages in organic evolution, mass extinctions and biotic recoveries, crustal plate motions and attendant episodes of crustal deformation, and long-term climatic changes.

Chronostratigraphy; Fossil Record; Historical Geology; Radioactivity; Stratigraphy