Geologic time scale

Geologic science has contributed to modern thought the realization of the immense time involved in the Earth's history. So vast is this time span that the term geologic time is used to distinguish it from other kinds of time.

Development of Historical Geology

The ancient civilizations were very indefinite regarding time periods. For Strato and Eratosthenes, invertebrate fossils were evidence only of ancient seas having existed. Herodotus associated vertebrate fossils with Greek mythology, concluding that large fossilized bones were remnants of battles between giants and their gods. During the mid-seventeenth century, the science of historical geology began to branch from the trunk of natural philosophy and develop its own identity. The first step toward that development was a new understanding of fossils. In the fall of 1666, fishermen fishing off the west coast of Italy caught a great white shark. Word of this unusual fish spread to the Medici court in Florence, where Grand DukeFerdinand II ordered the head cut off and brought for examination to Neils Stensen (known as Nicolaus Steno), a young Danish doctor serving the court. Steno recognized the strong resemblance of the shark's teeth to tongue stones. He made the intuitive leap that contrary to common belief, tongue stones did not grow in the ground but had their origin in the heads of sharks. The problem was to account for the transposition of the teeth from the shark's head into the solid rock that enclosed them. Through a series of critical observations and deductions, Steno arrived at three basic tenets of modern geology: Layered rocks result from sediments settling out of the water, the oldest strata are on the bottom, and the strata originally are deposited in an essentially horizontal position. Steno published these conclusions in Prodromus, his great work of 1669.

The English natural philosopher Robert Hooke gave Steno's ideas a wide hearing in lectures he presented before the Royal Society of London in 1667-1668. Hooke's main contribution to the budding science of historical geology was his support of the fossils’ organic origin, the extinction of species, the change within species over time, and the theory that subterranean forces have caused the continents to rise and fall with respect to the sea. English natural philosopher Thomas Burnet's very controversial Sacred Theory of the Earth (1681-1689) sparked further debate about the origin and changes in the Earth's surface features. Burnet called his theory sacred because it would justify by reason the biblical doctrines, specifically the Fall and the Universal Deluge. Burnet held that the Earth was around 4,000 years old and that the Universal Deluge occurred about 1,600 years later. Benoît de Maillet, a French diplomat, proposed in Telliamed (1748) a theory that put the age of the Earth at more than two billion years. Maillet based his theory on the sun's life expectancy and on observations of the fall of sea level. In this work, he supported two fundamental ideas: the belief that terrestrial life originated in the sea and Aristotle's “infinite age of the Earth,” which became Maillet's “vast amount of time” to build mountains layer by layer from strata once submerged.

Another description of the Earth's age appeared in the efforts of a second Frenchman, Georges-Louis Leclerc, the Comte de Buffon. Beginning in 1749, Buffon published a comprehensive multivolume work with the modest title Natural History, in which he divided the history of the Earth into seven epochs. Buffon's contribution to the question of age was an empirical study of the cooling rates of iron. In his own foundry, he heated to incandescence and then cooled iron balls of different diameters. He recorded the time for each ball to cool, extrapolating from his results the time it would take for a ball the size of the Earth to cool to the current level: 96,670 years and 132 days. Buffon generated a second timetable based on sedimentation rates observed in oceans. The variable deposition rates reported revealed a considerable range of time. The longest estimate placed the Earth's age at nearly three million years and had life appearing between 700,000 and one million years. These ideas circulated widely because of Buffon's strong influence on the intelligentsia of his time, which included correspondence with Benjamin Franklin and Thomas Jefferson. Buffon contributed two important ideas that helped to build a sense of time. First, he expanded the age of the Earth to millions of years. Second, he showed that scientists could understand past geologic events by observing the causes of change that are in operation today.

and Faunal Succession

In 1785, James Hutton published “Theory of the Earth” in the Transactions of the Royal Society of Edinburgh. The ideas expressed in this essay and his later elaborations of them are the beginning of modern geology. Hutton concluded that if humans could measure the rate at which erosion destroys lands, that rate could be used to calculate the time needed to form the strata observed in the field. The rates Hutton observed were so small, however, that he questioned the possibility of measuring them in a year or even a lifetime.

The vastness of the time required to describe the Earth's history came to Hutton as he observed what modern geologists call an unconformity (a gap of time in the geologic record). Hutton observed this sequence of strata at Siccar Point near St. Abbs, Scotland, where a sequence of horizontal strata rested on a sequence of vertical strata. He concluded that if all strata began as horizontal, a vast amount of time must have elapsed to produce the present configuration. According to Hutton, first, the sediments had to be deposited in a marine environment, where they solidified. Next, due to the Earth's internal heat, the horizontal sediments rose above sea level to a vertical position and eroded. Then, they were submerged in their vertical position, and a new deposition placed horizontal strata on top of the vertical strata. Finally, the whole structure again rose and eroded to expose the structure. Accounting for this history required an enormous time factor. In Hutton's closing essay, he suggested an infinite time frame: “The result, therefore, of this present inquiry is that we find no vestige of a beginning, no prospect of an end.”

In the late eighteenth and early nineteenth centuries, a new principle for determining the geologic ages of fossiliferous strata emerged from field studies in England and France. This was the principle of faunal succession. In a simplified form, it stated that within sequences of strata, different kinds of fossils succeed one another in a definite order. The Englishman William Smith most graphically demonstrated this principle. Smith was a self-taught surveyor and civil engineer working on constructing canals in England in 1794. During these excavations, he discovered that each formation revealed a distinctive fossil species. Extrapolating this knowledge to other regions allowed him to identify and predict the stratigraphic sequence. In 1814, Smith consolidated his findings in what geologists often consider the first geologic map. The next year, he published his major work, Delineation of the Strata of England and Wales, with Part of Scotland. Smith was recognized in his own time as the father of English geology. His contribution to dating was to utilize fossils to determine the relative ages of strata.

At the beginning of the nineteenth century, European geologists had begun a systematic classification of fossiliferous strata into coherent units or periods. The first of the geologic time periods appeared in 1799 with the work of Alexander von Humboldt, who applied the name Jurassic to a coherent sequence of fossiliferous limestone strata found in the Jura Mountains of Switzerland and France. Just to the west of these mountains, the limestones and their associated fossils dip under a dominantly chalky sequence studied by the Belgian geologist Omalius d’Halloy. He gave it the name Cretaceous in 1822. The pattern of naming rock units, based on the lithology and associated fossils, for the geographic region in which they were first described continued through the nineteenth century.

Definition of Period Boundaries

The original boundaries of these periods were neither distinct nor easily translated outside the holotype regions. Methods for defining the boundaries resulted from a dispute in England over a sequence of strata that Adam Sedgwick and Sir Roderick Impey Murchison described in 1835. Murchison based his chronological sequence on the fossil order he observed and named it the Silurian. Concurrently, Sedgwick relied on lithology to establish a sequence of strata that lay below the Silurian and was, therefore, older. He called it the Cambrian. Initially, the two periods seemed to create an order for these strata. However, under closer examination, the periods overlapped, and a dispute developed between Sedgwick and Murchison over the commonly held strata. The answer appeared in 1879, after their deaths when Charles Lapworth separated the systems based on fossils. Lapworth collected evidence that illustrated that the Cambrian-Silurian sequence contained three distinct fossil assemblages and resolved the dispute by removing the lower Silurian from its previous classification and renaming it the Ordovician. The discrimination of fossil assemblages provided the key to distinguishing other increments of geologic time, such as the Devonian and Permian periods.

By the middle of the nineteenth century, geologists recognized that the rock units they studied in one location were not universal geographically. Their systems were highly variable in lithology and thickness from region to region. The distinctiveness of the fossils within each group enabled them to translate from one geographic area to another. The power of this approach was that the order of the sequence was not random but predictable. For example, bryozoans and corals characterize the Ordovician period the world over. Each has its own distinctive suite of fossils. Recognition of the sequencing of life-forms through time had a profound effect on Charles Darwin. Indeed, Darwin's paleontological investigations led him to focus on the idea of a species changing through time. Later, in his On the Origin of Species by Means of Natural Selection (1859), Darwin pointed out that natural selection was a viable concept if and only if enough time had elapsed for its operation.

In 1862, Lord Kelvin made the first attempt to calibrate the geologic time scale. Working with the thermodynamic laws of heat production and radiation, he calculated the cooling rates of the Earth and sun. Kelvin's calculations set a “natural” upper limit to their ages and indicated that Earth had solidified from the original molten state between 20 million and 400 million years ago. This was much less than the time scale advocated by the uniformitarians. Geologists thus found themselves no longer in a position to assume unlimited time; their theories had to fit the time interval established by Kelvin's thermodynamic studies. The resolution of the time interval came in 1896 with the discovery of radioactivity by Antoine-Henri Becquerel, Pierre Curie, and Marie Curie. This was the beginning of a chain of events that revolutionized science and expanded the geologic time range into thousands of millions of years.

Radioactive Dating

In 1902, Pierre Curie announced that radioactive minerals constantly radiate heat. Two years later, Ernest Rutherford established that the amount of heat they radiate is proportional to the number of alpha particles they emit. John Joly provided additional support for Rutherford's discovery in his 1909 publication, Radioactivity and Geology. Joly demonstrated that the heat from radioactive decay within the Earth could alter the Earth's actual cooling rate to make it appear younger than it really was. Kelvin's calculations of 20 to 400 million years did not include the masking effect of internal radioactive heat, and thermodynamics alone, therefore, no longer established the boundaries. Geologists and biologists legitimately could claim a longer time interval for the evolutionary process. Continued investigations revealed that radioactive minerals might serve not only as sources of heat but also as clocks to date the rocks that contained them.

In 1905, Lord Rayleigh and Sir William Ramsay calculated an age of 2,000 million years for a specimen containing uranium. The American physical chemist Bertram Boltwood, noting that uranium ores always contain lead, speculated that lead might be the end product of a uranium decay series. By comparing the ratio of lead to uranium in forty-three minerals, he calculated their ages and obtained results ranging from 400 to 2,200 million years. Boltwood's results were the first quantitative proof of the Earth's age. Then, in 1913, Frederick Soddy and, independently, Kasimir Fajans demonstrated that radioactive elements can have the same chemical properties but slightly different atomic masses. Their discovery of isotopes was the next step in establishing intervals on the time scale. This development enabled researchers to measure the decay rate or half-life of a radioactive isotope.

By the late 1920s, Francis William Aston had begun to use mass spectrometry to make significant improvements in isotopic analysis. Alfred Otto Carl Nier continued to improve the mass spectrometric technique in the 1930s and 1940s, providing the most accurately determined values of uranium-lead ratios found in naturally occurring uranium minerals. Earlier, in 1913, the English geologist Arthur Holmes had made the first attempt at a quantified time scale using Boltwood's calculations of radioactive decay. In a 1948 paper, he developed an expanded time scale based on Nier's more recent values. By the 1960s, J. L. Kulp was continuing Holmes's investigations, introducing the rubidium-strontium decay series to date rock samples.

Mass spectrometric investigations continued throughout the twentieth century, and by the mid-1970s, the discovery and dating of Precambrian rocks in Greenland, South Africa, Australia, and Canada yielded ages of about 3.7 billion years. The Earth appears to be even older. Its age seems bracketed somewhere between these ancient terrestrial rocks and the lunar rocks, which date from 4.6 billion years. Isotope analysis of meteorites supports this same age range. Measurements of the oldest aged rocks include the 4.28 billion-year-old rocks from Hudson Bay, northeastern Canada, and remnants of the Murchison meteorite found in Australia, dated seven billion years ago.

In 2022, the date boundary of the Cambrian and Ediacaran periods was modified from 541 to 538.8. When discoveries lead to such changes, they almost always impact the numerical date but rarely result in major changes in period names or order.

Principal Terms

brachiopod: a bivalved filter feeder; clams and oysters are the modern equivalents

bryozoan: a colonial marine animal very much like modern sponges

faunal succession: the sequence of life-forms, as represented by the fossils within a stratigraphic sequence

geologic map: a map illustrating the age, structure, and distribution of rock units

holotype: the definitive example of a specimen with which all others are compared

lithology: the mineral composition and texture of a rock

paleontology: the science of ancient life-forms and their evolution as studied through the analysis of fossils

stratigraphic sequence: a set of rock units that reflect the geologic history of a region

tongue stones: an ancient colloquial term used to describe what are now recognized as fossil sharks’ teeth; if viewed from the convex side, a large shark's tooth might resemble a tongue turned to stone

trilobite: a many-legged arthropod named for its three symmetrical lobes; the principal index fossil for the Cambrian period (538.8-488 Ma)

unconformity: a surface that separates two strata; it represents a gap in time in which no geologic records remain

uniformitarianism: the general principle that the Earth's past history can be interpreted in terms of what is known about present natural laws, as these processes differ neither in degree nor in kind

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