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August 2, 2019

One of the distinctive characteristics of geological thinking is the concept of deep time. Earth is 4.54 billion years old, a span of time that is hard for many people to grasp. What did John Wesley Powell know about geologic time, and what have we learned since?

In the 19th Century

illustration of James Hutton standing by rock formation
Portrait of James Hutton, by artist John Kay (Credit: Library of Congress)

Powell was a self-taught geologist, but he was well read and certainly knew the geologic thinking of the times. He would have been aware of the work of William Thompson, more commonly remembered as Lord Kelvin, who, in 1862, had calculated an age of the Earth based on thermodynamic modeling, with the goal of identifying how long it would take to cool a molten Earth to current temperatures. Thermodynamics was well understood, and Lord Kelvin’s calculations were an early attempt to take a quantitative approach to the question “how old is the Earth?” Lord Kelvin’s cooling-magma calculations yielded an age of 20–400 million years. Geologists thought that estimate was too small, based on geologic principles, and biologists were skeptical that evolution could happen on that time scale (Darwin’s On the Origin of Species had been published in 1859), but Lord Kelvin’s calculations were the most rigorous attempted at that time. Let’s take a look at those ideas before returning to the late 19th century.

People have been making careful observations of rocks and fossils for centuries. Way back in the late 1400’s, Leonardo di Vinci noticed shell fossils in the Alps and concluded that this land must once have been under the ocean. He was hardly the only early thinker to contemplate geologic change — Aristotle had formed similar conclusions centuries earlier, as had early Chinese and Persian scholars. Nicholas Steno, 1669, gets credit for first writing down basic geological principles as laws. The law of original horizontality says that sedimentary layers are deposited in horizontal layers. The law of superposition says that older rocks are on the bottom and younger rocks are on the top. The law of cross-cutting relationships says that something that cuts across a boundary must be younger than that boundary. These basic principles of stratigraphy  — or the branch of geology concerned with the study of rock layers — allow observers to put geologic events in order to tell the story of relative time, but they don’t provide an absolute age.

In 1788, James Hutton proposed what would become the foundational principle of geology — uniformitarianism. This relatively simple but powerful principle states that the processes that we see happening on Earth today are the same processes that shaped Earth in the past, and that they will continue into the future. This idea has profound implications for geologic time. As Hutton saw it, if sand is deposited on a river point bar at a rate of about 2–5 cm per year, then vast sandstones must also have been deposited at that very slow rate. At the iconic unconformity at Siccar Point in Scotland, Hutton said of geologic time that it had “no vestige of a beginning. No prospect of an end.”

The next significant leap in our understanding of geologic time came from William Smith, a road surveyor in England and an amateur paleontologist. He noticed that fossils occurred in distinctive assemblages, or groups, and furthermore that these fossil assemblages always appeared in the same order everywhere they were seen. In 1815, Smith developed the law of faunal succession, which states that species become extinct and are replaced by new species, and that characteristic fossils of a certain age are that age everywhere. 

The 1820s-1850s was a remarkably fertile period for paleontology, and the work of scientists around the world developed a geologic time scale based on characteristic fossil assemblages. As much of this work was done in England, many of the names of geologic periods derive from locations in England — e.g. Devonian for Devon, Cambrian for Cambria, the Latin name for Wales. The first geologic time scale was published in 1841 — Powell certainly would have known of it and used it in his geologic trips to the West. He also understood the concept of unconformities (gaps in the geologic record that represent missing time); he was the first geologist to sketch the Great Unconformity in the Grand Canyon.

In 1869, the same year Powell launched his expedition, English scientist Thomas Huxley challenged Lord Kelvin’s calculation, asserting that his base assumptions were incorrect. It turned out that he was right about that, but we wouldn’t have the evidence for another 30 years. It was the discovery of radioactivity in 1896 that changed the heat-flow picture dramatically. French physicist Henri Becquerel made the discovery while testing whether uranium absorbed, then emitted, sunlight when the effect was not diminished on a cloudy day. In the next few decades major advances were made in geochronology. Radioactive decay solved the heat-flow question — with a source of heat in the cooling Earth, geologic time could be much longer — and gave us a tool for measuring time.

click on image for full description
The image on the right is a stratigraphic section of Grand Canyon by John Wesley Powell (1875). “A” is the metamorphic basement complex (Early Proterozoic Vishnu Group), with igneous intrusives labeled “a”; “B” is the Grand Canyon Supergroup (Middle and Late Proterozoic); “C” indicates the Paleozoic strata; “x” and “y” delineate the major unconformable contacts. The image on the left is a recent photograph of Grand Canyon from Walhalla Plateau, with the red line showing the Great Unconformity that was first noted by Powell (Credit: Ann Scott, USGS. Public domain.)


In the 21st Century

The current chart from Divisions of Geologic Time, 2018
The current chart from Divisions of Geologic Time, 2018.​​​​​​​ (Public domain.)

Our current understanding of geologic time is based on continued radiometric dating work throughout the 20th century, as well as data from new technologies. Based on radiometric dating of the oldest rock he could find, English geologist Arthur Holmes in 1913 calculated an age of 1.6 billion years, the first such estimate based on radiometric dating. The next forty years would be pivotal. Chemists and physicists discovered hundreds of radioactive isotopes and their half-lives*. Technology advanced to make measurement more precise at smaller volumes. Geologists scoured the Earth for rocks to date, looking at the boundaries of the already defined geologic time scale, and looking for the oldest rocks they could find. It was 1956 when Clair Cameron Patterson, an American geochemist, used lead-lead dating to determine that the Canyon Diablo meteorite was 4.55 billion years old. Since meteorites were formed at the same time as Earth, but have not been constantly recycled since, this date was accepted as the age of Earth, and is very close to accepted age today.

As with any scientific endeavor, new technology allows us to better refine what we know about the way the world works. The late 20th century brought technological advances in the usability and availability of highly sensitive instruments such as mass spectrometers and the Sensitive High Resolution Ion Microprobe-Reverse Geometry, or SHRIMP-RG, which analyzes zircon and other elements to provide numerical values (in years) to the age of rocks. The more we learn, the more refined the absolute dates of the geologic time scale become. Every few years, the Divisions of Geologic Time, a chart showing major geological units and their estimated ages, is updated by the International Commission on Stratigraphy, which includes input from USGS geologists. Perhaps in the next century, we will be able to pinpoint the exact month or day of geologic events!


*A little science lesson on radiometric dating: A half-life is the length of time it takes for half of the number of original, or parent, isotopes to decay to a stable daughter isotope. It is a fundamental characteristic of a particular isotope. For example, uranium-235 is an unstable parent isotope that decays to lead-207, with a half-life of 710 million years. 

Let’s consider a shorter time frame — say you had a delicious cake and you wanted to stretch out eating it over as many days as possible, so you vow to only eat half of the cake each day. On day one, you would eat half the cake and have half left (and probably go to bed with a bellyache). On day two, you would eat half of the remainder, one-quarter, and have one-quarter left. Day three, one-eighth. Day four, one-sixteenth. And so on. In this example, the half-life is one day. You can tell how old the cake is by measuring how much you have left; if you have one-eighth left, it is three half-lives old, or three days. 

Now back to rocks: you might ask, “But how do you know what fraction you have left if you don’t know how much you started with?” Fortunately, you are not eating the lead-207! It is still in the rock, so you can measure how much of each isotope you have. If seven-eighths of the total is Lead-207 and one-eighth is uranium-235, how old is the rock? Answer: three half-lives old, or 2.13 billion years old.

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