Deep Time: _{Measuring the age of the earth}

Take a minute to think about what you were like a year ago. If someone saw a picture of you today and a picture of you a year ago, they would most likely be able to recognize you. The city and neighborhood you live in probably look pretty much the same. Although there would be small changes, both you and your surroundings would be easily recognizable.

Now, think back 20 years. Maybe you weren’t yet born or were a baby or small child. If someone saw a picture of you today and a picture of you from 20 years ago, they might have to work a little harder to see the resemblance. Then, they would find the same birthmark, recognize the same eye color, and start to see that the two photos are of the same person. Your neighborhood and city could look a bit different—new buildings, new streets, stoplights, more people—but still recognizable as the same place.

Now go back 100 years. You probably don’t exist yet, probably not your parents, either. Some relatives might look a little like you, but many things about them may be different: the clothes they wear, the things they do, the ways they get around. It’s harder to find the things that are the same as today. If your family has lived in the same place for 100 years, you might see familiar items in a 100-year-old photograph; if your family has moved around, it would be hard to find the threads.

One more leap: go back a thousand years. Can you trace your family back that far? Who was living where you live now?

Now, rather than thinking about your own life, think about Earth. And rather than thinking about years, consider millions of years. A million or so years ago, Earth looked a lot like it does today. On North America, the ice sheets were bigger, but you would easily recognize the same continental shape, mountain ranges, and rivers we have today (Figure 1, left panel). Twenty million years ago, the continent would look a little different (Figure 1, middle panel). You would recognize it, but you would probably notice some changes: where the Hudson Bay is today, there would land, and Alaska would be connected to Asia.

Your view of Earth 100 million years ago would be even more confusing. Eventually, you might recognize North America and realize there was a sea dividing it in two (Figure 1, right panel). And one thousand million—one billion—years ago? You might wonder if it’s the same planet at all.

Comparing one year of human history to one million years of Earth history is one way to start thinking about “deep time,” the 4.54 billion years over which Earth has evolved. As we’ll explore, our understanding of the extent of deep time has also evolved as our cultures, ways of thinking, and the technologies available to scientists have changed over time.

The earliest recorded calculations people made to determine the age of our planet come from religious texts. Hindu scriptures known as the Vedas from approximately 100 BCE describe a cyclic nature of time, with repeated cycles of creation and destruction over billions of years. One day in the life of Brahma, a Hindu demi-god, is equivalent to 4.32 billion human years, and his night is an additional 4.32 billion human years. We are living in the first day of Brahma’s 51st year, which produces a total age of the universe of almost 156 trillion years. Though not widely known by non-Hindus, these scriptures and their calculations put a very long timescale on the age of Earth: billions to trillions of years.

Starting in the 1500s, many Christian scholars sought to determine the date of Creation in the Christian Bible by carefully reading the chronology of events and the lineages of families whose generations are described. These calculations led to a range of estimates for the time of Creation (what Christians considered the beginning of the Earth) from 5530 BCE to 3752 BCE. The differences between the age estimates were due to different assumptions about timespans like the length of an average king’s reign, and the time between generations. One scholar’s estimate received more attention than others: Archbishop James Ussher was a dedicated royalist, loyal to the English monarchy and well-connected to English royalty. In 1650, Ussher published a volume in Latin with a title that translates as “The Annals of the World” (Figure 2). In that book, Ussher claimed that Earth was created at nightfall on October 22, 4004 BCE. Because of his position, Ussher’s claim was incorporated into the King James Bible in 1701, the official translation of the Bible for the Church of England. A King James Bible was present in most Christian, English-speaking households at the time. Thus, the concept that Earth was less than 6000 years old became widely accepted at the time.

Ussher’s calculations may be the one that reached the widest audience at the time and throughout history. But all of the Christian scholars’ calculations were based on the same primary assumption: that Earth and humans had existed for the same amount of time. In other words, they were all using the cycles of human history, primarily birth, marriage, and death, to determine the age of Earth.

The mid- to late-1700s was a time of extensive exploration of the planet and investigation of the natural world by Europeans. Rather than focusing on human history, these investigators focused on observing the processes happening around them. One such person was James Hutton, born in Edinburgh, Scotland, in 1726. Hutton wandered a bit as a student, starting university studies in Edinburgh, then continuing in Paris, and finishing in the Netherlands with a degree in medicine in 1749. Despite his degree, he never practiced as a doctor. Instead, he continued to wander. Hutton had inherited a farm South of Edinburgh from his father (Figure 3). Shortly after graduating, he visited it and decided to devote himself to farming.

Hutton spent two years learning modern farming techniques from a farmer in Norfolk, England. In 1754, he returned to his farm and introduced several new practices to improve yields and reduce labor. Over the next ten years, Hutton conducted experiments on soil and minerals and observed the process of soil erosion in his farmlands. His experiments and observations made it clear that fertile soil was a product of the breaking down of rocks, yet this soil was constantly being removed from the farm by wind and water and taken to the sea by rivers. If the process of erosion continued, he realized, eventually all of the soil would be removed and the farm would be underwater. In order to generate new soil, he thought, there had to be an internal force that lifted mountains that could be eroded.

Hutton developed the theory that these processes happened in cycles—erosion and breakdown of mountains, then the formation of soil on land and carrying of sediments to the sea where they were deposited, then the deep burial of these sediments causing a build-up of internal heat that caused uplift of new mountains, and then a new cycle of erosion and deposition would begin again (Figure 4). He concluded that the time needed to go through each cycle was immense, far beyond human timescales. In the 1780s, he wrote down his ideas with some observational evidence to support them. On two Fridays in March and April 1785, he presented his “Theory of the Earth” to the Royal Society of Edinburgh (Hutton, 1785).

Throughout his presentation, he referred to the need for long periods of time. He repeatedly described the slow processes of sediment deposition, and how the deposition of all the sedimentary layers currently seen on Earth would take very long amounts of time. He noted the evidence for animals that existed long before humans (fossils in rocks) and that these provided “a measure for the computation of a period of time extremely remote.” Hutton also said that sediments had to be “cemented by the heat of fusion” and required extreme heat and amazing force to achieve “every degree of departure from a horizontal towards a vertical position.” He did not think it was possible to calculate the amount of time over which these cycles had been occurring. Hutton ended his presentation with a statement that has become famous: “The result, therefore, of our present enquiry is, that we find no vestige of a beginning—no prospect of an end.”

Many of those present for Hutton’s reading (and those who read about it later) were skeptical of his conclusions. What Hutton presented directly contradicted the prominent viewpoints at the time. It was a new way of thinking and reasoning about Earth’s age that sought information from Earth itself instead of human records. Over the next three years, Hutton collected observations, primarily from around Scotland, to further support his theory of the Earth, the cyclical nature of the processes of erosion, deposition, and uplift, and Earth’s immense age. In 1788, along with two Royal Society members who had been convinced by his arguments, Hutton traveled by boat along the Scottish coast east of Edinburgh, looking for an outcrop that would help bolster his theory. He found it at Siccar Point, a rocky stretch of shoreline where vertically-oriented gray rock layers were overlain by tilted red sandstone layers (Figure 5).

Hutton and his companions felt the exposure at Siccar Point was the strongest evidence yet for his proposed cycles of erosion, deposition, and uplift, which had to have happened at least twice to produce this particular landscape, as illustrated in Figure 6. It did not matter that he did not know how old each layer was: the relationship between them required much, much longer than any previous estimates for the age of Earth based on human timescales.

The critical reviews of Hutton’s theory continued, however, and a particularly extensive review from a founder of the Royal Irish Academy prompted Hutton to write an expanded version of “Theory of the Earth” that included his observations from Siccar Point and other locations. Two volumes were published in 1795, and Hutton had more volumes planned. Unfortunately, Hutton died in 1797 before publishing the additional volumes and before his ideas gained widespread acceptance.

One of Hutton’s companions from the trip to Siccar Point, John Playfair, sought to give Hutton the recognition he deserved for his work. In 1802, Playfair published Illustrations of the Huttonian Theory of the Earth. In it, he restated many of Hutton’s findings in more straightforward language and added his own observations from around the world (Playfair, 1802). Playfair clarified Hutton’s famous statement: "To assert that… we see no mark, either of a beginning or an end, is very different from affirming, that the world had no beginning, and will have no end.” Instead, Playfair argued, we simply lack the data to determine Earth’s absolute age, though we can be certain of the antiquity and relative ages of layers of the Earth.

Over the next few decades, geologists across Europe made observations, conducted experiments, and argued about Earth’s age. They continued trying to reconcile the evidence from Earth processes with the words of Christian scripture and their capacity for understanding deep time.

Neither Hutton nor Playfair put a definitive number on the age of the Earth, but both insisted that it was immense—beyond human knowing. Others were not convinced that the age could not be known and sought evidence from other types of investigations. In the mid-1800s, William Thomson (later Lord Kelvin), a physicist at the University of Glasgow who had shown an early aptitude for mathematics and physics, took a physics-based approach to calculating the age of the Earth. In 1862, he wrote, “For eighteen years it has pressed on my mind, that essential principles of Thermo-dynamics have been overlooked” by geologists in considering the age of the Earth (Thomson, 1862). In mentioning these principles of thermodynamics, Thomson was referring in part to the work of Joseph Fourier, a mathematician. In 1822, Fourier published an equation that described the rate of heat loss from a solid body. Fourier’s equation related the change in temperature with depth to time and included a thermal diffusivity constant (a number that represents how fast a substance cools) that varies by the composition of the solid. Experimental results showed that the equation accurately predicted heat loss in a homogenous solid if there were no additional heat sources.

Thomson considered Earth a homogeneous solid body and assumed it had cooled from an initially molten state to its present temperature through the process of conduction. By that logic, he could use measurements of the change in temperature with depth into the Earth (known as the geothermal gradient) with an assumed starting temperature to calculate the age of the Earth.

By the early 1860s, the geothermal gradient had been measured at several locations, with measurements ranging from ~12°C/km to ~75°C/km. He assumed an initial temperature of 3900° C and incorporated the thermal diffusivity constant for common rock types. Using these data, Thomson calculated a range of ages for the Earth from 24 million to 400 million years. To further constrain this range, he brought in his earlier calculations that the sun could not radiate at its current rate for more than 100 million years. Based on this reasoning, Thomson reduced his claim for the age of the Earth to the low end of his calculated range, promoting 24–100 million years as the maximum age.

Like Archbishop Ussher 200 years earlier, William Thomson was extremely well-connected and influential despite his young age (he was only 38 in 1862). As a result, his maximum age of 100 million years became widely accepted. His stature and influence only grew when he was elevated to the House of Lords and took the name Kelvin in 1892. Around that time, Kelvin revisited his calculations with a much lower estimate for the initial temperature derived from rock melting experiments and more accurate thermal properties of rocks, revising his age estimate down further. He wrote, “…we know have good reason for judging that it was more than 20 and less than 40 million years ago, and probably much nearer 20 than 40,” (Kelvin, 1897). This much younger estimate was met with considerable skepticism on the part of the geologists who were investigating Earth processes.

In 1895, one of Thomson’s former assistants at the University of Glasgow, John Perry, published a set of letters in Nature in which he corresponded with colleagues about Kelvin’s calculations (Perry, 1895). Perry took great pains in these letters to emphasize that Kelvin was impeccable in his calculations and made no mistakes in arriving at the answers. Instead, Perry addressed Kelvin’s primary assumption: that the Earth was a homogenous solid. He performed a series of calculations based on two alternative assumptions, illustrated in Figure 7:

1. That the Earth has a crust of some thickness that is of different composition and less conductive than the interior, and
2. That the interior of the Earth is not completely solid but a “honeycomb mass of great rigidity, partly solid and partly fluid”.

In assuming the presence of a crust (middle panel of Figure 6), Perry calculated that the age of the Earth would be two to six times Kelvin’s youngest age or 40 to 120 million years. In assuming the presence of a partly fluid interior and using the same rock properties that Kelvin used, Perry concluded that the Earth would be 56 times Kelvin’s age or just over one billion years old. Using recent measurements of the conductivity of different rock types and assuming that their conductivity increased with increasing temperature, Perry suggested that an age of the Earth anywhere between 2 and 3 billion years would be consistent with the data (right panel in Figure 6). The fluid component allowed for the process of convection to occur, which slowed the loss of heat through conduction.

Perry’s arguments were compelling (and ultimately correct) but were met with disdain for his claim that the interior of the Earth could contain a fluid component. Perry greatly admired Lord Kelvin and did not push the argument further. Regardless, his combination of thermodynamics with different theories about Earth’s interior helped reconcile the laws of physics with Earth processes to conceptualize the age of the Earth.

Both Kelvin and Perry had made another assumption in their calculations: that no additional heat source was present in Earth's interior. This assumption also turned out to be incorrect. Ultimately, the heat produced from radioactivity was too small to make much of a difference in the calculations, but radioactive decay had another, more important impact on measuring the age of the Earth.

The year after Perry’s recalculations were shared with the world, the French physicist Henri Becquerel discovered that a compound containing uranium produced energy that exposed a photographic plate while it was in complete darkness. Becquerel conducted additional experiments using a variety of uranium-bearing substances and found that what he called “uranic rays” were emitted by all of these substances. The rays could penetrate paper, cardboard, and metal sheets, and they did not diminish over time. These findings piqued the curiosity of a young scientist, Marie Curie, who sought to quantify uranic rays for her dissertation work (Figure 8). In 1897, she designed a procedure that allowed her to measure the energy of the activity produced by the uranic rays (like clicks on a Geiger counter) and compare it to the weight of the sample. Through a series of experiments, Curie found that the activity was directly proportional to the amount of uranium in the sample. In addition, the activity was not affected by temperature, combining with other elements, or exposure to light. Curie termed the elements that produced these rays “radio-active” in a paper she co-authored with her husband, Pierre, in 1898. A key component of their findings was that radioactivity was an inherent property of the element uranium. Uranium was always radioactive regardless of the compounds it was in.

Marie Curie also identified other radioactive elements in her experiments and worked to isolate them to understand their properties better. She processed tons of waste from a uranium mine to isolate thorium, polonium, and radium, producing pure samples to determine each element's atomic mass and radioactivity. In 1903, Pierre Curie and his student, Albert Laborde, found that a vial containing a sample of the element radium was warm. Although the sample was small, there was enough for Pierre Curie and Laborde to measure the heat produced. Based on their measurements, they calculated that a given quantity of radium could raise the temperature of the same quantity of water from its freezing point to its boiling point in an hour. Radium (and other radioactive elements) were present in minerals throughout Earth. In making this discovery, therefore, Pierre Curie and Laborde had shown that there is an additional heat source in the Earth—the radioactive decay of elements.

Despite the importance of this finding, the newly identified heat source did not substantially impact Kelvin’s calculations. There simply is not enough radioactive material in Earth’s interior to produce enough heat to extend the age of the Earth. However, the discovery of the radioactive decay of elements had an even more profound implication for the scientists interested in determining Earth's age.

While the Curies worked in Paris, two physicists at McGill University in Montreal, Canada, ran experiments on radioactive materials and how they changed over time. Ernest Rutherford and Frederick Soddy made several important observations when they isolated the radioactive gas emitted by thorium, which they called “emanation.” First, they observed that the radioactivity of the gas decreased exponentially, reducing by half every 54.5 seconds (Figure 9). When they measured the activity of other short-lived elements, they found the same exponential pattern but at different time intervals. These intervals eventually became known as the “half-life” of an element. The half-life was just under a minute for the gas they were measuring, later identified as an isotope of radon.

Rutherford and Soddy ran another series of experiments in which they removed solid decay products from a thorium sample. They observed that, as the activity of one of these products (later determined to be an isotope of radium) decreased exponentially, the activity of its decay product (later determined to be an isotope of thorium) increased (Figure 10). They interpreted this finding to mean that the decay of one isotope was producing the other. Today, we call these “parent-daughter pairs.” This decay process from radium to thorium also gave off a nonradioactive, stable gas product. Rutherford and Soddy hypothesized that it was helium, which was confirmed a few years later.

Another scientist, Bertram Boltwood, working out of his private chemistry lab in Connecticut, observed that minerals containing uranium also invariably contained the element lead. He obtained samples from a range of rocks whose relative ages were known and demonstrated that the amounts of lead and helium in the minerals was greater in the rocks thought to be older. Lead is not radioactive, so unlike uranium, thorium, and radium, it does not decay to other elements. Because of this proportional relationship, Boltwood proposed that lead was an end-product of the radioactive decay of uranium.

In 1905, Rutherford proposed a new method for determining the age of a mineral, based on knowledge of the end products of radioactive decay and the rates at which they were produced. He suggested that any helium present in a mineral was the product of radioactive decay, and the rate at which it was produced could be determined. Therefore, the amount of helium trapped within a mineral’s structure could be measured and used to calculate when the mineral formed. Rutherford acknowledged that some helium would likely escape, so any calculated age should be considered a minimum. Because of this potential helium loss, Rutherford also proposed that comparing the amounts of uranium and lead within a mineral would likely be a more accurate method for determining its age, as “the lead formed in a compact mineral has no possibility of escape” (Rutherford, 1906).

Rutherford’s proposal provided the basis for the field of geochronology (the study of dating Earth materials) and led to the development of several techniques to date rocks , minerals, geologic events, and Earth itself. Scientists immediately began putting his proposed idea to the test, using the uranium-helium and the uranium-lead methods to calculate ages. By 1911, several rocks from locations around the world had been dated to be well over one billion years old.

Other scientists took Rutherford’s idea in a different direction. They used estimates of the abundances of uranium and lead in Earth’s crust to calculate the age of the Earth. One of those scientists was Arthur Holmes, an early advocate for using radioactive decay methods and author of a book titled The Age of the Earth. In the second edition of his book, published in 1927, he estimated that one million grams of average crustal rock contains 7.5 grams of lead, 6 grams of uranium, and 15 grams of thorium. He assumed that all of the lead in the crust is produced through radioactive decay of uranium and thorium. Based on these assumptions, he calculated the age of the Earth to be just over 3 billion years (Holmes, 1927).

Holmes and others soon recognized that his simple estimates and assumptions were not accurate. Importantly, scientists demonstrated the existence of so-called “primordial” lead that had not been produced through the radioactive decay of uranium, in contrast to one of Holmes’ primary assumptions. In addition, they were able to identify different “reservoirs” of primordial lead defined by their proportions of three lead isotopes, meaning that the concept “average crustal rock” wasn’t a useful estimate. Finally, simply measuring the amounts of lead isotopes accurately proved challenging, as lead was a widespread contaminant in the mid-1900s when cars ran on leaded gasoline and most paints contained lead.

Clair Patterson, a geochemist, sought to address all of these issues when he arrived at Caltech in 1953. He built a “clean lab,” which would prevent any outside air from entering the room. And rather than focusing his efforts on Earth materials, he measured the amount of different isotopes of lead in meteorites, assumed to have formed at the same time as Earth. In 1956, he published his results that the age of several meteorites (and thus the Earth) was 4.55 ± 0.07 billion years (Patterson, 1956). He also defined what was he meant by “the age of the Earth”—the time since Earth had achieved its present mass. Patterson’s methodology and age have withstood the test of time, and the modern accepted age of the Earth is 4.54 billion years.

As the field of geochronology has evolved, new techniques have emerged, allowing geologists to look back in time more precisely. Scientists have gained more insight into the early period of Earth history and the timing of events over its 4.54 billion years. For example, some are working to better understand conditions on the early Earth and exactly when the cycles Hutton emphasized began. The cyclic nature of Earth’s processes and the very long time over which those cycles act are the foundations of Earth science.

Recognizing Earth’s very old age requires grappling with the concept of deep time. Instead of thinking in days or weeks, we must think in millions of years. The time scale is so far beyond the human experience, it is difficult to grasp. James Hutton was reluctant to describe an actual period of time, preferring instead to refer to “immense” amounts of time. He and other scientists struggled to reconcile mounting evidence for the old age of the Earth with the views of others around them and their own beliefs. Geologists today easily talk about 10,000 years ago like it was yesterday, and a million years ago like it was last month. A hundred million years ago might require some thought: dinosaurs were abundant, an inland sea split North America in two (Figure 1). A billion years ago, Earth would have looked much different, just as your neighborhood and family would look different if you went back a thousand years.

The concept of deep time influences how we think about our planet and ourselves. It allows us to envision how constant, slow processes, like the movement of tectonic plates a few millimeters per year, can produce the tallest mountains in the world from the sea floor over 70 million years. Deep time also helps us recognize the importance of rare events like meteorite impacts and massive volcanic eruptions that cause rapid changes in Earth’s climate and ecosystems. Developing your own sense of deep time takes, well, time. It does not happen overnight, even for people who end up becoming geologists. But cultivating a sense of deep time can help you put the processes that shape Earth and its life in context.