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Evolution: Education and Outreach volume 3s — Cite this article. Metrics details. Earth scientists have devised many complementary and consistent techniques to estimate the ages of geologic events. Annually deposited layers of sediments or ice document hundreds of thousands of years of continuous Earth history.
Gradual rates of mountain building, erosion of mountains, and the motions of tectonic plates imply hundreds of millions of years of change. Radiometric dating, which relies on the predictable decay of radioactive isotopes of carbon, uranium, potassium, and other elements, provides accurate age estimates for events back to the formation of Earth more than 4. Historians love to quote the dates of famous events in human history. They recount days of national loss and tragedy like December 7, and September 11, And they remember birthdays: July 4, and, of course, February 12, the coincident birthdays of Charles Darwin and Abraham Lincoln.
We trust the validity of these historic moments because of the unbroken written and oral record that links us to the not-so-distant past. Geologists also love to quote historic age estimates: about 12, years ago, when the last great glaciation ended and humans began to settle North America; 65 million years ago, when the dinosaurs and many other creatures became extinct; the Cambrian boundary at million years ago, when diverse animals with hard shells suddenly appeared; 4.
But how can we be sure of those age estimates? Earth scientists have developed numerous independent yet consistent lines of evidence that point to an incredibly old Earth. But first, a warning: it is difficult for anyone to conceive of such an immense time span as 4. The oldest humans the current record according to Guinness is held by a French woman who lived to celebrate her nd birthday fall far short of living for 4. All of recorded human history is much less than 4.
Yet, geologists claim that Earth formed half-a-million times longer ago than that. The answer lies in the testimony of the rocks. Rocks reveal their ages of formation in several ways and provide Earth scientists with their most reliable clocks. The most straightforward geologic timekeepers are rock formations with annual layers. Annual tree rings provide a familiar analog Fig.
Dating by tree rings, or dendrochronology, provides a continuous record back more than 20, years. Tree rings also preserve data on forest fires, climate change, and other environmental conditions. Source: Wikicommons; photograph by Mark A. The most dramatic varve deposits, such as a meticulously documented 13,year sequence in glacial lakes in Sweden, occur as thin alternating light and dark layers, representing coarser-grained spring sediments and finer winter sediments, respectively.
Ancient varved deposits sometimes preserve much longer time spans: the finely laminated Green River shale in Wyoming features continuous vertical sections with more than a million such layers Fig. Annual rock layerings, or varves, occur as the rate of sedimentation changes from season to season. Source: Wikicommons; photograph by Benutzer Grabenstedt. The Green River Shale in Wyoming and Utah displays more than a million annual layers, or varves, in places.
Photo courtesy of David Haggard. The oldest annual layers are extracted from ice cores, whose laminae arise from seasonal variations in snowfall Fig. A 2,meter ice core from East Antarctica revealsannual layers of accumulation, year-by-year, snow layer by snow layer. And those annual layers rest atop another 2, meters of ice, which sit on vastly older rocks. The obvious conclusion is that at least a million years is needed to for many surficial deposits of sediment and ice. Earth must be much older than that, but how old?
Ice cores from Antarctica and Greenland reveal hundreds of thousands of years of snow accumulation. First, how old is the big island of Hawaii? The massive Hawaiian Islands rose from the Pacific as volcanoes periodically added layers of lava Fig. From modern-day eruptions, we know that active volcanoes grow by perhaps a meter every century. The highest point on the big island of Hawaii is Mauna Kea at 4, meters above sea level. However, the volcano rises approximately 10, meters above the ocean floor, so a rough calculation gives its age:. The Hawaiian Islands are a chain of volcanoes, each formed from layer after layer of lava.
A new island dubbed Loihi is now forming south of the big island. This is a rough estimate, to be sure, but it jives well with other methods that date the big island of Hawaii as about a million years old. The other islands that string out to the northwest, each with now-dormant volcanoes, are progressively older and a new island, dubbed Loihi, is already forming as volcanoes erupt on the ocean floor southeast of the big island. You can do a similar calculation to date the Atlantic Ocean, which is about 3, kilometers wide and grows wider every year. These continents were once ed into the supercontinent Pangaea; the Atlantic Ocean formed when Pangaea split down the middle and formed a divergent boundary, now marked by the Mid-Atlantic Ridge Fig.
New crust forms along the Ridge, as Europe and Africa move away from the Americas. Exacting satellite measurements over the past two decades reveal an average spreading rate of 2. The Atlantic Ocean formed when the supercontinent Pangaea began to split apart about million years ago. Source: USGS. This rough estimate of about million years is close to other measurements of the age of the Atlantic.
It is remarkable to imagine that a great ocean, a seemingly permanent feature of our home planet, is so transient in the context of Earth history. A third simple calculation reveals even longer time spans. The Appalachian Mountains are now gently rounded and relatively low—mostly below 3, meters high Fig. But geological evidence reveals that they once were the grandest mountain chain on Earth, rivaling the Himalayas in ruggedness and height with some peaks at more than 10, meters.
Ever so gradually, erosion has worn the Appalachians down to their present state, but how long might that process take? The gently rounded Appalachian Mountains a were once the tallest and most rugged mountain rage in the world, rivaling the modern Himalayas b. Hundreds of millions of years of erosion were required to achieve their present appearance. The volume of this impressive mountain is thus:. Now, imagine a stream that flows down the side of this mountain. Mountain streams carry silt and sand downwards—a key factor in erosion.
All of these sediments came from higher up the mountain, which is constantly being eroded away. To estimate how long a mountain might survive against erosion, consider a mountain with six principal streams.
A typical stream might carry an average of one-tenth of a cubic meter of rock and soil a few shovels full per day off the mountain, though the actual amount would vary considerably from day to day. Over a period of a year, the six streams might thus remove:. That means every year on the order of cubic meters of material, or about 20 dump trucks full of rock and soil, might be removed from a mountain by normal stream erosion.
If the mountain streams remove about cubic meters per year, then the lifetime of the mountain can be estimated as the total volume of the mountain divided by the volume lost each year:. This estimate is certainly rough and not directly applicable to any specific mountain. Nevertheless, a few hundred million years is but a small fraction of a few billion years. How can we say Earth is 4. The physical process of radioactive decay has provided Earth scientists, anthropologists, and evolutionary biologists with their most important method for determining the absolute age of rocks and other materials Dalrymple ; Dickin Trace amounts of isotopes of radioactive elements, including carbon, uranium, and dozens of others, are all around us—in rocks, in water, and in the air Table 1.
The rest of the uranium will have decayed toatoms of other elements, ultimately to stable i. Wait another 4. Radiometric dating relies on the clock-like characteristics of radioactive decay. In one half-life, approximately half of a collection of radioactive atoms will decay. Source: NCSE. The best-known radiometric dating method involves the isotope carbon, with a half life of 5, years.
Every living organism takes in carbon during its lifetime. At this moment, your body is taking the carbon in your food and converting it to tissue, and the same is true of all other animals. Plants are taking in carbon dioxide from the air and turning it into roots, stems, and leaves. But a certain small percentage of the carbon in your body and every other living thing—no more than one carbon atom in every trillion—is in the form of radioactive carbon As long as an organism is alive, the carbon in its tissues is constantly renewed in the same small, part-per-trillion proportion that is found in the general environment.
All of the isotopes of carbon behave the same way chemically, so the proportions of carbon isotopes in the living tissue will be nearly the same everywhere, for all living things. When an organism dies, however, it stops taking in carbon of any form.
From the time of death, therefore, the carbon in the tissues is no longer replenished. Like a ticking clock, carbon atoms transmute by radioactive decay to nitrogen, atom-by-atom, to form an ever-smaller percentage of the total carbon. Scientists can thus determine the approximate age of a piece of wood, hair, bone, or other object by carefully measuring the fraction of carbon that remains and comparing it to the amount of carbon that we assume was in that material when it was alive.
If the material happens to be a piece of wood taken out of an Egyptian tomb, for example, we have a pretty good estimate of how old the artifact is and, by inference, when the tomb was built. The result: the two independent techniques yield exactly the same dates for ancient fossil wood. Carbon dating often appears in the news in reports of ancient human artifacts. In a highly publicized discovery inan ancient hunter was found frozen in the ice pack of the Italian Alps Fig. The technique provided similar age determinations for the tissues of the iceman, his clothing, and his implements Fowler Carbon dating revealed that he died about 5, years ago.
Photo courtesy South Tyrol Museum of Archaeology, www. Carbon dating has been instrumental in mapping human history over the last several tens of thousands of years.
When an object is more than about 50, years old, however, the amount of carbon left in it is so small that this dating method cannot be used. To date rocks and minerals that are millions of years old, scientists must rely on similar techniques that use radioactive isotopes of much greater half-life Table 1. Among the most widely used radiometric clocks in geology are those based on the decay of potassium half-life of 1. In these cases, geologists measure the total of atoms of the radioactive parent and stable daughter elements to determine how many radioactive nuclei were present at the beginning.
Thus, for example, if a rock originally formed a long time ago with a small amount of uranium atoms but no lead atoms, then the ratio of uranium-to-lead atoms today can provide an accurate geologic stop watch. When you see geologic age estimates reported in scientific publications or in the news, chances are those values are derived from radiometric dating techniques. In the case of the early settlement of North America, for example, carbon-rich campfire remains and associated artifacts point to a human presence by about 13, years ago. Much older events in the history of life, some stretching back billions of years, are often based on potassium dating.
This technique works well because fossils are almost always preserved in layers of sediments, which also record periodic volcanic ash falls as thin horizons. Volcanic ash is rich in potassium-bearing minerals, so each ash fall provides a unique time marker in a sedimentary sequence. The rise of humans about 2. Paleontologists rely on radiometric dating to determine the ages of fossils, such as this million-year-old trilobite, Ameura majorfrom near Kansas City, Kansas.
Photo courtesy Hazen Collection, Smithsonian Institution.Carbon-14 dating cannot be used for
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How Old is Earth, and How Do We Know?