By dating the rocks in Earth's ever-changing crust, as well as the rocks in Earth's neighbors, such as the moon and visiting meteorites, scientists have calculated that Earth is 4. Related : How big is Earth? Related : What's the speed of Earth around the sun?
Scientists have made several attempts to date the planet over the past years. They've attempted to predict the age based on changing sea levels, the time it took for Earth or the sun to cool to present temperatures and the salinity of the ocean. As the dating technology progressed, these methods proved unreliable; for instance, the rise and fall of the ocean was shown to be an ever-changing process rather than a gradually declining one.
And in another effort to calculate the age of the planet, scientists turned to the rocks that cover its surface. However, because plate tectonics constantly changes and revamps the crust, the first rocks have long since been recycled, melted down and reformed into new outcrops. Scientists also must battle an issue called the Great Unconformity, which is where sedimentary layers of rock appear to be missing at the Grand Canyon, for example, there's 1.
There are multiple explanations for this uncomformity; in early , one study suggested that a global ice age caused glaciers to grind into the rock , causing it to disintegrate. Plate tectonics then threw the crushed rock back into the interior of the Earth, removing the old evidence and turning it into new rock.
In the early 20th century, scientists refined the process of radiometric dating. Earlier research had shown that isotopes of some radioactive elements decay into other elements at a predictable rate.
By examining the existing elements, scientists can calculate the initial quantity of a radioactive element, and thus how long it took for the elements to decay, allowing them to determine the age of the rock.
But rocks older than 3. Greenland boasts the Isua supracrustal rocks 3. Samples in Western Australia run 3. Research groups in Australia found the oldest mineral grains on Earth. These tiny zirconium silicate crystals have ages that reach 4. The first of these referred to the rate of heat loss from the earth and the length of time it would have taken to form its solid crust. The second referred to such topics as the detailed shape of the earth bulging slightly at the equator and the dynamics of the earth-moon system.
The third referred to the heat of the sun, particularly the rate at which such heat is being lost, compared with the total amount of energy initially available. The first argument was completely undermined after taking into account the amount of heat generated by radioactive decay.
The second depended on highly dubious theories of formation of the earth and moon and plays relatively little role in this compilation. The third, which by the end was the most acute, presented a problem that outlasted the controversy itself. He did not need to wait long. In Sir Arthur Eddington came up with the answer: the fusion of hydrogen into helium. One referred to the depth of the sediments and the time they would have taken to accumulate; the other referred to the salinity of the oceans, compared with the rate at which rivers are supplying them with sodium salts.
In hindsight, both theories were deeply misguided, for similar reasons. They assumed that current rates—of sediment deposition and of salt transport by rivers—were the same as historical rates, despite the evidence they had that our own age is one of atypically high geologic activity.
Worse, they measured inputs but ignored outputs. The rock cycle, as we now know, is driven by plate tectonics, with sedimentary material vanishing into subduction zones.
And the oceans have long since approached something close to a steady state, with chemical sediments removing dissolved minerals as fast as they arrive. Nevertheless, by the late 19th century the geologists included here had reached a consensus for the age of the earth of around million years. Having come that far, they were initially quite reluctant to accept a further expansion of the geologic timescale by a factor of 10 or more. And we should resist the temptation to blame them for their resistance.
Radioactivity was poorly understood. Different methods of measurement such as the decay of uranium to helium versus its decay to lead sometimes gave discordant values, and almost a decade passed between the first use of radiometric dating and the discovery of isotopes, let alone the working out of the three separate major decay chains in nature. The constancy of radioactive decay rates was regarded as an independent and questionable assumption because it was not known—and could not be known until the development of modern quantum mechanics—that these rates were fixed by the fundamental constants of physics.
It was not until , when under the influence of Arthur Holmes, whose name recurs throughout this story the National Academy of Sciences adopted the radiometric timescale, that we can regard the controversy as finally resolved. But carbon is not the only element that can be dated— a whole host of others exist. In uranium-lead dating, for instance, the radioactive decay of uranium into lead proceeds at a reliable rate.
Based on the very old zircon rock from Australia we know that the Earth is at least 4. But it could certainly be older. Scientists tend to agree that our little planet is around 4. Colin Schultz is a freelance science writer and editor based in Toronto, Canada.
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