In the nineteenth century, prominent scientists such as Charles Lyell, Charles Darwin, Sir William Thomson (Lord Kelvin), and Thomas Huxley, were in continual debate about the age of the earth.
The discovery of the radioactive properties of uranium in 1896 by Henri Becquerel subsequently revolutionized the way scientists measured the age of artifacts and supported the theory that the earth was considerably older than what some scientists believed.
All radioactive dating is based on the fact that a radioactive substance, through its characteristic disintegration, eventually transmutes into a stable nuclide.When the rate of decay of a radioactive substance is known, the age of a specimen can be determined from the relative proportions of the remaining radioactive material and the product of its decay.In 1907, the American chemist Bertram Boltwood demonstrated that he could determine the age of a rock containing uranium-238 and thereby proved to the scientific community that radioactive dating was a reliable method.Uranium-238, whose half-life is 4.5 billion years, transmutes into lead-206, a stable end-product.Boltwood explained that by studying a rock containing uranium-238, one can determine the age of the rock by measuring the remaining amount of uranium-238 and the relative amount of lead-206. The long half-life of uranium-238 makes it possible to date only the oldest rocks.
This method is not reliable for measuring the age of rocks less than 10 million years old because so little of the uranium will have decayed within that period of time.This method is also very limited because uranium is not found in every old rock.It is rarely found in sedimentary or metamorphic rocks, and is not found in all igneous rocks.Another method for dating the rocks of the earth's crust is the rubidium-87/strontium-87 method.Although the half-life of rubidium-87 is even longer than uranium-238 (49 billion years or 10 times the age of the earth), it is useful because it can be found in almost all igneous rocks.Perhaps the best method for dating rocks is the potassium-40/argon-40 method.