What type of information do scientists use to determine the approximate age of the earth?

Answer 1

The age of the earth has been arrived at, slowly and step by step, as scientists refine existing methods and discover new ones.

Early in the nineteenth century, Charles Lyell examined the great volcano of Etna on Sicily and studied the historical records of frequent eruptions. He noticed that each time it erupted, a new layer of lava would be added, causing the mountain to grow at a measurable rate. By knowing the height of the volcano, its approximate rate of growth and the frequency of eruptions, Lyell determined that the volcano must be several hundred thousand years old. At the edge of the volcano, under the first lava flows, he found fossil shells that were virtually identical to the shells of molluscs still found in the Mediterranean Sea. From this, he deduced that the fossils were geologically recent, that a hundred thousand years was geologically short and that the age of the earth must be immense. Because of Lyell's research, it was no longer possible to consider the earth to be only a few thousand years old.

In 1862, Lord Kelvin, Professor of Natural Philosophy at Glasgow University and regarded by his contemporaries as the greatest physicist of his day, announced that he had calculated the time it would take the world to cool down from its molten state. He calculated that this was between 20 and 400 million years. Later, Kelvin refined his calculations to within the range 20 to 100 million years and, later still, to some 20 to 40 million years. With the discovery of radioactivity, it was soon realised that the uranium present in the earth prolongs its cooling almost indefinitely. So. Kelvin's method only gives a minimum age, but by no means a true estimate.

Samuel Haughton, an Irish geologist, calculated that sediments were deposited on the ocean floor at the rate of "one foot in 8,616 years". He then calculated a minimum duration of around 2000 million years. Unwilling to accept such a long period, he scaled it back, by a factor of 10, to just 200 million years. This may not be an important dating method today, but it does lend confirmation to other dating methods.

Once the radioactive decay of uranium was discovered, it was only necessary to determine the rate of helium production and the amount of helium that had accumulated, to calculate the age of a rock. In the early years of the twentieth century, Lord Rutherford established the age of a rock as 500 million years, by measuring the amounts of radium and helium present. Strutt soon realised that some of the helium would have escaped as the rocks were crushed for analysis, leading to false short estimates of the ages of the rocks. The rock must have been even older, but the proof of that had to wait.

It was established that lead was the stable element resulting from radioactive decay of uranium. Since lead is not a gas, Holmes decided to determine the age of rocks by using the ratios of uranium and lead. Using rocks from the Devonian age, Holmes calculated that the Devonian age was at least 370 million years ago.

In 1922, Dr Aston discovered a new lead isotope which must have been the end-product of a new isotope of uranium. That isotope of uranium was isolated as uranium 235, which was present in very small proportions to uranium 238 and decayed at a much faster rate. Rutherford used this new information to arrive at an age of 3400 million years for the earth. Scientists were now getting close to the real age of the earth.

Iron meteorites contain almost no uranium, so any lead in them would be 'ordinary' or primeval lead, the amount of which could be used as the uncontaminated estimate for the earth itself, provided that the earth and meteorites had a common ancestry. In 1953, Fritz Houtermans used material from a meteorite to calculate the age of the earth as 4500 million years, plus or minus 300 million. In the same year, Patterson independently produced figures of 4510 and 4560 million years, using a basalt and a granite sample. In 1956, Patterson proved that the earth and meteorites had a common ancestry, thus validating the results.

Holmes felt that it was unsound in principle to rely on meteorites for calculating the age of the earth. He announced that, from terrestrial evidence, he had dated the earth to 4,500 million years, plus or minus 100 million years.

The oldest things so far found on earth and that were formed here are zircon crystals found in Western Australia, that are more than 4 billion years old. Moon rocks have been found to be just over 4 billion years old, evidence that the earth and the moon were formed at the same time and from the same material.