Which technique has been used by scientists to determine that the earth is at least 4.5 billion years old?

Answer 1

Various methods have been used to date the earth itself, particularly, but not limited to, various radiometric techniques.

In 1862, Lord Kelvin 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, but later refined his calculations to within the range 20 to 100 million years. With the subsequent discovery of radioactivity, it was soon realised that the uranium present in the earth could have prolonged its cooling for as long as necessary to harmonise with other methods.

In the early years of the twentieth century, 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 - they were really even older than the initial estimates.

In the late 1930s and early 1940s, using the new techniques and the world's most advanced mass spectromoter, Alfred Nier dated some rocks at up to 2570 million years old. Essentially all that was needed from now on was to find the oldest possible rocks on earth and calculate the time in which the earth's crust existed in a molten state before the rocks were formed. The oldest so far found on earth are zircon crystals found in Western Australia, that are more than 4.4 billion years old.

The oldest known solid constituents of meteorites formed within the solar system are 4.567 billion years old, giving a probable upper limit for the age of Earth.