Stuff, New Zealand: Draining the land on two farms in Scotland, James Hutton (1726-1797) noticed layers of rock.
In some cases these layers or strata contained the fossilised remains of unknown creatures.These and other geological observations led him to a revolutionary theory – the formation of the Earth was not a one-off event but takes place continuously.
Processes such as erosion and sedimentation were constantly changing the land.He concluded that many of these changes must take place over huge spans of time, geological time.
In 1785 Hutton published his ideas in a pioneering book, “Theory of the Earth”.In 1862 the most influential scientist, Lord Kelvin published calculations in which he concluded the Earth was approximately 20 million years old.This was Kelvin’s revised estimate after his original estimate of 100 million years.He assumed that the Earth began as a molten sphere and his calculations established how long the Earth would take to cool to its present temperature.Some scientists were sure that 20 million years was too short but couldn’t prove otherwise and for about 40 years Lord Kelvin’s was the best estimate.
There are two key reasons why Kelvin’s estimate was dreadfully wrong.
The first was revealed in 1896 by Henri Bacquerel when he discovered the phenomenon of radioactivity.Radioactive atoms are unstable because the nuclei of these atoms have an excess of energy.The energy can be released as particles and/or radiation and occurs spontaneously in a process called radioactive decay.The link between radioactivity and the age of the Earth is that radioactive decay releases heat.Atoms of the same chemical element but which have different numbers of neutrons in their nuclei are called isotopes.Four radioactive isotopes are present in the Earth’s mantle, thorium-232, potassium-40 and two of uranium, uranium-235 and uranium-238.These are in sufficient quantities that their radioactive decay contributes half the flow of heat from the Earth’s interior to the surface.The other half is heat left over from the formation of the Earth.Lord Kelvin’s estimate of the heat loss rate of the Earth did not account for the “radiogenic” heat, nor did it account for the huge internal convection currents that these heat sources generate.
The second key factor in helping to determine an accurate age of the Earth was Ernest Rutherford’s principle of radioactive half-lives.It is impossible to predict when a particular radioactive atom will decay but the average time required for a quantity of a radioactive isotope to reduce to half its mass is called its half-life.
The range of half-lives is enormous, for example, the half life of lutetium-176 is 38 billion years; polonium-214’s half life is only 164 microseconds.
Geological samples revealed the four radioactive isotopes present in the Earth and by knowing their half-lives, their relative concentrations and the relative concentrations of their decay products, it is possible to calculate the age of the rock.
In 1904 a young Rutherford was giving a lecture at the Royal Institution.
In the audience was the imposing figure of Lord Kelvin.
Rutherford wanted to suggest that Kelvin’s estimate of the age of the Earth was wrong.
He decided to mention it late in the lecture, as he watched Kelvin starting to fall asleep.
Rutherford quickly stated that Kelvin’s estimate would be improved if the heat of radioactive decay was incorporated – as he spoke he “saw the old bird sit up, open an eye and cock a baleful glance at me!”
Early calculations based on the half-lives of radioactive isotopes were flawed and have been revised using input from later discoveries.
Today the best estimate of the age of the Earth is obtained from chondrite meteorite samples.
These are non-metallic meteorites formed from various dust types present in the very early stages of the solar system.
These meteorites are pristine and, unlike the Earth’s interior, have not been modified by melting and acidification, nor have they been differentiated into layers from the effects of temperature gradients and gravitational forces.
The current most accurate estimate of the age of the Earth is 4.543 billion years – accurate to within 100 million years.