Earth Story 1. The Time Travellers (13 of 20)

Earth Story

(AUDIENCE LAUGHING) McKENZIE: Rutherford realised that various elements inside the Earth were radioactive, like uranium and thorium and potassium, and that these generated an important amount of heat, and that this completely changed the basis of Kelvin's calculation, because instead of the Earth cooling all the time it actually had heat sources in it and that you couldn't any longer use that argument to estimate the age of the Earth.
MANNING: Rutherford had removed a central plank of Kelvin's argument.
Not all the heat inside the Earth was left over from its formation.
Instead heat was continuously being generated within the planet by radioactive decay.

McKENZIE: But on the other hand, what this then allowed you to do was to use the decay of these things, right, to not make an estimate, but actually measure the age of the Earth.
MANNING: Rutherford realised that radioactivity was slowly transforming the Earth's crust.
Hidden inside every rock were minerals containing elements such as uranium.
As time passed, radioactive decay was gradually turning the uranium into lead, changing the chemical composition of the rock.
This inexorable process begins the moment a rock forms and new minerals crystallise within it.
Rutherford suggested that by carefully measuring the chemistry of these minerals, scientists should be able to tell how long ago the rock had formed.
So, after 200 years of controversy and speculation, the age of the Earth would be found in a few grains of dust.
There's a story that when Rutherford was working in Canada, he went up to a colleague one day with a sample of rock.
"How old do you think this is?"
he inquired.
"Oh, about 10 million years," was the reply.
"I can prove to you," said Rutherford with glee, "that this rock is more than 500 million years old."
And that was far older than current estimates of the age of the Earth.
And it is indeed remarkable how, as radioactive dating techniques developed, how quickly scientists' estimate of geological time and the age of the Earth expanded.