Earth Story 1. The Time Travellers (14 of 20)

Earth Story

McKENZIE: What Rutherford did, really, at a stroke, was to lengthen geological time by a factor of something like 100.
And this was greeted by the geologists with a great sigh of relief.
And it is really one of the major achievements, right, of the 20th century, that we now can date rocks and minerals and things of that kind with greater and greater accuracy and see how the whole history of the solar system and the Earth has unrolled.
MANNING: The techniques that Rutherford pioneered have been extended and refined by scientists like Stephen Moorbath at Oxford University.
...sort of lifelong love-hate relationship.

MANNING: Radioactive dating has become the geologist's most powerful tool.
And you've got really here an atomic clock, I mean, which runs at a very constant rate.
You know that uranium will always decay to lead at the same rate, no matter what the temperature or the pressure or...
Absolutely constant, it never changes, in no known physical or chemical process.
- No matter how hot or cold it is.
- Nothing at all.
It 's completely invariable.
The rate of radioactive decay is always the same.
MANNING: It's a universal clock.
And that's vital, because to finally determine the age of the Earth, scientists needed a rock with a very special history, a rock left over from the time when the Earth was forming.
This rather inconspicuous-looking object, it's part of a meteorite which fell in Mexico, at a place called Allende in February, 1969.
And it is actually the oldest known object that we know of that exists on Earth.
It 's the oldest object that can be held by human hands.
It has an age of 4,566 - plus or minus two - million years.