Geology Reference
In-Depth Information
enough heat to prolong that cooling for longer than Kelvin
could possibly imagine, and for as long as geologists and biol-
ogists might need it. The coffin lid banged shut.
And so it was uranium, as old as the Earth herself, that must
surely be the Mother of Time, giving birth to the Daughters of
Decay. Uranium was the clock geologists had been looking for
which would allow them to tell geological time. So slow is her
rate of decay, her ticking clock, that she requires four and a half
billion years, almost the total age of the Earth, to be reduced by
half her original amount, so today there is half the quantity of
uranium left in the world as there was when the Earth first
formed. In another four and a half billion years, half of what
is here today will be gone. Of the daughters she conceives,
radium will decay to half its original amount in 1600 years, a
timescale we can begin to comprehend when we realise that
1600 years ago the Romans were leaving the shores of Britain
after their four-hundred-year occupation. Radium leaves behind
radon, which is highly unstable and intensely active. So rapidly
does it give up energy and liberate helium that it decreases by
half every four days. In its place arises another product of even
more transitory nature, its activity falling by half every three min-
utes. And so on, continuing down a long succession of trans-
formations, the shortest of which has a half life of less than a
second! It hardly exists at all. But the most remarkable feature
about all these radioactive phenomena is that they continue
unceasingly: for year after year, decade after decade, century
after century, millennium after millennium, the spontaneous
production of helium goes on, accompanied by an unfailing
evolution of heat.
At last the stage was set for geologists to escape from the
physicists' strait-jacket of twenty million years. The clock had
been found, all they had to do now was learn how to tell the
time.
