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What do we mean by ‘the age of a rock’?
It’s very simple actually. We mean the time since it was last
molten. If some rock gets re-melted then
its clock is reset. (We’re talking
‘igneous’ rock here, granite and its cousins.
We’ll cover sedimentary rock, sandstone, limestone etc., later.)
This definition causes some anomalies of course. For example, if magma flows up through a
volcano and melts some of the rock it passes through, then that rock is
rejuvenated. But the rest of the area,
that didn’t get so hot, is not.
How do you measure the age of a rock? By radioactivity.
Most rock contains minute specks of radioactive
elements. Granite is notorious for
it. One of the most useful for ancient
rock is the ‘stable’ version of uranium, U238.
Even U238 decays slowly – into lead.
And over thousands of millions of years, this lead accumulates alongside
the uranium that is left.
If the rock is re-melted, then the lead is flushed
away; and the process starts again.
Physicists can measure the amount of both
elements. They can work out how much
uranium there was originally, and how long it took for that amount of lead to
accumulate from that sized speck of uranium.
Now you may think that all sounds a bit dodgy. Many scientific techniques sound pretty dodgy
when described baldly like that.
But in fact
there is a whole community of specialists whose lives revolve around just
applying each of them. They are not
interested in what the results mean – that’s for their ‘customers’. The only thing that interests them is whether
the answers that they give are right or not.
These specialists check their results, cross check
them and then double check them. In this
case they might well find more than one radioactive element in a sample. They can then check that they both give the
same answer. And so on. Sometimes they can check against other
evidence. The geologists can often say
which of two layers of rock is older, and maybe even roughly by how much.
Unfortunately you can’t apply these techniques to
sedimentary rocks like sandstone. You
can measure the ages of individual grains of sand or gravel, but it doesn’t
tell you much. What you want to know is
when that layer of material was laid down.
This can be more detective work than science. But it can often be done.
Fossils can be magic.
For example, the patterns on snail shells tend to change gradually over
time. Palaeontologists study these
changes, and can produce calendars which show which patterns were around at
various times. If the geologists find in
a sample of rock, a shell sporting a certain pattern, the palaeontologists can
often tell them how long ago its snail was alive.
Again, you might think that this technique sounds
rather dodgy. So did the geologists –
until they found that the answers fitted in with the rest of the picture that
they were building up. Now it’s a
tried-and-tested tool.
© C B Pease, December 07