That's a good question. There are tiny bits of
radioactive chemical elements in rocks and
fossils. Scientists can tell how old the rocks
and fossils are by seeing how much radioactivity
is left. Radioactivity has a half-life -
that's the time for half of the radioactivity to
decay [disappear]. For radioactive carbon - C-14
- the half life is about 6,000 yrs, so after 6,000
yrs, there's half as much C-14, and after about
12,000 yrs, there's a quarter as much C-14.
C-14 is good for finding the age of things that
are less than about 70,000 yrs.
Some radioactive elements have much longer
half-lives, so they're good for dating things up
to millions or even a billion yrs old.
Another way to date fossils is to look at
the layer of rock where they're found Older
rock layers get buried by younger rock layers, so
the rock layer on the bottom is the oldest, and
the rock layer on the top is the youngest.
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