Geology Reference
In-Depth Information
(40 to 50°C) hotter with every mile below ground. We can surmise from this that the schist
was approximately ten miles below the surface when it formed, twice as far down as Mt.
Everest is tall. The bottom of the canyon exposes the roots of an ancient mountain range,
visible today only because of the erosion of the overlying rock that had to have lain above
the canyon walls to turn all that sand and mud into solid rock in the first place.
How long ago did the schist form below those ancient mountains? More than a billion
years ago, although no one can tell just by looking at the rocks. Using the right tools, the
age of a rock can be read like a geologic clock because radioactive isotopes decay at a
fixed rate. Radiometric dating is based on the fact that younger rocks have more of the
initial parent isotopes of their radioactive elements and older rocks have proportionately
more of the daughter isotopes produced by radioactive decay. If you know the half-life of
an isotope—how long it takes for half the remaining amount to decay—then the ratio of the
parent-to-daughter isotope now in a rock tells you how long ago the rock crystallized.
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