The Earth's rocky outer crust solidified
billions of years ago, soon after the Earth formed.
This crust is not a solid shell; it is broken up into
huge, thick plates that drift atop the soft, underlying
mantle.
The plates are made of rock and drift all over the globe;
they move both horizontally (sideways) and vertically
(up and down). Over long periods of time, the plates
also change in size as their margins are added to, crushed
together, or pushed back into the Earth's mantle. These
plates are from 50 to 250 miles (80 to 400 km) thick.
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The map of the Earth is always changing;
not only are the underlying plates moving, but the plates
change in size. Also, the sea level changes over time
(as the temperature on Earth varies and the poles melt
or freeze to varied extents), covering or exposing different
amounts of crust |
