Geologists came to the conclusion in the 1960's
that the Earth's rigid outer layer (crust and outer, rigid layer of the
mantle) was not a single piece, but was broken up into about 12 large
pieces called plates. The red lines on the map of the world above
- Convergent boundaries - two plates collide to form mountains or
a subduction zone.
- Divergent boundary - two plates are moving in
opposite directions as in a mid-ocean ridge.
- Transform boundary - two
plates are sliding past each other as in the San Andreas fault of
California. A transform boundary is like a tear in the Earth's crust.
These plates move very slowly across the surface of the Earth as though
they were on a conveyor belt. The convection currents in the much
hotter mantle continually move the plates about 1/2 to 4 inches per
When the plates move they collide or spread apart allowing the very
hot molten material called lava to escape from the mantle. When
collisions occur they produce mountains, deep underwater valleys called
trenches, and volcanoes. As mountains and valleys are being formed
natural disasters such as earthquakes and volcanic activity can occur
which has affected humans for thousands of years.
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