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Plate Tectonics - How does the Earth's Crust get its Shape?

The Earth's crust is very thin compared to the size of the planet.
It is thinner than an eggshell is compared to the size of an egg.
The Earth's crust is solid and lies on top of the mantle.
The mantle contains both hot solid rock and magma.

Some scientists once believed that the features of
the Earth's surface (hills, valleys, plains and mountains)
were caused by the Earth shrinking as it cooled down
from its original state. Now scientists believe that the
Earth's features are caused by the movement of tectonic plates.

What are Tectonic Plates?

The Earth's crust and the upper mantle are cracked into six large
(and many smaller) pieces. These pieces are called plates.
The plates are given names according to the land masses that
are on top of them. For example, there is the Pacific Plate,
Eurasian Plate, African Plate, North American Plate and so on.
You do not need to remember these names.

Why do Tectonic Plates Move?

The plates are always moving, on convection currents
in the hot mantle. This is called continental drift.
Heat is generated in the mantle by natural processes of
radioactive decay. The plates move very very slowly,
only a few centimeters per year on average.
Where the plates collide or grind past each other,
rocks are stretched, crushed, deformed and sometimes melted.
This is called tectonic activity (see the rock cycle).

The theory of plate tectonics and continental drift was first
suggested by Alfred Wegener in 1912 but it was not
accepted for many years as the evidence for it was open to question.
At that time there was no known way that continents could move
and Wegner was considered to be an outsider by other geologists.
The theory became widely accepted after sea floor spreading
showing magnetic reversal in rocks was discovered in the 1960's.

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