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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 the Tectonic Plates?
The Earth's crust is broken into
six large and many smaller pieces.
These pieces are called plates. The plates
are given names according
to the land masses that they
carry. 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 the
1920s 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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