When the earth cooled, much of the matter
made of heavy elements sank to the center. The lightest materials
tended to float on the top of heavier ones. These lighter elements
became the continents on our world.
When I think of the continents, I think of
mats of soap bubbles floating on water. Of course, the continents
are solid, and they float on rock, but when the thick rock beneath
them moves, they move right along with it.
The elements had not been
sorted when the earth became
molten, and it took time for
the magma to move, and to
bring the lighter material
to the top by a process
Scientists think that after
the earth cooled down, the
steam in the atmosphere
condensed and rain fell for
many, centuries. Some
of this water had been mixed
up with the rocks and had
been released by the
outgasing of the erupting
volcanoes. Some of the
water was brought in by the
comets that crashed into our
world. Perhaps the
earth was completely covered
by water. It is
thought that, in the early
days, only about 10% of the
earth was covered by land
Scientists today think in terms of the Tectonic
Plate theory. This theory was advanced in 1915 by a German climatologist
called Alfred Wegener. Although he advanced a number of well
researched arguments, it took some time for his theory to be
Scientists today think that when the earth
cooled the surface cracked to become about twenty huge rafts
of rock, called tectonic plates. The continents lie on top of
some of the plates, and the plates move slowly over the surface
of the mantle. At one time all the continents were joined together,
forming a huge continent called Pangea. They drifted apart, and
eventually came to be where they are today. The continents are
still on the move, though. The world will have a different map
in the distant future.
What Makes the Continents
In the deepest parts of the
ocean there are huge cracks between the plates of the crust.
Hot lava comes up through these cracks, forming long ridges of
jumbled, rocky mountains deep under the sea. As lava continues
to come up, the new rock pushes the older, cooler rock away.
New rock is formed, and the slowly moving rocks make the ocean
wider. This is called sea-floor spreading. Plate
boundaries in which the plates move away from each other are
called divergent boundaries.
The earth is a sphere, and the rock cannot
just keep forming and moving at divergent boundaries. Eventually
it has to go somewhere. As it goes, it carries the continental
plates along with it, and the continents ride on top.
This is called continental
continental plates will meet. When they meet, the rocks coming
out from the ocean ridges push the continents together. These
places are called convergent boundaries. What happens
One plate can slide under another plate. This
is called subduction. As the rock goes deeper,
the heat in the earth will begin to melt the rock. Volcanoes
may be formed as the hot rock rises.
Sometimes two continental plates collide.
As the rocks are pushed together, they pile up. (Think of pushing
two wet towels together. See how they make ridges and form a
mound?) As time goes by, the rocks pile up higher and higher.
As the mountains get steeper, erosion works faster.
Think of what must be happening to the rocks
inside this collision. They are being subjected to a lot of heat
and pressure, and being bent and folded. This sounds like a good
place for metamorphic rocks
1999, 2003. Elizabeth Anne Viau
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