World Builders™                                                                           Session Two  --  Geology             

                   Tectonic Plates    

The continents are floating on huge plates of rock that rest on the hot mantle of the earth.

Tectonic Plates

     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 called convection.  

     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 accepted.

    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 Move?

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 drift.  Eventually, 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 now?

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 to form.

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