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Why are there plates? Why isn't the Earth's crust one piece?
Answer 1:

You probably know that the earth is not solid, just the very center and the crust. Between these is magma--a liquid. You probably know that the magma is very very hot and the heat comes from pressure of material on the outer parts of the planet. This means that the magma near the crust must be a little cooler than the magma closer to the center of the planet. Remember that cooler material is more dense and hotter material is less dense. Denser material sinks while less dense material rises. So this means that magma near the crust is cooling, becoming more dense and sinking until it heats up again and rises. All of this means that the magma under the crust is moving. That means that the crust is floating on moving magma. The moving magma pushes the crust around like water pushing a raft down a river but the magma is not all moving in the same direction! So different parts of the crust are being pushed in different directions. These pushes or forces are great enough to crack up the crust into plates. The plates push up against each other and form the different geology that you see like mountains, underwater trenches, faults, volcanoes, and on and on.

Answer 2:

This is a question that is still being researched on Earth and evidence from other planets like Mars and Venus are helping to unravel what is really happening with the plates.Most scientists agree that the main reason for the plates is because the Earth has a tremendous amount of heat in its core that can't get out efficiently by simply conducting through the rocks. This heat is so great that the rocks of the mantle become able to slowly move in big rotations that bring hot material up from the core near the surface where it meets the bottom of the more solid, cold crust and flows underneath the crust before cooling enough that it sinks back to the core to be heated again. The areas where the mantle rises near the surface often form spreading zones like the Mid-Atlantic Ridge where new crust is formed. The mantle sinks again at subduction zones where a plate dives beneath another one down towards the core. As the mantle moves along the bottom of crustal plates, the plates move with it and at the surface, this makes the motion of plates. The function of this moving mantle is to more efficiently allow heat from the center of the Earth to escape. If there were no plates, then places where the mantle was going up and meeting the bottom of the crust would become hotter and thinner until they started moving apart. Mars is smaller than the Earth and has a cooler core. Mars is suspected to have only one plate but. Why do you think this would be the case? If Venus is close to the same size as the Earth, do you think it would have many plates? Can you think of any other ways that heat could get out of a planet's core?

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