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The earth's crust is actually a two-component layer. The lithosphere is a thin layer of rock (average density of 2.7 grams per cc) and "floats" on top of a plastic-like layer called the asthenosphere. Plastic-like materials are weird - they deform under stress but don't really break. A glacier is a good example of a material that moves and flows plastically. The convective heat currents in the mantle impinge on the asthenosphere causing deformation and subsequent movement of the lithospheric plates.
This process can be simulated in your kitchen by putting some jello in a bowl and putting some peebles on top of the jello. As you shake the bottom of the bowl, the jello deforms but doesn't break and the rocks that float on the jello collide. (apologies to real geologists for this analogy).
As a result of plate movements, interesting things occur at plate boundaries. In general you don't want to live near a plate boundary as the earth is active there. About 75% of the world's population does live near these boundaries.
There are three types of plate boundaries:
Local Manifestations of Plate Tectonics:
Some Other Internet Resources on Plate Tectonics:
Another overview of all of this
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