What's a "Rock Cycle"?
We may think that we are ahead of the game when we take our aluminum cans downtown to the recycling center. As it turns out, the earth has been recycling for billions of years. Instead of being a never-changing cold rock whizzing through space, our planet is constantly reworking itself, though we can't see a complete cycle in any one place. We get a hint of this every so often when a volcano violently erupts or an earthquake shakes buildings to the ground. These events tell us that something is going on underground.
In fact, the earth is made up of plates put together much like a baseball that is stitched together at the seams. These plates move past each other (San Andreas Fault, California), and sometimes will actually slip above or below another (offshore Japan and Alaska).
As a sinking plate goes lower and lower beneath another
plate, the massive heat and pressure that it experiences actually causes
the rock to melt. This molten rock (or "lava") comes bubbling to the surface,
and can form tremendously violent volcanoes. This lava, once it gets near
the surface, will eventually cool and once again form rock.
Sometimes, instead of one plate sliding under another, they will actually collide and push each other upward, forming the world's tallest mountains (The Himalayan Mountains are an example). When this happens, the intense pressure on the rocks will cause them to change into entirely different rocks, as they melt and recrystallize due to the heat and pressure put on them.
This, of course, has been a very simple version of what happens over time. But this rock cycle gives us a feel for some of the most common paths.
The Paleontological Research Institution