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.

The Rock Cycle
This diagram illustrates some of the more common methods of rock formation and alteration. The diagram also tells us one important fact about the Earth - it is constantly changing

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.

Two types of plate collisions. left: oceanic plate, because it is more dense, will slide beneath continental crust. right: when continental crust collides with continental crust, they have nowhere to go but up.


Over millions of years, water, ice and wind will flatten these mountains, and send the broken bits of rock downhill towards the ocean. Hundreds of millions of years later, the cycle repeats itself, as these tiny bits of rock, mostly reduced to sand, silt and clay, once again find themselves at the bottom of the ocean, sliding under another piece of the earth's crust.

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.

click here to learn about sedimentary rocks

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