Before the first Earth Day or recycle bin, Mother Nature already had a recycling program in place. Every drop of water, every molecule of air have been endlessly recycled since day one. Most of us don’t know, though, that she also recycles the solid parts of the planet, in a process called The Rock Cycle.
Geologists lump all the rocks on the earth into three kinds; igneous, metamorphic, and sedimentary. A rock is assigned to a type based on how it formed: igneous rocks crystallize from super-hot liquid, metamorphic rocks have been squashed deep in the earth, and sedimentary rocks form from bits and pieces of eroded rocks at the surface. We already know that water cycles continually: it falls as rain, runs down a river to the ocean, and evaporates into the clouds only to become rain again. Just as water cycles, the elements that make up rocks are also on a sort of endless treadmill – the rock cycle just moves a lot, lot slower.
Visualize the rock cycle as a triangle with each of the three rock types at a corner. Between the corners are different processes needed to form new rock types. Start with the corner labeled igneous: between igneous and sedimentary rocks are the processes of erosion and sedimentation. Erosion, the action of wind, water, and ice, breaks an igneous rock into little bits. This “sediment” is moved by wind and water, then deposited with other sediment and finally buried. The result is a sedimentary rock.
Keep going around the triangle, and you’ll find the processes of heat, pressure, and stress. If our sedimentary rock becomes very deeply buried, it can encounter enough heat and pressure to change its internal structure, though not quite enough heat and pressure to melt it. The change is called metamorphosis, and creates the third type of rocks: metamorphic.
Keep raising the heat, however, and our metamorphic rock may finally melt after all. The melted rock forms a super-hot liquid called magma. If this pool of magma gets a little too cool, however, it crystallizes like water freezes to ice. The resulting frozen magma becomes… an igneous rock! It may have taken hundreds of millions or even billions of years, but we’ve now gone all the way around the rock cycle.
Rocks don’t always complete the entire cycle. For instance, sedimentary rocks can be melted to magma without having undergone metamorphosis. A sedimentary rock can be eroded and the resulting sediment deposited as a new sedimentary rock; or an igneous rock can be re-melted to make an entirely new igneous rock. Mother Nature is not picky, just as long as everything gets recycled.
One of the most impressive things about the rock cycle is how long each step may take. Mother Nature must be very patient about this recycling program; because each process can take millions of years to complete. A chunk of rock may well sit, unchanged, for hundreds of millions of years awaiting the processes that will take it to its next step in the cycle. Sometimes the change happens quickly, but it can be quite slow: the oldest known rocks on earth, igneous rocks about 4.3 billion years old, have already been waiting for a very long time.
Sources: National Association of Geology Teachers, Science Daily, Center for Educational technologies
