Rocks at the surface of the earth are of many different ages. They range from over three billion years old to less than one million years old. We know that one of the laws of physics is that under ordinary circumstances matter can neither be created or destroyed. We can then ask the question, from what matter are new, younger rocks formed? The answer is--from older rocks. Older rocks are broken up, and their materials go to make younger rocks. These younger rocks may in turn be altered by various processes and their matter goes to make still younger rocks, and on and on. This process of forming younger rocks from older rocks is clearly cyclical. Older rocks are recycled to make younger rocks, just as old aluminum cans are recycled to make new aluminum cans or old newspapers are recycled to make new cardboard boxes. We call this the ROCK CYCLE, just as we could have a CAN CYCLE, or a PAPER CYCLE.
There are a number of PROCESSES by which rocks may be altered and reconstituted as new rocks. On the diagram to the right, the processes are all in small letters, the PRODUCTS or ROCKS formed by these processes are in capital letters. Weathering and erosion at the earth's surface can break down a rock into small bits. These can be deposited as sediments that become sedimentary rocks. Burial, with rising pressure and temperature, can alter any rock to form a metamorphic rock. Continued rise in temperature can eventually melt any rock until it is molten (called a magma). When the molten rock cools it forms an igneous rock. Metamorphic rocks can form from either sedimentary or igneous rocks. The sedimentary particles from which a sedimentary rock is formed can be derived from a metamorphic, an igneous, or another sedimentary rock. All three rock types can be melted to form a magma. Thus, the cycle has continued over the ages, constantly forming new rocks, breaking those down in various ways, and forming still younger rocks.