Igneous, Sedimentary, and Metamorphic: What Are the Differences and What Is the Rock Cycle?

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As a kid, I had trouble remembering what an igneous rock was and how to visually distinguish one from the sedimentary or metamorphic kind. After teaching earth science for a few years, I've become pretty good at it and have come up with ways to remember who's who in the rock world.

Let's start off with the cycle

Current theory states that the earth has an inner core and an outer core. On top of the outer core is the biggest area called the mantle. The lithosphere, or the crust, floats on top of the mantle. The last layer is the atmosphere. The lithosphere, or crust, is broken up into plates called tectonic plates. Some of the plates spread apart from each other, and some of the mantle comes up into the lithosphere, hardens, and forms new crust. Some of the plates come together and parts of the plates are subducted back into the mantle.

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This forming of new crust and destruction of old crust is what powers the rock cycle, and creates the first type of rocks: igneous.

Igneous

Newly formed rocks are called igneous rocks. Igneous means "it came from fire." Igneous rocks are formed by cooling magma, or lava, which comes from the mantle of the earth.

There are two kinds of igneous rocks. Some of the rocks are formed and cooled under the earth. Others are formed on top of the earth and in contact with air.

Extrusive

Extrusive igneous rocks form on the surface of the earth's crust. Because they are on the crust, they cool quickly and are fine-grained. Crystals haven't had time to form. You can remember extrusive rocks because they extrude to the surface. Basalt is formed by the extrusive process.

Intrusive (plutonic)

Intrusive rocks cool below the surface, inside of the earth's crust. Since they're not exposed to air, they cool much slower than extrusive rocks. This slow cooling time gives crystals time to form and intrusive rocks have a coarser grain. Granite is formed by the intrusive process.

Sedimentary

Sedimentary rocks are formed by sedimentation, either in air or in water. This type of rock is known for it slayers. There are three types of sedimentary rocks: clastic, chemical, and organic.

Clastic sedimentary

When the igneous rocks make it up to the surface, they start to weather. Both chemical and mechanical weathering break down the rock into smaller rock and some dust. Clastic sedimentary rocks form when layers of other, broken down rocks compress together. Sandstone is a classic clastic sedimentary rock.

Organic sedimentary

These are created by living organisms such as coral and mollusks. Coal and oil shale are formed by the organic sedimentary process.

Chemical sedimentary

Chemical sedimentary rocks precipitate out of a supersaturated solution. Salt, gypsum, and limestone are created this way.

Metamorphic

Metamorphic rocks are formed by either sedimentary rocks or igneous rocks. The other rocks are subjected to intense pressure, heat, or chemical change and are changed into a new rock. Marble is a metamorphic rock.

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