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What are the Different Types of Rocks?

Take a close look at a small rock. Can you tiny pieces of different colors and apes in it? These pieces are called minerals. Most rocks are a mixture of different minerals. For example, granite consists mainly of quartz and two types of feldspar. It may also contain mica and hornblende.
Scientists who study rocks are called geologists. Geologists have classified all the kinds of rock into three main groups, according to how the rock was originally formed. These groups are igneous rock, sedimentary rock and metamorphic rock.

Igneous Rocks

Molten rock that bursts from the mantle under the earth’s crust cools down and becomes solid. This is called igneous rock. We can clearly see crystal shapes in some igneous rocks, such as granite and basalt. Crystals are clear solids made from atoms arranged in an orderly pattern. An atom is one of the basic units of matter.

Sedimentary rocks

Small pieces of rock are washed down into the sea. These pieces settle in layers, or strata. Over millions of years, more and more layers press down on each other and turn the bottom layers into hard, sedimentary rock.
Sometimes, the remains of dead plants and animals turn to stone in the layers of sedimentary rocks. These remains, called fossils, tell us about life on earth long ago when the rocks they were trapped in were formed.

Metamorphic rocks

Both igneous and sedimentary rocks can be changed into a different kind of rock, called metamorphic rock. This process is often caused by intense heat and pressure underground. For example, the sedimentary rock called limestone can be turned into a metamorphic rock, called marble, in this way.


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