The Three Categories of Rock

igneous rock Image:

igneous rock


Passionate about learning and expanding her awareness of the world, Valerie Varnuska of Westbury, NY, studies a variety of scientific concepts. Valerie Varnuska pursues a particular interest in geology and in the types of rocks that form on Earth.

Geologists classify rocks into three main types. The first, igneous rock, develops when liquid magma from deep inside the earth begins to cool. This hardens the magma into a solid state.

When it cools inside the earth, magma becomes a type of igneous rock known as intrusive rock. This rock can eventually surface through erosion of the rock layers above it. Other deposits of magma come to the surface as liquid lava from a volcano and cool at exposure to the atmosphere.

Igneous rock forms the majority of the earth’s surface. On top of this lies a layer of pebbles, sand, shells, and other materials of various origin. When this debris collects and pieces attach to one another, it creates sedimentary or secondary rock. Often soft and occasionally fragile, it is by far the most likely rock type to contain fossils.

The final type of rock, metamorphic, occurs when an igneous or sedimentary rock comes into contact with intense heat and pressure. This effects structural change and transforms the rock into something extremely sturdy and erosion-resistant. The process of change may also cause the metamorphic rock to have a layered appearance, while minerals that collect on its surface may develop into crystals.


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