The Rock Cycle

The types of rocks and how they were formed.

Igenous-Formed from lava or magma below-[intrusive] or above-[extrusive] the earth's surface.


For thousands, even millions of years, little pieces of our earth have been eroded--broken down and worn away
by wind and water. These little bits of our earth are washed downstream where they settle to the bottom of the rivers, lakes, and oceans. Layer after layer of eroded earth is deposited on top of each. These layers are pressed down more and more through time, until the bottom layers slowly turn into rock.

Metamorphic-formed from heat and pressure under earth's surface.

The Rock Cycle Part 1

There are three main types of rocks: igneous, sedimentary, and metamorphic. Each of these types of rocks are formed in different ways and each type of rock can be changed into each of the other types of rock. Geologists call this process the Rock Cycle. Essentially the rock cycle is the process that makes and recycles rocks.

Most rocks on earth began as igneous rocks. Let's trace a possible rock cycle for newly formed igneous rocks. Igneous rocks are formed from magma. Magma cools and solidifies into rock. When igneous rocks are exposed on the surface, time and weather break the rock down into smaller and smaller pieces. This process is called weathering and erosion. Wind and water carry the smaller pieces of igneous rocks into piles called sediment beds. Over time the sediment beds get buried and the pieces of rock become cemented together to form a new type of rock called a sedimentary rock.

The Rock Cycle Part 2

Our igneous rock has turned into a sedimentary rock. If our sedimentary rock is exposed at the surface, it can be eroded away and eventually changed into new sedimentary rock. However, if our sedimentary rock gets buried deep in the Earth, heat and pressure essentially bake the rock, changing it into something new. This process is called metamorphosis, and the new rock is called a metamorphic rock. Metamorphosis can happen to igneous rocks as well.

Metamorphic rocks can also be weathered and eroded and eventually changed into sedimentary rocks. Or, if metamorphic rock is forced deeper into the Earth, the rock can melt and become magma. If the magma cools and hardens it will form into igneous rock. Igneous rocks and sedimentary rocks can also be forced deep into the earth and melt into magma. Once magma cools it forms igneous rocks.