How rocks are formed

The earth’s crust is made up of rocks. They are formed in three ways:

Sedimentary Rocks

Some sedimentary rocks are made only of rock fragments. Example sandstone is made from sand.

Millions of years ago trees and marsh buried in swamps. Then they decayed and turned into coal.Chalk and limestone are formed from the remains of plants and sea creatures.

Igneous Rocks

Magma which rises into the crust cools very slowly, it may take thousands of years to cool.

This allows crystals to form. Granite is an igneous rock with crystals.

Metamorphic Rocks 

Metamorphic rocks are existing sedimentary or igneous rocks whose structure and appearance have been changed by great heat or pressure or both of these.

The rocks may be “baked” by coming into contact with magma.

Earth movements may create great pressure and cause changes in the rock.

This is marble. It was limestone which was changed by heat or pressure or both.

Some marbles are pure white. This white marble has grey veins. Some white marbles has pink veins. 

Thin layers of salt or sand in the limestone from these veins when the limestone is changed into marble. Other impurities in limestone may change it into green, brown, black marble.