Rocks

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What is a rock?

A rock is any hard substance found in nature which is made of minerals and formed by geological processes.

Geological processes are natural processes which change things on or beneath the surface of the earth. Examples of geological processes are volcanic activity and erosion.

Types of rock

There are three major types or groups of rock. The three major types or groups of rock are igneous, sedimentary and metamorphic rocks. Each type of rock is formed in a different way. Let us look at each one separately.

Igneous rocks

These rocks are formed when molten rock (melted rock) cools and hardens. For example, when a volcano erupts hot molten rock known as lava pours out of the volcano. When the lava cools down it hardens to form igneous rocks. There are many different igneous rocks. Look at the pictures below. The coin in the pictures is there to give you an idea of the actual size of the rocks.

This is **basalt**. It is sometimes used in concrete and when building roads.

This is **obsidian**. It is also called volcanic glass because it looks like black glass.

This is **granite**. It is often cut and polished and used to make fancy countertops and tiles.

Sedimentary rocks

Sedimentary rocks form when material like mud, sand and small stones are compacted over time until it hardens to form rock. This usually happens in bodies of water like the sea. The material which forms sedimentary rock is often built up in layers. These layers can sometimes be seen in the rock. There are many different kinds of sedimentary rock.

Sandstone is formed from compacted sand. You can see layers of darker and lighter sand.

This is red sandstone. You can see some of the grains of sand in the rock.

This is **mudstone**, formed from compacted mud. You can see the shape of the shells of animals that were trapped in the mud before it hardened.

This rock is made up of small stones and mud. It is called conglomerate.

Metamorphic rocks

When rocks are placed under great pressure or very high temperatures (or both), they can change into completely different types of rock. These changed rocks are called metamorphic rocks. Look at the pictures of metamorphic rocks below.

**Marble** is formed from limestone which has been placed under great pressure and heat.

**Diamonds** are formed when carbon is placed under great heat and pressure.

Slate is formed when mudstone or shale is placed under great heat and pressure

Watch the video below about the different types of rock.

Take the Rock Quiz!