Types of rocks and their properties

​​In a nutshell

There are three types of rocks that form in different ways and have different properties, making them useful for various purposes. 

Definitions

Rock types

There are three main types of rock that you need to know. These are explained below.

Rock type	Examples	Formation
Igneous	Granite, Basalt, Obsidian	​This rock type is formed from magma cooling and hardening.
Sedimentary	Coal, Chalk, Sandstone	​This rock type is formed from sand, mud and other small rocks, as well as plant and animal remains are compressed over a long period of time.
Metamorphic	Anthracite, Marble, Slate	This rock type is formed from igneous and sedimentary rock, when it is heated to really high temperatures and under huge pressure.

Rock properties 

Different rocks have different properties that make them suitable for different purposes. 

​​Hardness

Some rocks are harder than others and because of this they can be used for different things. Hard rocks are often used for building, while softer rocks have different purposes.

Example

	Chalk is a soft rock and can be used as a writing tool.
	Granite is a hard rock and is often used for buildings.

Permeability

Some rocks are more permeable than others, meaning they are suitable for different functions.

Example

	Slate is an impermeable rock, so it is used to build roofs to keep out rain water.
	Limestone is a permeable rock and is used in water purification methods. Water comes into the rock (1), rock particles collect particulates and clean the water (2) and then water is pushed out by the rock (3).

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