Igneous Rocks - Formation

Deep within the ground is molten rock called magma. Sometimes this bursts through the surface in the form of volcanoes.

Igneous rocks are formed when molten magma cools and solidifies.

This may happen within the ground, or in the case of volcano lava, above ground.

magma

intrusive igneous rock

extrusive igneous rock

Igneous Rocks – Rate of Solidification

The appearance of igneous rocks is affected by how fast they cool.

Rocks formed within the ground are known as intrusive igneous rocks . They cool slowly and this gives time for large crystals to grow.

Rocks formed once the magma has burst through the surface are called extrusive igneous.

They cool quickly.

As a result crystals are much smaller.

Igneous Rocks - intrusive

granite

A hard grey rock consisting of randomly arranged crystals.

pumice

Extrusive igneous rocks, like pumice and basalt, do not have large crystals and tend to be less dense than intrusive rocks like granite.

Igneous Rocks - extrusive

Igneous Rocks – Properties

Igneous rocks are hard.

They contain crystals. The the size of the crystals depends on how quickly the rock solidified.

The crystals are disordered (unlike some metamorphic rocks.)

They never contain fossils.

Igneous rocks consist of a disorderly collection of crystals