🌋 Igneous Rocks
Igneous rocks are rocks that form when molten rock cools and becomes solid.
The word igneous comes from a Latin word meaning “fire”, because these rocks are
formed from very hot molten material inside or on the surface of Earth.
1. What is the molten material?
There are two important terms:

Magma → molten rock found below Earth's surface.

Lava → magma that reaches the Earth's surface through a volcano.
When magma or lava cools, it changes from a liquid state into a solid rock. This
process is called solidification.
Simple sequence:
Magma → cooling → solidification → Igneous rock
or
Lava → cooling → solidification → Igneous rock
2. How are igneous rocks formed?
There are two main ways igneous rocks form.
A. Intrusive igneous rocks
These form inside the Earth.
1. Deep underground, rocks can become extremely hot and melt, producing
magma.
2. The magma moves into cracks or spaces between existing rocks.
3. Because it is deep underground, the surrounding rocks keep it relatively
insulated.
4. The magma therefore cools slowly.
5. Slow cooling gives crystals plenty of time to grow.
6. Large mineral crystals develop, producing a coarse-grained rock.
Example: Granite
Granite contains visible crystals of minerals such as quartz and feldspar.
Other examples include:

Gabbro

Diorite
Remember:
Inside Earth + slow cooling = large crystals
B. Extrusive igneous rocks
These form at or near Earth's surface.
1. Magma rises toward the surface.
2. During a volcanic eruption, it may come out as lava.
3. The lava is exposed to much cooler surroundings.
4. It cools quickly.
5. Minerals have less time to form large crystals.
6. The resulting rock usually has very small crystals or sometimes a glassy
appearance.
Examples:

Basalt

Obsidian

Pumice
Remember:
At Earth's surface + fast cooling = small crystals
3. Why does cooling speed affect crystal size?
This is one of the most important ideas about igneous rocks.
Imagine making ice cubes:

If water freezes quickly, there is less time for large structures to develop.

If cooling happens more slowly, there is more time for crystals to grow.
The same general idea applies to minerals in cooling magma.
Cooling
Where?
Crystal size
Example
Slow
Underground
Large
Granite
Fast
Earth's surface
Small
Basalt
Cooling
Where?
Crystal size
Example
Very fast
Surface
Very tiny/no visible crystals
Obsidian
4. Important examples of igneous rocks
� Granite

Intrusive

Forms from slowly cooling magma underground.

Has large, visible crystals.

Common minerals include quartz and feldspar.

Often used as a building and decorative stone.
� Basalt

Extrusive

Forms when lava cools quickly at Earth's surface.

Usually has small crystals.

Commonly forms from volcanic lava.
� Obsidian

Extrusive

Lava cools extremely quickly.

It has very little time to form visible crystals.

It has a glassy appearance.
� Pumice

Extrusive

Forms from volcanic material containing many trapped gas bubbles.

It can have many holes and may be light enough to float on water.
� Gabbro

Intrusive

Cools slowly underground.

Has relatively large crystals.

It is similar in composition to basalt but forms in a different location and
cooling environment.
5. Granite and basalt — an important comparison
Feature
Granite
Basalt
Type
Intrusive
Extrusive
Forms
Underground
At/near surface
Material
Magma
Lava
Cooling
Slow
Fast
Crystal size
Large
Small
Appearance
Crystals usually visible
Crystals usually difficult to see