Lesson 5: Igneous Rocks

1/8/15

Igneous Rocks are rocks formed by molten, or melted rock as it

cools and hardens.

This process can occur fairly quickly when

molten rock is forced to Earth’s surface, or slowly deep beneath

Earth’s crust.

Discovery Education Video:

Igneous Rocks

Intrusive igneous rocks are formed from magma (molten rock beneath

Earth’s surface).

These rocks cool slowly and will often have large mineral crystals present.

Extrusive igneous rocks are formed from lava (molten rock that is forced to

Earth’s surface).

These rocks cool quickly and will often have few, if any, mineral crystals present.

Crystals often form in igneous rocks when the molten rock cools slowly enough for

individual minerals to crystallize together.

Quartz (white)

Mica (black)

Feldspar (pink)

Many types of igneous rocks will be full of small holes. These are caused by gasses

escaping from the molten rock as it cools.

Common Igneous Rocks:

Granite Rhyolite

Common Igneous Rocks:

Gabbro Basalt

Common Igneous Rocks:

Pumice Scoria

Common Igneous Rocks:

Obsidian Peridotite

Key Questions:1) How are igneous rocks formed?2) What is the difference between intrusive and extrusive igneous rocks?3) What causes small holes to form in some types of igneous rocks such as pumice?4) What features can help you to determine whether a rock sample is an igneous rock?