1. Igneous Rock

2. Igneous Rock
Igneous rocks form when magma (beneath the surface) or lava (reaches the surface) cools and hardens
Igneous comes from the Latin word ignis, which means fire
There are two types of igneous rock, intrusive and extrusive

3. Intrusive Igneous Rocks
Intrusive igneous rocks form when magma hardens beneath Earth’s surface
As magma rises, it cools, allowing elements to combine and form minerals
Minerals grow in size, forming a solid mass of interlocking crystals
Granite is a common intrusive igneous rock

4. Extrusive Igneous Rocks
Extrusive igneous rocks form when lava hardens at the Earth’s surface
Lava is similar to magma, except that most of the gases have escaped in lava
Rhyolite
Basalt
Obsidian

5. Classification of Igneous Rocks
Texture and composition are two characteristics used to classify igneous rocks
Texture describes the appearance of an igneous rock based on its size, shape, and arrangement of its interlocking crystals
Composition of igneous rocks are based on the proportions of light and dark minerals in the rock

6. Classification of Igneous Rocks
Texture describes the appearance of an igneous rock based on its size, shape, and arrangement of its interlocking crystals
There are four types of texture:
Coarse-grained
Fine-grained
Glassy
Porphyritic

7. Coarse-grained Texture
Slow cooling of magma allows ions, charged atoms, to move large distances within the magma
Slow cooling results in the formation of large crystals
Igneous rocks with large crystals show coarse-grained texture
Gabbro
Diorite

8. Fine-grained Texture
Rapid cooling of magma or lava causes ions to lose their motion and quickly combine
Rapid cooling results in the formation of many small, interconnected mineral grains
Igneous rocks with small grains show fine-grained texture
Rhyolite
Andesite

9. Glassy Texture
When lava spews onto Earth’s surface, there may not be time for the ions to arrange themselves into crystals
The ions are randomly distributed causing a glassy texture in the igneous rock
Obsidian (compact glass)
Pumice (frothy glass)

10. Porphyritic Texture
Minerals that crystallize from magma do not form at the same rate or the same time
Some crystals will become large while others even start to form
Igneous rocks with large crystals and fine-grained minerals show a porphyritic texture
Andesite

11. Classification of Igneous Rocks
Composition of igneous rocks are based on the proportions of light and dark minerals in the rock
There are four types of composition
Granitic
Basaltic
Other Compositional Groups

12. Granitic Composition
Made almost entirely of the light-colored silicate minerals, quartz and feldspar
Most granitic rocks contain 10% dark silicate minerals, often biotite and amphibole

13. Basaltic Composition
Made almost entirely of the dark-colored silicate minerals and plagioclase feldspar
Basaltic rocks are rich in the elements magnesium and iron

14. Other Compositional Groups
Rocks with a composition between granitic and basaltic rocks have an andesitic composition
Named after the common volcanic rock andesite
Contains at least 25% dark silicate minerals
Rocks composed entirely of dark silicate minerals, iron and magnesium, is referred to as ultramafic
Ultramafic rocks are rare at Earth’s surface because they make up the mantle.
Peridotite

15. Igneous Rock Summary
Igneous rocks form when magma or lava cools and hardens
Intrusive rocks form from magma deep within Earth
Extrusive rocks form from lava at Earth’s surface
Igneous rocks can be classified according to texture and composition

16. Table 1 on page 74