1. Igneous Rocks What are they and how do they form?
Mt. Rushmore

2. Half Dome, Yosemite, CA
Ansel Adams

3. IGNEOUS ROCKS
Some igneous rocks form from cooled lava that has exited Earth’s crust
Hawaii

4. Half Dome, Yosemite, California
Some Igneous rocks cool from magma inside Earth and later become exposed through pressure, uplift, and erosion!!!
Half Dome, Yosemite, California

5. Igneous
Think of “ignite”
To light a fire
Igneous rock comes from the heat within Earth: Geothermal

6. How are Igneous Rocks Classified?
Igneous rocks are classified by:
1. Mineral Composition
2. Crystal Size (Texture)

7. Igneous Rock Formation
Most Igneous rocks form directly from Magma which is hot, molten rock
Granite is the most common igneous rock, it cooled from magma
Igneous rocks can also form from lava

8. 3. First Classification of Magma:
Felsic
Is rich in Silica (SiO2)
Small amounts of Ca, Fe, Mg (Calcium, Iron, & Magnesium)
Slow-moving, thick= HIGH VISCOSITY (resistance to flowing)
Crystallizes to light-colored minerals
Resulting rocks have relatively low density
Felsic rocks make up the continents

9. Examples of FELSIC igneous rocks
Pumice
Rhyolite

10. 4. Second Classification of Magma:
Mafic
Low amounts of Silica (SiO2)
Fe and Mg rich (Iron and Magnesium)
Fast-moving, thinner= LOW Viscosity: less resistance to flowing, more fluid
Crystallize to dark-colored minerals
Resulting rocks have relatively high density
Mafic rocks make up the ocean floors

11. Mafic Igneous Rock examples:
Basalt (ocean floor)
Gabbro

12. 5. Crystal Size It takes time to grow crystals
As magma cools, it crystallizes
If magma cools slowly, what size crystals will form?
Large Crystals
If Magma cools quickly, small crystals will form because they have little time to grow.

13. Igneous Rock Obsidian cools so fast that no crystals have time to form
A.K.A. Volcanic glass
Obsidian is dark in color due to iron oxides.

14. 6. Igneous Rocks
Granite
Intrusive
Igneous rock that crystallizes below Earth’s surface, from magma
Intrusive= Inside Earth
Large Crystal size
Notice how Granite has large crystals, it cooled slowly from magma deep beneath Earth’s surface

15. Igneous Rocks
Extrusive
Igneous rock that crystallizes or forms at Earth’s surface, from lava
Extrusive= exited Earth
Basalt
Small Crystal size
Notice how basalt has small crystals, it cooled quickly at Earth’s surface