1. Igneous Rock Formation
Molten rock comes from depth (less dense so works its way to surface) - full of gases (H20, C02, S02) plus elements in silicates
Eruption - gases escape as pressure lessons
Magma cools and hardens beneath the surface (intrusive; plutonic)
Crystallization - process of cooling and solidifying
Lava cools and hardens on surface (extrusive)
In volcanic eruptions on or under the earth’s surface

2. Intrusive Igneous Rocks
Form when magma hardens beneath Earth’s surface
Coarse grained due to slow cooling
Examples- Granite, Diorite, Gabbro, Periodotite
(image on left is diorite)

3. Extrusive Igneous Rocks
Form when lava hardens above the Earth’s surface, when most of the gases have escaped
Fine grained due to rapid cooling
Examples- Basalt, Rhyolite, Scoria
(image on right is basalt)

4. Classification of Igneous Rocks
Texture- size, shape, arrangement of crystals
Composition- proportions of light and dark minerals in the rock
(images from top to bottom: obsidian, diorite, basalt)

5. Coarse Grained = Large Crystals
Intrusive rocks
Magma cools slowly
Ions have time to move large distances within magma
Few centers of crystal growth develop

6. Fine Grained: Fast Cooling
Extrusive rocks
Magma or lava cools rapidly resulting in small, interconnected mineral grains
The ions in the melted material lose their motion and quickly combine

7. Glassy Texture Fast Cooling, Extrusive
No time for the ions in the lava to arrange themselves into a network of crystals
Obsidian (top); pumice (bottom)

8. Porphyritic Texture: Different Sized Crystals
Minerals that do not crystallize at the same rate or time in magma --different sized crystals
Inside volcano some magma never reaches the surface--two waves of crystallization
Large crystals called phenocrysts (visible-crystals) in a matrix of fine grained crystals

9. Granitic Composition 0%-25% dark Felsic Light colored Quartz, feldspar
10% dark silicate materials-- with magnesium, iron
70% silica (light silicates)
Example- Rhyolite
(also granite)

10. Basaltic Composition 45%-85% dark minerals Plagioclase feldspar
Rich in magnesium and iron
Darker and denser (because of iron)
Mafic
Ocean floor = basalt
Example- Basalt
(also gabbro)

11. Andesitic Composition
Between granitic and basaltic
25%-45% ‘dark’
25% dark silicate minerals-- amphibole, pyroxene, biotite mica
Other dominant mineral:
Plagioclase feldspar
Example- Andesite
(also diorite)

12. Ultramafic Composition
85%-100% dark
Olivine and pyroxene
Almost entirely dark silicate minerals
Peridotite rock. Rare at E’s surface but composition of much of mantle