1. Igneous Rocks
Rocks that crystallized directly from a melt

2. Igneous rocks Form as magma cools and crystallizes
Rocks formed inside Earth are called plutonic or intrusive rocks
Rocks formed on the surface
Formed from lava (a material similar to magma, but without gas
Called volcanic or extrusive rocks

3. Igneous rocks Crystallization of magma
Fractionation: mineral separation in a magma chamber based on crystallization temperatures
Ions are arranged into orderly patterns
Crystal size is determined by the rate of cooling
Slow rate forms large crystals
Fast rate forms microscopic crystals
Very fast rate forms glass

4. Igneous rocks
Classification is based on the rock's texture and mineral constituents
Texture
Size and arrangement of crystals
Types
Fine-grained – fast rate of cooling
Coarse-grained – slow rate of cooling
Porphyritic (two crystal sizes) – two rates of cooling
Glassy – very fast rate of cooling

5. Classification of igneous rocks

9. Mafic magma can become felsic magma as it cools
Mafic magma can become felsic magma as it cools! Bowens Reaction Series tells us that: As a magma chamber cools, the mafic minerals crystallize first, then felsic ones As minerals crystallize out, the gas is left behind in the felsic melt…

10. The main factor contributing to crystal size is rate of cooling
The main factor contributing to crystal size is rate of cooling! Textures based on crystal size: Aphanitic: almost no visible crystals Porphyritic: large crystals in a matrix of smaller crystals Phaneritic: interlocking large crystals Vesicular: full of holes (vesicles) Glassy: resembles glass Pyroclastic: made up of fragments (often ash and pumice)

11. Intrusive (Plutonic) Mafic- roots of plutons
Gabbro
Intermediate- near the base of plutons
Diorite
Felsic- most plutons (very common!)
Granite

12. Extrusive (volcanic)
Mafic- shield volcanoes, cinder cones, ocean basalts
Basalt
Scoria
Intermediate- composite cones
Andacite
Dacite (more felsic)
Felsic- domes and pyroclastic deposits
Rhyolite
Tuff
Obsidian and pumice

13. Fine-grained igneous texture

14. Course-grained igneous texture

15. Porphyritic igneous texture

16. Obsidian exhibits a glassy texture