1. How does igneous rock form? From magma which cools and hardens.
IGNEOUS ROCKS
How does igneous rock form?
From magma which cools and hardens.

2. How does magma form?
When temperature increases above the melting point of the minerals in the rock
Rocks melt at lower temps under lower pressure (excess pressure removed)
When fluid such as water is added (decreases the melting point)

3. Partial melting Different minerals have different melting points
The composition of the magma changes as different minerals melt at different temperatures
This process is called partial melting

4. Fractional crystallization
Opposite process of partial melting
Chemicals in magma combine to form minerals, each mineral has a different freezing point.
Highest freezing points (freeze at the highest temperature) crystallize first.
As minerals crystallize they remove specific chemicals from the magma changing its composition

5. TEXTURES Determined by size of their crystals and grains
Size of crystals and grains determined by the cooling rate of the magma

6. Intrusive Igneous Rock Extrusive Igneous Rock
Cools deep inside the crust
Slowly cools and hardens
Hint: think INterior
Cools at Earth’s surface
Hint: think EXterior

7. Coarse-Grained Igneous Rock
Example: QUARTZ-DIORITE
Composed of large mineral crystals.
Cools slowly allowing minerals to form large well developed crystals.

8. Fine- Grained Igneous Rock
Example: BASALT
Composed of small mineral grains that you can’t see with the naked eye.
Magma cools too rapidly to form large crystals.

9. Glassy Igneous Rock Example: OBSIDIAN
Highly viscous magma that cools too quickly for crystals to grow.

10. Porphyritic Igneous Rock
Example: VOLCANIC BRECCIA
This texture forms when magma cools slowly at first, then cools more rapidly as it nears Earth’s surface.
Larger crystals are imbedded within the smaller ones.

11. Vesicular Texture Igneous Rocks
Example : PUMICE
When magma contains a large amount of gases and cools rapidly, the gases become trapped as bubbles in the rock.
This produces a rock full of holes called vesicles

12. Composition of Igneous Rocks
Mafic
Felsic
Intermediate

13. FELSIC Light colored Large proportion of Silica
Main mineral components- potassium feldspar, and quartz
INCLUDES granite, rhyolite, obsidian and pumice

14. Mafic Dark in color Lower silica; Rich in Fe and Mg
Main mineral components: Plagioclase feldspar and Pyroxene minerals.\
INCLUDES basalt and gabbro

15. INTERMEDIATE
May contain lower proportion of silica then felsic rocks but higher proportions than mafic
Made up of Plagioclase feldspar, hornblende, pyroxene, biotite mica
Includes diorite and andesite

16. INTRUSIVE IGNEOUS ROCKS
Batholiths- spread over at least 100 km2 (Half Dome, Yosemite)
Stocks- spread over less than 100 km2 (Wilson Peak, CO)

17. Laccoliths – as magma flows between rock layers and spreads upwards, it sometimes pushes the overlying rocks into a dome
Sills and Dikes - When magma flows between layers of rock and hardens a sill forms parallel to the rock layers
When magma forces itself through cracks in existing rock layers or create new ones, a dike forms

18. EXTRUSIVE IGNEOUS ROCKS
Volcanic neck
Solidified central vent of a volcano