Download CHAPTER FIVE: IGNEOUS ROCKS

CHAPTER FIVE: IGNEOUS ROCKS
I. What are Igneous Rocks?
A. Origin of Igneous Rocks
1. Form from molten earth material
a) Magma: inside the Earth
b) Lava: on the Earth’s surface
B. Intrusive Igneous Rocks
1. Rocks that form below the Earth’s surface
2. Form from magma that cools and hardens
a) usually have large mineral grains
3. Appear at the Earth’s surface in two ways
a) the layers of rock covering them have been removed
OR
b) forces in the Earth have pushed the rock upward.
C. Extrusive Igneous Rocks
1. Rocks that form on the Earth’s surface
2. Form when lava flows over the Earth’s surface allowing it to cool and
harden.
3. These rocks show little or no visible mineral grains
a) have a fine grained texture
b) Ex. Obsidian, pumice, scoria
D. Factors that Affect Magma Formation
1. Temperature
a) geothermal gradient: temperature increases with depth
2. Pressure
a) pressure increases with depth
b) increased pressure increases melting point
3. Water Content
a) more water makes it melt easier
E. How rocks melt
1. Partial melting
a) some minerals in a rock melt while others remain solid
b) each mineral has a unique melting temperature
2. Fractional Crystallization
a) occurs when magma cools
b) crystallizes in the reverse order it melts
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F. Bowen’s Reaction Series.
1. As magma cools minerals form in a predictable pattern
2. Different minerals always form first
a) the minerals will depend on the composition of the magma
b) Quartz is often the last mineral to crystallize
II. Classifying Igneous Rocks
A. Mineral Composition
1. Mafic Igneous Rocks (Basaltic)
a). Characteristics
Dense
heavy
Dark-colored
b)Form from magma rich in iron and magnesium
2. Felsic (Granitic)
a) light colored
b) lower density
c) Magma is thick and stiff
d) contains a lot of silicon & oxygen
e) Granitic magma builds up a great deal of pressure causing
violent eruptions. Ex. Mt. St. Helens
3. Intermediate (Andesitic)
a) composition in between Basaltic & Granitic
b) more dense----------------------------------------------less dense
mafic
intermediate
felsic
c) most of the moon is Andesite, also makes up the Andes Mts.
B. Grain Size
1. Cooling Rates
a) lava cools quickly forming little or no crystals (extrusive rock)
b) magma cools slowly allowing large crystals to form (intrusive
rock)
C. Texture
1. Describes crystals
a) are all large or small?
b) are sizes mixed?
c) porphyritic texture: two different mineral grain sizes
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III. Igneous Rocks as Resources
A. Pegmatites
1. Vein deposit
a) form as magma intrudes through rock
2. Very large crystal sizes
a) very slow cooling produces large crystals
B. Kimberlites
1. Form from magma deep in the crust
a) 150-300km down
b) pressure in the Earth forces them up
2. Contain most of the world’s diamonds
*You must also know your vocabulary
* The test will contain several application style questions.
*There will be some open-ended questions also for completion (these are worth more)
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