Geology Study Guide
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Discern between Magma and Lava.
Magma is molten rock deep in the Earth’s crust. Lava is magma flowing out onto the Earth surface.
Compare and contrast intrusive and
extrusive igneous rocks (name specific examples of each).
Intrusive Igneous Rocks form deep within the Earth’s crust, cool slowly, and form large crystals while Extrusive Igneous Rocks form on the Earth’s surface, cool quickly, and form small crystals.
- Intrusive Igneous Rocks:
- Granite, Gabbro
- Extrusive Igneous Rocks:
- Rhyolite, Basalt
List and briefly describe the three factors affect
Rate of cooling
When igneous rocks cool slowly, they form large crystals. When igneous rocks cool rapidly, they form small crystals.
Amount of silica present
Increase silica, viscosity increases, smaller crystal size
- Decrease silica,
- viscosity decreases, larger crystal size
Amount of dissolved gases
- Gases decrease
- viscosity, increase crystal grain sizes
What two characteristics do we use to classify igneous rocks?
Texture and Mineral Composition
Compare and contrast various igneous rocks, based on texture and composition.
Granite: Phaneritic, Felsic
Rhyolite: Aphanitic, Felsic
Diorite: Phaneritic, Andesitic (Intermediate)
Andesite: Aphanitic, Andesitic (Intermediate)
Gabbro: Phaneritic, Basaltic (Mafic)
Basalt: Aphanitic, Basaltic (Mafic)
List the various igneous textures and explain their origins.
- Phaneritic (visible)
- Large Crystals
- Aphanitic (Not Visible)
- Small Crystals
- Porphyritic (Both)
- Large and Small Crystals
- Magma chamber moves as it cools toward the Earth’s surface
- Made up of Silica, cools rapidly
- No Crystals
- Fragments of rock from a volcanic eruption
- Chucks of ash
- Super Large Crystals (greater than one centimeter)
Describe the various igneous compositions.
- Felsic (Granitic)
- High in silica (less that 25% dark silicates)
- Intermediate (Andesitic)
- Mix of Felsic and Mafic (25% - 45% dark silicates)
- Mafic (Basaltic)
- Rich in iron and magnesium (45%-85% dark silicates)
- Less than 10% feldspar (greater than 85% dark silicates)
How do you determine the composition of an igneous rock?
- Light colors: Felsic
- Dark Colors: Mafic
- Green: Ultramafic
Discuss silica content, its role as an indicator of
composition, and its role in magma behavior.
The higher the silica content, the more viscous the magma is.
Explain/draw Bowen’s Reaction Series and note how it relates to the composition of igneous rocks.
Model that predicts the order minerals crystallize.
List/discuss 4 factors that affect the melting of
Heat – Higher temperature, more melting
Pressure – Increasing pressure increases melting temperature, less melting. Decreasing pressure decreases melting temperature, more melting. (e.g. decompression melting)
Volatiles – Water lowers pressure, melts better.
Time – Some rocks must be exposed to high temperatures for a long period of time.
- Different minerals crystallize at different temperatures. When crystallization occurs,
- the magma’s composition changes when the melt separates several elements to
- form crystallized minerals. The last minerals to melt are the first to crystallize.
Rising magma chemically evolves when collecting dissolved elements from the surrounding rock in the magma chamber.
Briefly explain the concept of partial melting and how it relates to magma formation.
The incomplete melting of rock. As rocks heat up, minerals with the lowest melting point melt first. If melting is complete, magma composition will approach the composition of the rock.
In general is magma more felsic or mafic than its parent rock? Why?
Tectonically, where might we expect to find mafic magmas, intermediate magmas, and felsic magmas?
- Mafic – Hot Spots
- Intermediate – o/c convergent boundaries
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