igneous rocks. i.igneous rocks are formed from molten material. - the term igneous is derived from...
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IGNEOUS ROCKS
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I. Igneous rocks are formed from molten material.
- The term igneous is derived from Latin term meaning “from FIRE”!!
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A. Extrusive igneous rocks are formed when moltenrock cools above ground. Usually they are formedafter the material has been erupted by a volcano.
1. This molten materialcools quickly.
2. No crystals are visibleto the eye.
ENVIRONMENT
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B. Intrusive igneous rocks are formed when molten rock cools below ground.
1. These rocks cool very slowly.
2. There is lots of time for visible crystals to grow
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Cools quickly Cools slowlyCools above ground Cools below groundNo visible crystals Visible crystals
EXTRUSIVE INTRUSIVE
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TEXTURE
• Coarse:
– INTRUSIVE igneous rock composed of large mineral grains.
• GRANITE
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TEXTURE
• Fine:
– EXTRUSIVE igneous rock composed of fine-grained minerals.
• BASALT
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TEXTURE
• Glassy:
– HIGHLY VISCOUS, silica-rich magma cools rapidly, no crystals.
• OBSIDIAN
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TEXTURE
• Vesicular:
– HIGHLY VISCOUS, large amount of dissolved gases, rock full of holes.
• PUMICE
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COMPOSITION• Mafic:
– Low silica– Rich in Iron and Magnesium
• DARK COLOR.
• BASALT
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COMPOSITION
• Felsic:
– High silica• LIGHT COLOR.
• RHYOLITE
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Igneous Rock Structures
• Intrusions vs. Extrusions:
- Form Underground
- Form on the surface of the Earth
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INTRUSIONS• Batholiths:
– At least 100 km2 of surface exposure.
• Means “Deep Rock”
• Sierra Nevada Mountains
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Exposed Batholiths
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INTRUSIONS• Laccoliths:
– Flat-bottom intrusion that pushes over-lying rock layers into an arc.
• Means “Lake of Rock”
• Black Hills–South Dakota
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INTRUSIONS• Stocks:
– Like a Batholith, but covers less than 100 km2.
• Devils Tower–Wyoming
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INTRUSIONS• Dikes:
– Igneous intrusion that cuts across rock layers.
• Black Canyon–Colorado
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INTRUSIONS• Sills:
– Hardened magma formed between parallel layers of rock.
• Salisbary Crags–Scotland
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EXTRUSIONS• Volcanic Necks:
– Solidified central neck of a volcano.
• Shiprock–New Mexico