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