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Earth & Space Science
Chapter 5
Igneous Rocks
The Rock Cycle
Formation of Igneous Rocks
• Two important factors:
– Composition
• Basaltic: low silica (quartz) content
• Andesitic: intermediate silica
• Rhyolitic/granitic: high silica
– Rate of crystallization is related to
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Temperature—increases with depth
Pressure—increases with depth
Water content—increase in water decreases melting point
Mineral content
Bowen’s Reaction Series
Texture of Igneous Rocks
Intrusive
• coarse-grained—see individual crystals
• form underground
• cool slowly
• ex: granite, gabbro, diorite
Texture of Igneous Rock
Extrusive
• fine-grained, vesicular, or glassy
• form at or near Earth’s surface
• cool quickly
• ex: scoria, rhyolite, obsidian, basalt, pumice
What determines texture?
• Demonstration…group game…
• More time (slower cooling rate) allows bigger
crystals to form.
• Intrusive rock cool more slowly, producing
coarse-grained rocks.
Composition of Igneous Rocks
Silica content determines color (except obsidian)
Light color (felsic)
• High silica
• Lower density
• Examples
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granite
rhyolite
pumice
obsidian
Composition of Igneous Rocks
Dark color (mafic)
• Low silica
• More iron and magnesium
• More dense
• Examples:
– Gabbro
– Basalt
– Scoria (vesicular basalt)
Why is silica important?
Silica content determines:
• What type of rock forms from the magma/lava
• The temperature at which the rock melts or
crystallizes
• Characteristics of volcanoes
– High silica: violently explosive (Yellowstone)
– Intermediate silica: moderate (Mt. St. Helens)
– Low silica: gentle, frequent (Kilauea, Hawaii)
Classifying Igneous Rocks
Uses of Igneous Rocks
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Metal ores/veins within igneous rock
Pegmatites (veins of other minerals)
Kimberlites (Diamond mines)
Construction
– Granite is very resistant to weathering
• Crystals interlock
• Composed of weather-resistant minerals
• Name something that is made of granite…