1. Earth & Space Science Chapter 5
Igneous Rocks

2. The Rock Cycle

3. 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
Temperature—increases with depth
Pressure—increases with depth
Water content—increase in water decreases melting point
Mineral content

4. Bowen’s Reaction Series

5. Texture of Igneous Rocks
Intrusive
coarse-grained—see individual crystals
form underground
cool slowly
ex: granite, gabbro, diorite

6. 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

7. 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.

8. Composition of Igneous Rocks
Silica content determines color (except obsidian)
Light color (felsic)
High silica
Lower density
Examples
granite
rhyolite
pumice
obsidian

9. Composition of Igneous Rocks
Dark color (mafic)
Low silica
More iron and magnesium
More dense
Examples:
Gabbro
Basalt
Scoria (vesicular basalt)

10. 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)

11. Classifying Igneous Rocks

12. Uses of Igneous Rocks 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…