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 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 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 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…