Igneous Rocks Fire’s Within

Why Should We Study Rocks? It’s like a good history book – it tells us Earth’s long history

Three Types of Rocks Igneous Rocks –Formed from crystallization of molten magma Sedimentary Rocks –Sediment of pre- existing rock Metamorphic Rocks –Changed rock

Igneous Rocks Parent Material Magma –Molten rock found at depth –Includes dissolved gases and crystals Lava –Magma that reaches Earth’s surface Crystallization –Formation of a crystalline solid from a liquid “Ignis” is Latin for Fire

Igneous Rock Classification Texture Composition Important characteristics that reveal a lot about the environment the rock formed.

Igneous Rocks Texture – Volcanic (extrusive) – Plutonic (intrusive) Size, shape, and distribution of crystals Function of its cooling history

Igneous Rocks Volcanic (extrusive) Rocks that form at the Earth’s surface. Cools quickly

Igneous Rock Texture Extrusive Vesicular – Air bubbles Basalt Pumice Obsidian Aphanitic – Fine grained Glassy – Glass like

Igneous Rocks Plutonic (intrusive) Rocks that form beneath the Earth’s surface Cools slowly

Igneous Rock Texture Intrusive Phaneritic – coarse grained Granite

Igneous Rock How was this rock formed? Dacite Two-stage cooling history Contains both coarse- and fine-grained crystals - Porphyritic

Igneous Rocks Mineral Composition Felsic Mafic – light-colored rocks – dark-colored rocks – composed mostly of Si and Al – i.e. granite – composed mostly of Fe and Mg – i.e. basalt

Igneous Rocks Naming Rocks

Origin of Magma How does solid rock melt? Heat – geothermal gradient (~25°C/km) Pressure – decompression melting Volatiles – addition of H 2 O

Magma Formation Partial Melting

Magma Evolution Magmatic Differentiation –Bowen’s Reaction Series –Assimilation –Crystal Settling –Magma Mixing

Magma Evolution Magmatic Differentiation Formation of one or more secondary magmas from a single parent magma

Magmatic Differentation Bowen’s Reaction Series Geologist N.L. Bowen found that minerals tend to form in specific sequences of in igneous rocks

Magma Evolution Magmatic Differentiation Assimilation –Incorporates surrounding foreign rock Crystal Settling –Heavier minerals settle at the bottom of the chamber Magma Mixing –One magma body intrudes another