1	Igneous Rocks
2	Igneous Rocks Formed when magma (molten rock) cools and crystallizes Two types Intrusive or plutonic Cool underground Extrusive or volcanic Cool above ground
3	Igneous Rocks Q: What do they both have in common?? A: Magma!
4	Key Concepts All rocks melt at different temperatures Magma is molten rock Magmas change composition as they rise, i.e. they EVOLVE
5	How Magma Forms Geothermal gradient Increasing temperature with increasing depth Partial melting of crust
6	How Magma Forms Pressure Mineral’s melting point increases with increasing pressure Pressure increases with depth Rocks that normally melt at the surface need higher temperature to melt underground Decompression melting: as hot mantle rock moves upward, the pressure decreases so that melting takes place
7	How Magma Forms Gas: water vapor Under high pressure, water breaks the silicon-oxygen bonds and causes the crystal to liquify. i.e. mineral’s melting temperature is decreased by water under high pressure
8	Magma Compositions Evolve Parent magma Source: Rock that has melted at the base of the crust, or in the mantle Composition: High in Fe, Mg. Low to no Si Daughter magma Source: Magma that has risen upward, & changed composition Composition: varying amounts of Fe, Mg & Si, depending on depth
9	From parent to daughter, a magma evolves
10	Magmas evolve Magma Mixing (not shown) Assimilation of Country Rock Differentiation of Magma Bowen’s Reaction Series Partial Melting of Crust of Lower Crust or Upper Mantle
11	1. Partial Melting Melting of Fe, Mg-rich upper mantle/lower crust produces Fe, Mg-rich magmas Called ultramafic (upper mantle) or mafic/basaltic (lower crust) Melting of Si-rich lower continental crust produces Si-rich magmas Called felsic, or intermediate Occurs in continental plate & subduction zones
12	2. Differentiation of Magmas Bowen’s reaction series is a well defined sequence by which minerals crystallize from a cooling melt High temperature minerals (Mg, Fe) crystallize FIRST As magma moves upward, it becomes richer and richer in silica (Si)
13	3. Assimilation of Country Rock Rising, hot magma can melt “country rock” Adds its minerals to the melt
14	4. Magma Mixing Rising bodies combine forming an intermediate composition
15	Magma Summary Felsic >65% < 35% continental settings, Granite, subduction zone Rhyolite Intermediate 50- 65% ~50% subduction zone Diorite, Andesite Mafic <50% >50% oceanic settings, Gabbro, intraplate-hot spots Basalt
16	Classifying Igneous Rocks
17	How do Extrusive and Intrusive Igneous Rocks Differ?
18	Intrusive vs. Extrusive Both are classified by texture and composition Texture: refers to the size of the individual mineral crystals within the rock, called ‘grains’ Extrusive rocks: fine-grained rocks (<1 mm) Forms small crystals that can only be seen when magnified (aphanitic texture) Intrusive rocks: coarse-grained rocks (>1mm) Forms larger crystals, visible to the eye (phaneritic texture)
19	How Textures Form Intrusive/Plutonic Formed as magma slowly rises through country rock Cools slowly so larger crystals form (atoms have time to align) = coarse-grained Extrusive/Volcanic Form crystals immediately after magma hits surface, because lava cools rapidly. No time for crystals to grow = fine-grained!
20	Origin of Intrusive/Plutonic Igneous Rocks
21	Igneous landscapes