GEO 101 1st Edition Lecture 6 Outline of Last Lecture 1 Divergent Plate Boundaries 2 Divergent Plate Boundaries Rifting 3 Driving Mechanisms 4 Testing the Theory Evidence 5 Importance 6 The Rock Cycle Outline of Current Lecture 1 Igneous rocks 2 Igneous Rocks Plate Tectonics 3 2 components of magma 4 Evolution of magma 5 Changing Magma s Composition 6 Textures 7 Textures of Igneous Rocks a Aphanitic b Phaneritic c Porphyritic Texture d Glassy Texture 8 Competition of Igneous Rocks 9 Viscosity 10 Mafic Magma 11 Felsic Magma These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute 12 Naming Igneous rocks a Felsic b Intermediate c Mafic 13 Bowen s Reaction Series 14 Intrusive Igneous Rock Current Lecture Ch 4 Igneous Rocks Igneous Rocks From rock that was once a melt Magma Melt at depth magma If magma solidifies intrusive plutonic rocks Magma at surface lava Lava extrusive volcanic rocks Igneous Rocks Plate Tectonics Convergent Boundaries Divergent Boundaries Hot Spots 2 Components of Magma Liquid portion melt Maybe solids Volatiles dissolved gases in melt e g H2O CO2 Evolution of Magmas Over time a volcano may have lavas of different compositions Changing Magma s Composition Crystallization Settling differentiation Assimilation Magma mixing Textures Texture size shape arrangement of minerals Rate of cooling most important factor in crystal size Slow rate fewer but larger crystals Fast rate many small crystals We name igneous rocks based on textures and composition Textures of Igneous Rocks 4 types 1 Aphanitic fine grained texture a Rapid rate of cooling b Microscopic crystals 2 Phaneritic coarse grained texture a Slow cooling b Large crystals 3 Porphyritic texture a Indicates 2 stages of cooling b Large crystals phenocrysts in matrix of smaller crystals groundmass 4 Glassy texture a Very rapid cooling b Obsidian Composition of Igneous Rocks 4 categories of igneous rocks based on composition In other words each igneous rock has a set of minerals that combine to make it It is the percentage of each of these minerals that makes the categories Compositions Felsic o Nonferromagnesian silicates o Feldspar and silica o Silica rich SiO2 65 o Continental crust Intermediate o Contain 25 ferromagnesian minerals o 53 65 silica Compositions Mafic o Ferromagnesian silicates Ca rich feldspar o Silica poor 45 52 o Oceanic crust volcanic islands Ultramafic o Rare o Entirely ferromagnesian silicates o 45 silica Silica content indicates composition 45 to 70 Si range in crust Viscosity m Resistance to flow flows slowly flows easily Viscosity Resistance to flow o Low flow easily e g o High flow slowly e g Silica content influences magma Mafic magma low low o Si content o E g Hawaii Felsic magma high high o Si content o E g Cascades Viscosity m Resistance to flow flows slowly flows easily Mafic magma Si content E g Hawaii Felsic magma Si content E g Cascades Naming Igneous Rocks Felsic 1 Rhyolite a Volcanic extrusive felsic b Aphanitic texture 2 Granite a Intrusive equivalent of Rhyolite b Phaneritic texture Intermediate 3 Andesite a Volcanic extrusive intermediate comp b Aphanitic texture 4 Diorite a Intrusive equivalent of andesite b Phaneritic texture Mafic 5 5 Basalt a Volcanic extrusive mafic b Aphanitic texture 6 Gabbro a Intrusive equivalent of basalt b Phaneritic texture c Most of oceanic crust 7 Obsidian a Dark colored b Glassy texture 8 Pumice a Volcanic extrusive b Glassy texture c Frothy 9 Pyroclastic Rocks a Ejected during eruptions b Varieties i Tuff ash sized fragments ii Volcanic breccia particles ash Bowen s Reaction Series All magmas derived from parent mafic magma As temperature decreases mineral crystallization occurs Minerals do not crystallize simultaneously from cooling magma instead crystallize in predictable sequence Intrusive Igneous Rock Magma emplaced at depth cools solidifies called pluton o Duke tabular discordant pluton o Still tabular concordant pluton o Laccolith Lens or mushroom shaped pluton Arches overlying strata upward o Batholith Largest pluton 100 km2 smaller bodies termed stocks