Chapter 4: Igneous Rocks

Introduction Igneous rocks = formed from “fire” Magma = completely or partially molten rock Lava = magma which reaches surface Melt = liquid portion of partially melted rock Crystallization = when magma cools, ions arrange themselves into orderly crystalline structures Extrusive = erupts on surface “Volcanic” from god Vulcan Intrusive = cools under ground “Plutonic” from god Pluto Volatiles = gases & liquids dissolved into magma Igneous rocks = formed from “fire” Magma = completely or partially molten rock Lava = magma which reaches surface Melt = liquid portion of partially melted rock Crystallization = when magma cools, ions arrange themselves into orderly crystalline structures Extrusive = erupts on surface “Volcanic” from god Vulcan Intrusive = cools under ground “Plutonic” from god Pluto Volatiles = gases & liquids dissolved into magma

Crystal Size Effected By … Cooling rate Amount of silica Amount of dissolved gases Igneous rocks classified by texture & composition Cooling rate Amount of silica Amount of dissolved gases Igneous rocks classified by texture & composition

Igneous Textures Aphanitic Vesicular Phaneritic Porphyritic Phenocrysts Groundmass Aphanitic Vesicular Phaneritic Porphyritic Phenocrysts Groundmass Glassy Viscosity Pyroclastic AKA Fragmental Pegmatitic

Aphanitic Texture Fine-grained “A” = not “Phaner” = visible Rapid cooling Characterized by color Light Intermediate dark Usually extrusive / volcanic Fine-grained “A” = not “Phaner” = visible Rapid cooling Characterized by color Light Intermediate dark Usually extrusive / volcanic

Vesicular Texture Type of aphanitic rock Vesicule = void left by gas bubble Spherical or elongate holes Usually form in upper layers of extrusive rocks Type of aphanitic rock Vesicule = void left by gas bubble Spherical or elongate holes Usually form in upper layers of extrusive rocks

Phaneritic Texture Coarse-grained “Phaner” = visible Slow cooling Minerals can be identified Usually intrusive Exposed at surface after much erosion Coarse-grained “Phaner” = visible Slow cooling Minerals can be identified Usually intrusive Exposed at surface after much erosion

Porphyritic Texture Large crystals in matrix of smaller crystals Phenocryst = large crystals Groundmass = surrounding smaller matrix Due to minerals forming at different temps & rates Large crystals in matrix of smaller crystals Phenocryst = large crystals Groundmass = surrounding smaller matrix Due to minerals forming at different temps & rates

Glassy Texture High SiO 2 content forms chains which impede movement of ions High viscosity e.g., Obsidian Quick cooling also prevents crystallization e.g., Pele’s hair High SiO 2 content forms chains which impede movement of ions High viscosity e.g., Obsidian Quick cooling also prevents crystallization e.g., Pele’s hair

Pyroclastic Texture AKA Fragmental Texture Forms from consolidation of rock fragments from violent eruption Large range in particle sizes Angular pieces AKA Fragmental Texture Forms from consolidation of rock fragments from violent eruption Large range in particle sizes Angular pieces Montserrat, 1996

Pegmatitic Texture Abnormally large crystals > 1 cm Late stages of crystallization Rich in volatiles Composition ~ granite Abnormally large crystals > 1 cm Late stages of crystallization Rich in volatiles Composition ~ granite

Igneous Compositions Composed primarily of silicate minerals Dark (Ferromagnesian) minerals Examples: olivine, pyroxene, amphibole, biotite mica Light (nonferromagnesian) minerals Examples: quartz, feldspars, muscovite mica Composed primarily of silicate minerals Dark (Ferromagnesian) minerals Examples: olivine, pyroxene, amphibole, biotite mica Light (nonferromagnesian) minerals Examples: quartz, feldspars, muscovite mica

Granitic vs. Basaltic Rocks Granitic Composition Light-colored silicates Rich in SiO 2 Up to 70% AKA Felsic From feldspar & silica AKA Silicic Major constituent of continental crust Granitic Composition Light-colored silicates Rich in SiO 2 Up to 70% AKA Felsic From feldspar & silica AKA Silicic Major constituent of continental crust Basaltic Composition Dark silicates & Ca-rich feldspar Low in SiO 2 ~45% AKA Mafic From magnesium & ferric (iron) Higher density than felsic rocks Major constituent of ocean floor & many islands

Other Compositions Intermediate Contains ~25% dark silicates AKA Andesite & rhyotlie Associated with dome- building & explosive eruptions Associated with subduction zones Intermediate Contains ~25% dark silicates AKA Andesite & rhyotlie Associated with dome- building & explosive eruptions Associated with subduction zones Ultramafic Rare composition Rich in Mg & Fe Poor in SiO 2 ~40% Composed entirely of ferromagnesian silicates e.g. olivine & pyroxene Peridotite & komatiite

Silica Indicates Composition Exhibits considerable range in crustal rocks 40% - 73% SiO 2 Influences magma behavior Granitic magmas More SiO 2 = more viscous => more explosive Basaltic magmas Less SiO 2 = more runny => more like fluid Exhibits considerable range in crustal rocks 40% - 73% SiO 2 Influences magma behavior Granitic magmas More SiO 2 = more viscous => more explosive Basaltic magmas Less SiO 2 = more runny => more like fluid

Igneous Rock Continuum

Silica Indicates Composition Exhibits considerable range in crustal rocks 40% - 73% SiO 2 Influences magma behavior Granitic magmas More SiO 2 = more viscous => more explosive Basaltic magmas Less SiO 2 = more runny => more like fluid Exhibits considerable range in crustal rocks 40% - 73% SiO 2 Influences magma behavior Granitic magmas More SiO 2 = more viscous => more explosive Basaltic magmas Less SiO 2 = more runny => more like fluid

Naming Igneous Rocks

Silicic Composition >25% Quartz ~40% Feldspar Intrusive = Granite Very abundant Associated w/ mountain building Extrusive = Rhyolite May contain glass & vesicles Less common than granite >25% Quartz ~40% Feldspar Intrusive = Granite Very abundant Associated w/ mountain building Extrusive = Rhyolite May contain glass & vesicles Less common than granite

Glassy Composition Obsidian Dark-colored glassy rock Forms when SiO 2 -rich lava cools quickly Pumice Forms when large amounts of gas escape Frothy texture Usually found with obsidian Obsidian Dark-colored glassy rock Forms when SiO 2 -rich lava cools quickly Pumice Forms when large amounts of gas escape Frothy texture Usually found with obsidian

Intermediate Composition 60%-67% SiO 2 Intrusive = Diorite Phaneritic Extrusive = Andesite Named for Andes Mts. Aphanitic / Porphyritic 60%-67% SiO 2 Intrusive = Diorite Phaneritic Extrusive = Andesite Named for Andes Mts. Aphanitic / Porphyritic

Basaltic Composition 45%-50% SiO 2 Composed of pyroxene & Ca-rich feldspar Intrusive = Gabbro Extrusive = Basalt Most common extrusive rock on Earth 45%-50% SiO 2 Composed of pyroxene & Ca-rich feldspar Intrusive = Gabbro Extrusive = Basalt Most common extrusive rock on Earth

Porphyritic Rocks Composed of fragments ejucted during eruption Varieties: Ash Flow = loose agglomeration of ash- sized particles from fallout in ash cloud Tuff = lithified ash layer Welded tuff = hot enough to fuse Volcanic Breccia = particles larger than ash, usually angular & poorly sorted Composed of fragments ejucted during eruption Varieties: Ash Flow = loose agglomeration of ash- sized particles from fallout in ash cloud Tuff = lithified ash layer Welded tuff = hot enough to fuse Volcanic Breccia = particles larger than ash, usually angular & poorly sorted

Origin of Magma Highly debated topic Formation of Magma from Solid Rock Role of Heat Role of Volatiles Role of Pressure Highly debated topic Formation of Magma from Solid Rock Role of Heat Role of Volatiles Role of Pressure

Evolution of Magmas Fractional Crystallization Crystal Settling Magmatic differentiation Assimilation Magma Mixing Fractional Crystallization Crystal Settling Magmatic differentiation Assimilation Magma Mixing

Bowen’s Reaction Series

Assimilation & Magma Mixing