GY111 Earth Materials Lecture 4 Igneous Rocks Types of Rocks Rock an aggregate of one or more minerals Igneous Rocks crystallize from a magma Sedimentary Rocks Clastic formed by the erosion of pre existing rocks Chemical Biochemical precipitated from chemical reactions Metamorphic Rocks formed by exposure to extreme heat pressure below the melting point Magma Magma is generated in the interior earth by heat from radioactive minerals Volcanic eruptions prove that magma exists near the surface of the earth Laboratory studies verify that common rocks will melt at the T P inside the earth Coarse grained igneous rocks prove that magma must cool slowly and the only way that that can happen is that the surrounding rocks must be almost as hot as the magma itself Intrusive Igneous Rocks Cool slowly at depths 1 km Form coarse grained textures Surrounding rock is termed country rock May contain portions of the country rock that fall into the original magma chamber forming a xenolith Extrusive Igneous Rocks Form on the Earth s surface Lava flow of magma onto the Earth s surface Pahoehoe ropy surface low viscosity Aa fragmental surface high viscosity Pyroclastic rocks form from the explosive eruption of volcanoes Ash particles of glass Tuff a rock composed of fragments of pre existing rock in an ash matrix Pumice a rock so full of voids vesicles that it can float in water S G 1 0 Obsidian massive volcanic glass Lava Flow Types Pahoehoe ropy Aa fragmented Igneous Textural Terms Aphanitic mineral grains in rock are too small to be identified with a hand lens rock cooled from magma rapidly Phaneritic minerals grains in rock are large enough to be identified with a hand lens rock cooled relatively slowly Phenocrysts crystals that are distinctly larger than surrounding mineral grains Porphyritic a texture where relatively large phenocryst mineral grains are surrounded by smaller grains View of Textural Types Aphanitic Phaneritic Composition Felsic light colored igneous rock relatively rich in Si Na and K Intermediate rock made up of equal proportions light and dark minerals Mafic dark colored rock relatively rich in Ca Fe and Mg Ultramafic dark colored rock relatively rich in Fe and Mg Note red is considered a felsic light color green is considered a mafic dark color Where Different Igneous Textures Form Common Igneous Minerals Classification of Igneous Rocks Based on Mineral Content Texture Magma Formation Magma formation is favored by increasing temperature and decreasing pressure Magma formation is favored by increasing H2O content because it effectively lowers the melting point of minerals in rocks Several tectonic environments favor magma formation Divergent boundaries Hot Spots pressure reduction in upwelling mantle Decompression melting Convergent boundaries increasing temperature and water content in subducting slab frictional heating Granite Melting Curves Experimental results with actual granite rock displays effect of pressure and water 10 Convergent solid 8 P Kbar decompression melt 35 km Divergent Dry melting curve Subduction 20 km 6 Wet H2O melting curve solid melt 4 500 600 T Deg C 800 Fractional Crystallization Controlled by Bowen s Reaction Series Discontinuous Series Continuous Series Palisades Sill Example of Fractional Crystallization Early high temp crystals settle to the base of the magma chamber Palisades Sill cont The end result is a layered intrusion different layers have different compositions Forms of Magma Intrusions Batholith discordant 100 km2 Stock discordant 1 and 100 km2 Pluton discordant 1 km2 Dike discordant tabular Sill concordant tabular Laccolith concordant shield shaped Lopolith concordant saucer shaped Intrusive Forms Note laccoliths and lopoliths are not shown in this schematic Plate Boundary Associations Divergent Divergent Boundaries production of ophiolite sequences Ultramafic mantle partially melts to form basalt and gabbro mafic rocks While in contact with ocean water the ocean crust is hydrated and altered chemically seawater alteration Plate Boundary Associations Convergent Subducted ocean lithosphere partially melts to produce intermediate and felsic magma The hydration of the ocean lithosphere dramatically lowers its melting point leading to abundant felsic to intermediate magma generation Volcanic Landforms Central Vent Eruptions Shield Volcanoes low viscosity lava flows Volcanic domes viscous lava extruded as a dome after major pyroclastic eruption Cinder cones small low viscosity eruptions that spatter small fragments of lava that solidify as cinders Stratovolcanoes high viscosity pyroclastic eruptions build a steep sided cone Craters Calderas explosive eruptions will blast a small crater at the summit of a volcano or a large caldera for more violent eruptions Diatremes rapid intrusion of a very low viscosity carbonate rich magma Diamond bearing diatremes are termed Kimberlites Volcanic Landforms cont Central vent eruptions Shield Lava dome Cinder cone Stratovolcano Composite Caldera Caldera Formation Result from very large pyroclastic eruptions Super Eruptions The Yellowstone Caldera is one example Fissure Eruptions Flood Basalts large outpourings of low viscosity basaltic lava fills in low areas Ash Flow deposits result from the fissure eruption of felsic magma to produce extremely large pyroclastic flows Yellowstone Columbia River Flood Basalts An example of a fissure eruption of mafic lava Hydrothermal Vents Water rich liquid at high temperature Under high pressure water may have a temperature of over 400 deg C and still be a liquid phase Geysers interaction between groundwater and a volcanic magma chamber Hydrothermal veins important economic mineral sources boil off from magma during fractional crystallization Global Patterns of Volcanism Divergent low viscosity mafic magma with little or no H2O generate shield volcanoes Iceland Convergent high viscosity intermediate and felsic magma with abundant H2O generate stratovolcanoes Cascade Range Hot Spot low viscosity dry mafic magma produces shield volcanoes under ocean lithosphere Hawaii high viscosity wet felsic magma under continental lithosphere Yellowstone Exam Summary Know intrusive geometry classes Know textural terms aphanitic phaneritic etc Know common rock forming silicates in felsic intermediate etc compositions Know the characteristics of Shield versus Composite Cone volcanoes Be able to diagram Bowen s Reaction Series and describe the Pallisades Sill as an example or fractional