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GY111 Physical Geology 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 Divergent decompression 10 solid 8 P Kbar melt 35 km Dry melting curve Convergent 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 Palisades Sill as an example or fractional crystallization. Be able to describe the conditions that lead to the formation of aa, pahoehoe, pumice, obsidian, welded tuff, scoria. Be able to explain why some volcanoes extrude low-viscosity lava whereas others tend to erupt explosively. Relate low- versus high-viscosity magma to types of plate tectonic boundaries.