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Lecture 2 Igneous Rocks

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Published on September 20, 2007

Author: Sharck

Source: authorstream.com

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Igneous Rocks :  Igneous Rocks Rock Cycle – describes the way the earth recycles itself over time Igneous Rocks:  Igneous Rocks Igneous Rocks – crystallize from magma (molten rock) Extrusive (volcanic) – fine-grained forming on surface of earth Intrusive (plutonic) – coarse-grained forming beneath earth’s surface Plutonic Rocks:  Plutonic Rocks Evidence for magmatic origin of intrusive rocks 1. Plutonic rocks are mineralogically and chemically identical to volcanic rocks. 2. Volcanic rocks are fine-grained indicating rapid cooling, plutonic rocks are coarse-grained indicating slow cooling. Plutonic Rocks:  Plutonic Rocks Evidence for magmatic origin of intrusive rocks (cont’d) 3. Experimental evidence indicates high temperatures and/or high pressures are needed to form observed minerals. Plutonic Rocks:  Plutonic Rocks Evidence for magmatic origin of intrusive rocks (cont’d) 4. Contacts between plutonic rocks and country rock are observed. These include 'Baked' zones, and 'Chill zones' along contacts between plutonic and country rocks. 5. 'Xenoliths' Pieces of country rock are found in plutonic rocks Plutonic Rocks:  Plutonic Rocks Igneous rock textures – range from very fine-grained to very coarse-grained (pegmatites) Plutonic Rocks:  Plutonic Rocks Igneous rocks are classified according to the grain size, mineralogy, and chemical composition Plutonic Rocks:  Plutonic Rocks Intrusive igneous structures – based on 1. Size - Is the intrusion large or small? 2. Shape - Does the intrusion have a particular geometric shape? 3. Depth of formation - Did the intrusion form at great depths or was it shallow (andlt;2 km)? 4. Orientation - Does the intrusion follow layering in the intruded county rock? Plutonic Rocks:  Plutonic Rocks Intrusive igneous structures : Volcanic neck - solidified throat of old volcano Plutonic Rocks:  Plutonic Rocks Intrusive igneous structures Dike - tabular, discordant, intrusive structure Sill - tabular, concordant, intrusive structure Plutonic Rocks:  Plutonic Rocks Intrusive igneous structures Diapirs – magma that moves up through crust Pluton - magma that crystallized deep within the crust (classified based on size): stock (less than 100 sq km exposed), batholith (greater than 100 sq km exposed) Igneous Rocks:  Igneous Rocks Geothermal Gradient - rate at which the temperature increases with increasing depth Hot spots (Mantle plumes) – high geothermal gradient Igneous Rocks:  Igneous Rocks Factors that control melting temp of rocks andamp; minerals Pressure - the higher the pressure, the higher the temperature required to melt a mineral or rock. Water content/pressure - the presence of water under pressure allows a mineral or rock to melt at a lower temperature than if the material were dry. Mineral mixtures - a mixture of two or more minerals can cause the temperature needed to melt either mineral alone to decrease Igneous Rocks:  Igneous Rocks Evolution of magmas Bowen’s Reaction Series - idealized crystallization sequence in which minerals form from a cooling magma (high temp to low temp) Mafic Intermediate Silicic Igneous Rocks:  Igneous Rocks Evolution of magmas Differentiation - process by which different ingredients separate from an originally homogeneous mixture Igneous Rocks:  Igneous Rocks Evolution of magmas Partial Melting - process by which a magma source is only partially melted Igneous Rocks:  Igneous Rocks Evolution of magmas Assimilation - process by which a magma melts away some of the country rock and incorporates the newly molten material into the magma Igneous Rocks:  Igneous Rocks Evolution of magmas (cont’d) Magma Mixing - process by which two magmas with different compositions combine to form a composition intermediate to the two original magma compositions Igneous Rocks:  Igneous Rocks Evolution of magmas (cont’d) Assimilation of crustal rock Melting of sedimentary rock Igneous Rocks:  Igneous Rocks Igneous processes in context of Plate Tectonics Divergent Boundaries - magma comes to surface and crystallizes on or in shallow crust (mid-ocean spreading ridges, primarily basalts andamp; ultramafics) Igneous Rocks:  Igneous Rocks Igneous processes in context of Plate Tectonics Intraplate - magma reaches surface or crystallizes on or in shallow crust via hot mantle plumes (oceanic - Hawaiian islands, basalts; continental - Yellowstone, silicic) Igneous Rocks:  Igneous Rocks Igneous processes in context of Plate Tectonics Convergent Boundaries - magma reaching the surface forms andesites, magma that crystallizes below the surface forms granites (subduction zones andamp; collision zones along plate boundaries) Volcanism and Extrusive Rocks:  Volcanism and Extrusive Rocks Lava - magma that reaches the surface as a result of volcanism (lava flows andamp; pyroclastic eruptions) Volcanism and Extrusive Rocks:  Volcanism and Extrusive Rocks Pyroclasts (tephra) - rock fragments resulting from rapid crystallization associated with an explosive eruption Volcanism and Extrusive Rocks:  Volcanism and Extrusive Rocks The explosiveness of volcanic eruptions are controlled by two factors: 1. The amount of gas trapped in the magma; 2. The ease or difficulty with which the gas can escape Volcanism and Extrusive Rocks :  Volcanism and Extrusive Rocks Ability of a gas to escape from the lava is controlled by the viscosity (resistance to flow) Lava viscosity is controlled by the temperature of the lava relative to its solidification temperature; the silica content; and of lesser importance the amount of gas dissolved in the magma Volcanism and Extrusive Rocks :  Volcanism and Extrusive Rocks Silica content andamp; temp Silica-rich (andgt;65% silica andamp; low temp ~700°C) - Rhyolite Intermediate (65-50% silica andamp; temp ~900°C) - Andesite Silica-poor (andlt;50% silica andamp; high temp ~1200°C) - Basalt Volcanism and Extrusive Rocks :  Volcanism and Extrusive Rocks Gases commonly found in lava includes: water vapor, carbon dioxide, sulfur dioxide, hydrogen sulfide, and hydrochloric acid Pyroclastic flows are dense mixtures of gas and pyroclastic debris that generally stay close to the ground Volcanism and Extrusive Rocks :  Volcanism and Extrusive Rocks Volcanic textures –Mineral grain size is controlled by rate of cooling, and viscosity. A slower rate of cooling and low viscosity results in larger mineral grains; rapid cooling and a high viscosity prevents development of large mineral grains Volcanism and Extrusive Rocks :  Volcanism and Extrusive Rocks Typical volcanic textures include: Glassy (obsidian) – high silica content with rapid cooling Porphyritic - presence of large crystals (phenocrysts) in a groundmass or matrix of finer-grained minerals or glass Volcanism and Extrusive Rocks :  Volcanism and Extrusive Rocks Typical volcanic textures include: Vesicular - cavities resulting from trapped gas bubbles (Scoria - highly vesicular basalt with more gas space than rock; Pumice - silicic lava where the gases are churned into a froth resulting in a rock with more air than rock Pyroclastic – particles range from dust and ash, to cinders, to bombs and blocks Volcanism and Extrusive Rocks :  Volcanism and Extrusive Rocks Rocks that consist primarily of pyroclastic material are called tuff (ash andamp; dust size particles) or volcanic breccia (cinders, bombs, blocks) depending on grain size Other volcanic features include: Vents – openings where volcanic material is released Crater – basin like depression at the summit of volcanic cone Flank eruption - eruption from vents along the side of a volcano Caldera - large volcanic depression remaining from either a volcanic explosion or collapse Volcanism and Extrusive Rocks :  Volcanism and Extrusive Rocks Volcanism and Extrusive Rocks :  Volcanism and Extrusive Rocks Major types of volcanoes Shield volcanoes - broad, sloping cones constructed of solidified basaltic lava flows with slopes between 2° and 10° from the horizontal Cinder cones - volcano formed from pyroclasts ejected from a central vent with slopes of about 30° Composite volcanoes - constructed of alternating layers of pyroclastics and lava flows, which can form with steep slopes and become very large Volcanism and Extrusive Rocks :  Volcanism and Extrusive Rocks Other volcanic features include: Volcanic domes - steep-sided, dome- or spine-shaped masses that form from viscous lava that solidifies in or very near a volcanic vent (example, Mount St. Helens) Flood basalts (Plateau basalts) - nonviscous mafic lava flows that consist of individual lava flows 10's of meters thick, that can cover thousands of square kilometers and attain a cumulative thickness of andgt;3,000 meters Volcanism and Extrusive Rocks :  Volcanism and Extrusive Rocks Other volcanic features (cont’d): Columnar jointing – tension fractures in basalt caused by contraction cooling after solidifying that form parallel, vertical columns that are usually six-sided Pillow basalts - pillow-shaped rounded masses that are formed closely together and are the result of underwater eruptions Volcanism and Extrusive Rocks :  Volcanism and Extrusive Rocks Most of the largest volcanoes occur in two regional belts, Circum-Pacific belt or 'Ring of Fire'; and Mediterranean belt, which includes Mt Vesuvius and Mt Etna

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