Chapter 6 – IGNEOUS ROCKS

How, Why & Where Rocks Melt Begins as solid Molecules warm & begin vibrating = softening Molecules may vibrate violently enough to break bonds

How, Why & Where Rocks Melt Heat & pressure inside Earth –Magma chamber –Geothermal gradient Continental vs. ocean crust Composition varies melting point Pressure increases melting point Water decreases melting point

Heat and pressure inside Earth –Fractional melt (partial melting) Mix of molten & solid rock –1 magma body may produce several different igneous compositions How, Why & Where Rocks Melt

Magma (Intrusive) –Molten rock below Earth’s surface Lava (Extrusive) –Molten rock when it reaches Earth’s surface

Magma & Lava –Composition Silica content varies (~ 45-75%) Water vapor & carbon dioxide –Temperature Temperature varies (~ 750°C – 1200°C) –Viscosity (resistance to flow) Varies in ability to flow Influenced by silica content & temperature How, Why & Where Rocks Melt

Tectonic setting –Characteristics influenced by location Oceanic, divergent margins –Hot, low viscosity basaltic lava Subduction (convergent) zones –Cooler, viscous lavas with more silica Ocean hot spots –Hot & basaltic; build giant shield volcanoes Continental hot spots –Cooler & granitic; high silica lava How, Why & Where Rocks Melt

Cooling and Crystallization Crystallization –Process where mineral grains form & grow in cooling magma (or lava) –Classified based on: 1.Texture (size of mineral crystals) –Volcanic (extrusive) = small grains due to rapid cooling –Plutonic (intrusive) = large grains due to slow cooling 2.Composition (silica content)

Rate of Cooling Extrusive Textures –Glassy Cools too rapidly to form crystals Example: obsidian

Rate of Cooling Extrusive Textures –Aphanitic Fine grained (small crystals) Examples: basalt, andesite, rhyolite

Rate of Cooling Extrusive Textures –Vesicular Form from trapped gas bubbles Examples: pumice, scoria

Rate of Cooling Extrusive Textures –Pyroclastic or fragmental Includes rock fragments Example: volcanic tuff

Rate of Cooling Intrusive Texture –Phaneritic Course grained (large crystals); slow cooling inside Earth Examples: granite, syenite, diorite, gabbro, peridotite

Chemical composition Igneous rocks subdivided into 4 categories based on silica content –Felsic –Intermediate –Mafic –Ultramafic

Igneous Rock Classification Common Igneous Compositions Composition Type % Silica Other Elements Magma Viscosity Temperature crystallization begins Igneous Rocks Produced Type of Igneous Rock Felsic>65%Al, K, NaHigh C GranitePlutonic RhyoliteVolcanic Intermediate55-65% Al, Ca, Na, Fe, Mg Medium C DioritePlutonic AndesiteVolcanic Mafic45-55% Al, Ca, Fe, Mg Low C GabbroPlutonic BasaltVolcanic Ultramafic<40% Mg, Fe, Al, Ca Very low>1200C PeridotitePlutonic KomatiiteVolcanic

Igneous Rock Classification

Fractional Crystallization Crystals separate from liquids during crystallization –Bowens reaction series –Predictable melting & cooling of minerals

Plutons and Plutonism Plutons –Any body of intrusive igneous rock, regardless of size or shape Massive vs. Tabular Concordant vs. Discordant

Plutons & Plutonism Batholith –Large, irregular shaped pluton –Massive & Discordant Laccolith –Mushroom- shaped pluton –Massive & Concordant

Plutons and Plutonism Dikes –Magma squeezes into cross cutting fracture & solidifies –Tabular & Discordant Sills –Magma intrudes between 2 layers; parallel to layers –Tabular & Concordant

Plutons & Plutonism Volcanic pipe = remnant Volcanic neck = remnant exposed via erosion

Economics of Igneous Rocks Uncommon uses: –Mining –Pumice stone –Lava soap –Fingernail files –Surgical tools