chapter_03_lec modified - Igneous Rocks Intrusive Activity...

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Steve Kadel, Glendale Community College Igneous Rocks, Intrusive Activity, and the Origin of Igneous Rocks Physical Geology 11/e, Chapter 3
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The Rock Cycle A rock is a naturally formed, consolidated material usually composed of grains of one or more minerals The rock cycle shows how one type of rocky material gets transformed into another Representation of how rocks are formed, broken down, and processed in response to changing conditions Processes may involve interactions of geosphere with hydrosphere, atmosphere and/or biosphere Arrows indicate possible process paths within the cycle
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The Rock Cycle and Plate Tectonics Magma is created by melting of rock above a subduction zone Less dense magma rises and cools to form igneous rock Igneous rock exposed at surface gets weathered into sediment Sediments transported to low areas, buried and hardened into sedimentary rock Sedimentary rock heated and squeezed at depth to form metamorphic rock Metamorphic rock may heat up and melt at depth to form magma Convergent plate boundary
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Igneous Rocks Magma is molten rock Igneous rocks form when magma cools and solidifies Intrusive igneous rocks form when magma solidifies underground Granite is a common example Extrusive igneous rocks form when magma solidifies at the Earth’s surface (lava) Basalt is a common example Granite Basalt
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Igneous Rock Textures Texture refers to the size, shape and arrangement of grains or other constituents within a rock Texture of igneous rocks is primarily controlled by cooling rate Extrusive igneous rocks cool quickly at or near Earth’s surface and are typically fine- grained (most crystals <1 mm) Intrusive igneous rocks cool slowly deep beneath Earth’s surface and are typically coarse-grained (most crystals >1 mm) Coarse-grained igneous rock Fine-grained igneous rock
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Special Igneous Textures A pegmatite is an extremely coarse-grained igneous rock (most crystals >5 cm) formed when magma cools very slowly at depth A glassy texture contains no crystals at all, and is formed by extremely rapid cooling A porphyritic texture includes two distinct crystal sizes, with the larger having formed first during slow cooling underground and the small forming during more rapid cooling at the Earth’s surface Pegmatitic igneous rock Porphyritic igneous rock
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