09 GEOS 1004 - GEOS 1004, SPOTILA Lecture 5, 9/8/09 IGNEOUS...

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GEOS 1004, SPOTILA Lecture 5, 9/8/09 IGNEOUS ROCKS Igneous rocks are common - for example, they make up most of the crust. Where do they occur, and how are they made? Most generally, igneous rocks are made along plate boundaries. Specifically; 1) subduction zone (the Ring of Fire volcanic arc); melting caused by = temperature increases with depth, addition of volatiles 2) mid-ocean ridge or “hot-spot” mantle plume (new sea floor, mid-Atlantic); melting caused by = “hot spot” mantle plume, melting caused by decompression Igneous rock classification 1) Is it volcanic, or is it intrusive? The proxy for this is; grain size of rock * depends on cooling rate; things cool slowly at depth, fast at the surface. intrusive = at depth, magma chamber; cools slowly, larger crystals, coarse interlocking texture volcanic (extrusive) = lava at the surface, cools fast, small crystals, ashy, glassy Phenocryst = large crystal in volcanic rock, that forms where? One chemical composition that can form 3 very different rocks, depending on rate of cooling: obsidian = very fast cooling, a glass rhyolite = fast cooling, volcanic rock, no crystals granite = intrusive rock, slowly cooled, big crystals 2) What is its composition in terms of silica content? felsic (silicic) = has the most Si, light in color, less dense
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This note was uploaded on 09/09/2011 for the course GEOG 1004 taught by Professor Staff during the Fall '07 term at Virginia Tech.

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09 GEOS 1004 - GEOS 1004, SPOTILA Lecture 5, 9/8/09 IGNEOUS...

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