GEOS 1004, SPOTILA
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