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GEOS100 igneous rocks cheat sheet
This is a draft cheat sheet. It is a work in progress and is not finished yet.
Igneous Rocks
Magma - Found at depths of 10-200km below surface; solidifies to form intrusive igneous rocks
Lava - Magma that has reached the surface; solidifies to form extrusive igneous rocks |
Igneous Rock Textures (grains)
Crystalline - Aphanitic - Fine grained (few crystals visible); relatively rapid cooling rate
Crystalline - Phaneritic - Coarse grained texture ; relatively slow rate of cooling; interlocking crystals; pegmatites
Porphyritic - Well formed crystals (phenocrysts); fine grained matrix (groundmass); initial stage of slow cooling, later stage of rapid cooling
Glassy Texture - Rapid cooling (quenched); conchoidal fracture; no apparent crystals (volcanic glass); felsic
Vesicular Texture - Trapped gases in cooling vesicles
Pyroclastic - Explosive erupted debris ;mixture of rock, glassy fragments, and ash |
Pegmatites - Igneous rocks with very coarse texture
Matrix/Groundmass - finer-grained, often microscopic, crystals in which larger crystals, called phenocrysts, are embedded.
Vesicles - the small holes left behind after lava cools and the gases release
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Igneous Rock Types
With cooling, magma becomes more felsic
Mafic
silica content around 50%
high concentration of Fe, Mg, Ca
high temp. of magma (1000-1200C)
major minerals: olivine, pyroxene, Ca plagioclase
Felsic
silica content: 65-77%
high consentration of Al, Na, K
lower temp. magmas (<850C) |
Igneous Rocks - Intrusive vs Extrusive
Intrusive/Plutonic |
Extrusive/Volcanic |
form from the cooling of melted rock (magma) |
form from the cooling of melted rock (lava) |
cool below the earth's surface |
cool above the earth's surface |
slow cooling of magma - large crystals (minerals) |
fast cooling of magma - small crystals, glassy, or pyroclastic |
form from the cooling of melted rock (magma) |
form from the cooling of melted rock (lava) |
Pyroclastic - made of volcanic materials (eg, pumice)
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