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Igneous Rocks Cheat Sheet (DRAFT) by

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

Igneous Rock Types

With cooling, magma becomes more felsic
silica content around 50%
high concen­tration of Fe, Mg, Ca
high temp. of magma (1000-­1200C)
major minerals: olivine, pyroxene, Ca plagio­clase
silica content: 65-77%
high consen­tration of Al, Na, K
lower temp. magmas (<850C)

Igneous Rocks - Intrusive vs Extrusive

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)