Igneous Rocks
Igneous Rocks
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Magma vs. lava
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magma - molten rock below surface
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lava - molten rock at surface
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gases can escape lava more easily than magma
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Crystallization vs. precipitation
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crystallization - crystals forming from magma or lava; occurs with cooling
(“freezing”)
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precipitation - crystals forming from aqueous (water) solution; due to
supersaturation of ions
Classifying Igneous Rocks
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Chemistry and mineralogy (see Figs. 4-16, 17)
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Silica (SiO2) makes up 40 - 70%
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Felsic - rich in feldspar and silica
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also called Silicic - SiO2 (silica) rich
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Ex: granodiorite
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Intermediate - intermediate in silica, Mg, and Fe compared to felsic and
mafic rocks
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Mafic - Magnesium and Fe rich; silica poor
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Ex: basalt, gabbro, Ca-plagioclase
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Ultramafic - more Mg and Fe rich, and more silica poor, than mafic rocks
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Ex: peridotite (pyroxene and olivine rock)
Classifying Igneous Rocks (cont.)
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Igneous Textures
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Texture - size, shape, and arrangement of grains (and voids)
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phaneritic - grains visible with naked eye
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aphanitic - grains not visible with naked eye
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porphyritic - large crystals (phenocrysts) in a fine-grained matrix (noun:
porphyry)
Classifying Igneous Rocks (cont.)
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Origins
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Intrusive
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rock crystallizes from magma within the crust
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Also called Plutonic rocks
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phaneritic texture
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Extrusive
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rock crystallizes from lava at Earth’s surface
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Also called Volcanic rocks
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aphanitic texture
Igneous Rock Types
Intrusive/ Extrusive (Fig. 4.3)
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granite/ rhyolite -abundant qtz and K-spar
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granodiorite/ dacite -qtz; plag>K-spar
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diorite/ andesite -plag, amph; minor pyx; little or no qtz
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gabbro/ basalt -pyx, plag; no qtz
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peridotite - mostly pyx and ol
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Decreasing silica, increasing Mg downward; becoming darker downward
Granite/Rhyolite
Granodiorite/dacite
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granodiorite/ dacite -qtz; plag>K-spar
Diorite/Andesite
Gabbro/Basalt
Other Extrusive Rocks
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Pyroclastic Rocks (fire particles)
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tuff - forms from volcanic ash
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volcanic breccia - forms from larger pyroclasts (broken fragments of rock)
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obsidian - glass (dark brown or black but silica rich)
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pumice - glass with abundant vesicles (“frozen” bubbles; froth)