Igneous Rocks

• Magma vs. lava
• magma - molten rock below surface
• lava - molten rock at surface
•  gases can escape lava more easily than magma
• Crystallization vs. precipitation
• crystallization - crystals forming from magma or lava; occurs with cooling (“freezing”)
• precipitation - crystals forming from aqueous (water) solution; due to supersaturation of ions

• Chemistry and mineralogy (see Figs. 4-16, 17)
• Silica (SiO2) makes up 40 - 70%
• Felsic - rich in feldspar and silica
• also called Silicic - SiO2 (silica) rich
• Ex: granodiorite
• Intermediate - intermediate in silica, Mg, and Fe compared to felsic and mafic rocks
• Ex: andesite, diorite
• Mafic - Magnesium and Fe rich; silica poor
• Ex: basalt, gabbro, Ca-plagioclase

• Ultramafic - more Mg and Fe rich, and more silica poor, than mafic rocks
•  Ex: peridotite (pyroxene and olivine rock)

• Igneous Textures
• Texture - size, shape, and arrangement of grains (and voids)
• phaneritic - grains visible with naked eye
• aphanitic - grains not visible with naked eye
• porphyritic - large crystals (phenocrysts) in a fine-grained matrix (noun: porphyry)

• Origins
• Intrusive
• rock crystallizes from magma within the crust
• Also called Plutonic rocks
• phaneritic texture
• Extrusive
• rock crystallizes from lava at Earth’s surface
• Also called Volcanic rocks
• aphanitic texture

• granite/ rhyolite -abundant qtz and K-spar
• granodiorite/ dacite -qtz; plag>K-spar
• diorite/ andesite -plag, amph; minor pyx; little or no qtz
• gabbro/ basalt -pyx, plag; no qtz
• peridotite - mostly pyx and ol
• Decreasing silica, increasing Mg downward; becoming darker downward

Granodiorite/dacite

• granodiorite/ dacite -qtz; plag>K-spar

Gabbro/Basalt

Other Extrusive Rocks

• Pyroclastic Rocks (fire particles)
• tuff - forms from volcanic ash
• volcanic breccia - forms from larger pyroclasts (broken fragments of rock)
• obsidian - glass (dark brown or black but silica rich)
• pumice - glass with abundant vesicles (“frozen” bubbles; froth)