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Igneous Rocks. Igneous Rocks. Magma- molten rock material Lava- magma that reaches the surface of the Earth Volcanic Rocks- from form lava at the surface Cool quickly so crystals are small Plutonic Rocks- cool beneath the surface Cool slowly so fewer and larger crystals. Igneous Rocks.
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Igneous Rocks • Magma- molten rock material • Lava- magma that reaches the surface of the Earth • Volcanic Rocks- from form lava at the surface • Cool quickly so crystals are small • Plutonic Rocks- cool beneath the surface • Cool slowly so fewer and larger crystals
Igneous Rocks • Textures: • Aphanitic- crystals too small to see by eye • Phaneritic- can see the constituent minerals • Fine grained- < 1 mm diameter (< sugar) • Medium grained- 1-5 mm diameter (< pea) • Coarse grained- 5-50 mm diameter • Pegmatitic- > 50 mm diameter • Porphyritic- bimodal grain size distribution • Glassy- no crystals formed • Vesicular- contains voids (once gas bubbles) • Fragmental- contains pieces of rock and mineral expelled from an explosive eruption
Classification based on composition (Mafic –Silicic) vs. texture
Classification based on composition (Mafic –Silicic) vs. texture
Igneous Rocks Orthoclase (pink) Biotite (shiny black flakes) Plagioclase (white) Quartz (gray, transparent, glassy-looking) Hornblende (black laths) Granite- Coarse-grained (plutonic) silicic
Igneous Rocks Plagioclase (white) Hornblende (black) Diorite- phaneritic intermediate “salt-and-pepper”
Igneous Rocks Plagioclase (gray) Pryoxene (blackish green) Gabbro- phaneritic mafic (dark)
Igneous Rocks Rhyolite- aphanitic silicic = light-colored (pink, tan, lt gray)
Igneous Rocks Porphyritic Andesite Andesite- aphanitic intermediate (green, gray, red)
Basaltic Scoria- aphanitic dark-colored with numerous vesicles
Igneous Rocks Basalt in thin-section under the polarizing microscope
Igneous Rocks Porphyritic Gabbro- coarse plagioclase phenocrysts in a fine-grained phaneritic groundmass
Igneous Rocks Glassy texture: obsidian (really silicic rhyolite although dark-colored)
Igneous Rocks Rhyolite Pumice- frothy vesicular, light color
Igneous Rocks Mafic xenoliths and silicic dike in granite