Igneous Rocks Igneous Rocks Magma molten rock material

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 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) Hornblende (black laths) Granite- Coarse-grained (plutonic) silicic Plagioclase (white) Quartz (gray, transparent, glassylooking)

Granite in hand sample and thin section

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- 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

Fragmental texture of andesitic volcanic breccia

Igneous Rocks Mafic xenoliths and silicic dike in granite