Download Chapter 8: Major Elements

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
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