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Geology of the Pacific Northwest

Geology-208

Week 2:  READING THE ROCKS OF WASHINGTON

Rock Forms

Igneous Rocks

Cooled and crystallized from underground magma or surface lava pushing through other rocks; contains silicon, oxygen, and other elements
INTRUSIVE / PLUTONIC
Rock
Texture
Origin
Color
Minerals
Granite
(pink, white, graphic, porphyritic, hornblende)
coarse-grained; may be porphyritic
cooled in two stages; slow-cooling over many Ka or Ma until solid
beneath earth's surface; various depths
light (felsic; >65% silica)
quartz (20%), K-feldspars (orthoclase & microcline), mica (muscovite or biotite), Na-plagioclase
Granodiorite
(granite family;also: pink, white)
coarse-grained; may be porphyritic;
cooled in two stages; slow-cooling over many Ka or Ma until solid
beneath earth's surface; various depths
light, medium (intermediate felsic)
quartz, amphibole, mica (biotite), Na-plagioclase, hornblende
Diorite
coarse-grained; may be porphyritic;
cooled in two stages; slow-cooling over many Ka or Ma until solid
beneath earth's surface; various depths
medium, dark (intermediate mafic; 55-65% silica)
Na & Ca-plagioclase, amphibole, hornblende, mica (biotite), pyroxene (augite)
Gabbro
coarse-grained; may be porphyritic;
cooled in two stages; slow-cooling over many Ka or Ma until solid
beneath earth's surface; various depths
medium (mafic; <10% silica)
Ca-plagioclase, olivine, pyroxene (augite), magnetite
Volcanic Breccia
coarse-grained; rock fragments (pyroclastic)
transitional, water
gray siliceous fragments in yellowish matrix (felsic)

EXTRUSIVE / VOLCANIC
Rock
Texture
Origin
Color
Minerals
Andesite
(amygdaloidal, porphyritic)
fine-grained; may be porphyritic w/ feldspar, mica (biotite), hornblende or pyroxine (augite)
associated with subduction zones
medium (intermediate mafic; 55-65% silica)
Na & Ca-plagioclase, amphibole, pyroxene (augite), mica (biotite)
Basalt
(porphyritic, amygdaloidal, vesicular)
fine-grained; fast-cooled, by atmosphere's cool temperature or by water
non-viscous lava moving great distances
dark (mafic)
CA-plagioclase, pyroxene (augite), olivine, magnetite, apatite
Dacite
fine-grained (cools too fast for minerals to grow); may be porphyritic

light, med. (intermediate felsic)
Na-pplagioclase, ambhibole, biotite mica, quartz
Rhyolite
(also: banded)
fine-grained (cools too fast for minerals to grow); may be porphyritic

light, med. (intermediate felsic)
quartz, orthocase, Na-plagioclase, mica (biotite)
Obsidian
(also: snowflake)
fine-grained; conchoidal fracture; glassy
viscous acid lava; rapid-cool
dark (mafic); silica-rich
glass; phenocrysts of quartz and feldspar
Pumice
fine-grained; vesicular; low density (floats in water)
lava, ash / volcanic (frothy rhyolitic volcanic lava)
medium (intermediate felsic)

Scoria
vesicular

dark (mafic)
Ca-plagioclase, pyroxene (augite), olivine
Tuff
(lithic, crystal)
fine-grained
pyroclast / volcanic (ash deposited in atmosphere)
medium (intermediate felsic)
lithic, vitric, or crystal formations


GLOSSARY


Send comments to Rob Viens (e-mail: rviens@bcc.ctc.edu) or call him at his Bellevue Community College office at (425) 564-3158. Office hours are by appointment.


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