Geology of the Pacific Northwest

Geology-208

Week 2: READING THE ROCKS OF WASHINGTON

Rock Forms

Metamorphic Rocks

Sedimentary and igneous rocks that have been heated and pressurized in mountain building process
ROCK / Grade	TEXTURE	FOLIATED	MINERALS	ORIGIN /Parent
FOLIATED: Laminated, parallel orientation or segregation of different minerals; platy or sheetlike structure that reflects the direction that pressure was applied:
Slate / low (green; black; with pyrite; with distorted fossil)	fine-grain; cleavage surfaces have dull luster; variable colors	Slaty Cleavage: breaks into flat plates	clay materials	mountain ranges / shale (low to moderate pressure)
Phyllite / low	fine-to-medium grain; satiny luster; gray or green color	Slaty Cleavage: breaks into flat plates	clay, mica, chlorite	mountain ranges / shale
Schist / med. (garnet, folded, muscovite, biotite, kyanite)	med.-to-coarse grain; large mica grains; sparkly surface	Schistosity: platy foliation due to mica grains	mica, garnet	mountain ranges / shale, siltstone, graywacke
Blueschist/ med.-high	med.-to-coarse grain; bluish-gray color; mostly without mica	Schistosity: platy foliation due to mica grains	blue amphibole	mountain ranges (subduction zones) / basalt
Gneiss / high (also: folded, augen, granular)	coarse-grain; mostly without mica; light & dark layers	Gneissic Banding (light & dark)	quartz, feldspar, amphibole, pyroxene (augite); may have biotite & garnet	mountain ranges / shale, siltstone, graywacke; felsic igneous
NON-FOLIATED:
ROCK / Grade	TEXTURE	FOLIATED	MINERALS	ORIGIN /Parent
Hornfels (cordierite, pyroxene, garnet, chiastolite)	fine-grain; dense; dark; similar to basalt		pyroxene, amphibole, plagioclase	contact aureoles / shale, siltstone, basalt
Greenstone	fine-to-medium grain; greenish gray/black		chlorite, amphibole, plagioclase	basalt, gabbro
Marble (green, blue, gray, olivine)	fine-to-coarse grain; crystalline texture; softer than glass		calcite [acid reaction]	contact aureoles / limestone
Quartzite (white, pink, tan, brown, red, purple)	fine-to-coarse grain; crystalline; variable color		quartz [scratches glass]	quartz, sandstone
Amphibolite	coarse grain; dark green/black		amphibole, hornblende, actinolite, biotite, pyroxene, garnet, tremolite, epidote, feldspar, chlorite	mountain ranges / basalt, gabbro
Meta-Conglomerate	coarse grain; sometimes deformed		rock fragments	conglomerate

GLOSSARY

Igneous Rocks • Metamorphic Rocks • Sedimentary Rocks • WEEK 2

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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.