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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
FOLIATED: Laminated, parallel orientation or segregation of different minerals; platy or sheetlike structure that reflects the direction that pressure was applied:
ROCK / Grade
TEXTURE
FOLIATED
MINERALS
ORIGIN /Parent
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



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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Page updated on 5 September 2002  • 
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