Geology of the Pacific Northwest

Week 4:  METAMORPHIC & SEDIMENTARY ROCKS

Metamorphic Rocks |  Sedimentary Rocks |  Resources

Metamorphic Rocks

Igneous, sedimentary, and previously metamorphosed rocks change texture and composition when subjected to differing temperatures and pressures.

1. TEXTURE:
• Foliated (layered) – There are three types of foliation:
• slaty cleavage (i.e., slate)
• schistosity (i.e., phyllite)
• banding (i.e., gneiss)
• Non-foliated – like igneous rocks, crystalline growth within a rock
2. CHEMICAL COMPOSITION
• The basic elements, although the structure has been changed, still make up the rock; those elements identify the parent rock (and original environment)

A rock's metamorphic grade (low, medium, or high grade) is related to the temperature and pressure at the time of contact or regional metamorphism. For instance, a low-grade metamorphic rock is a result of metamorphism at a low temperature and pressure.

See chart.

Sedimentary Rocks

All rocks tell a story. A rock's size tells something of its origin and how fast or slow the water moved that deposited the rock in its present location. A rock's roundness tells how long it traveled to get to its current location. The sorting (whether one size or various sizes) of the its constituent sediments tells whether the rock was transported by landslide or by glacier.

1. Texture
• Clastic (fragmental) – rock or fossil fragments
• Crystalline (water evaporation, crystal growth; i.e. rock salt)
• fine-grained
• coarse-grained
2. Composition
• Mineral makeup (its origin)

See chart.

RESOURCES

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.