Metamorphic Rocks
When a rock's environment changes because of plate tectonic movements or isostatic changes caused by erosion, the minerals in the rock are likely to undergo chemical change. Rocks that have experienced these changes (without melting) are metamorphic (changed) rocks. Most common metamorphic rocks were sedimentary rocks that have been buried and heated at a convergent plate boundary. Water-rich clay minerals in the rock undergo dehydration to form muscovite at low temperature, then biotite, garnet and feldspars as temperature increases. Regional metamorphism at plate boundaries will form broad bands of metamorphic rock stretching for thousands of km. The highest grade rocks will be at the center, and will mark the actual site of the collision. Other kinds of metamorphism are contact, caused by heat from intrusions, and hydrothermal metamorphism which is caused by the movement of water through dry igneous rocks. Both of these kinds can produce ores like the gold of the Sierras of California or the copper deposits near Lake Superior. Hydrothermal metamorphism is especially common in the ocean crust and mantle where hot ocean water circulates through cracks formed by extension and shearing.
I. Classification -- metamorphic grade
A. Temperature effects
B. Pressure effects -- Foliation, metamorphic layering
C. Water Content
D. Regional metamorphic rocks
1. Slate
2. Phyllite
3. Schist
E. Contact metamorphism
III. Underlying causes -- especially related to plate tectonics
A. Convergent boundaries
1. Regional Metamorphism
2. Contact Metamorphism
B. Metamorphic rocks in SE Pennsylvania -- Interpreting geologic history
Objectives
1. Describe the effect of temperature on clay minerals.
2. Describe the effect of pressure on the formation of micas.
3. Explain why regional metamorphic rocks are foliated while most contact metamorphic rocks are not.
4. Regional metamorphic belts are related to zones where lithospheric plates have collided. Sketch a map view of a typical belt of metamorphic rocks showing relative positions of foliatied rocks of different grade.
5. Explain how you might recognize an unfoliated metamorphic rock.