ANSWERS TO STUDY QUESTIONS CHAPTER 8 - Metamorphic Rocks

1. Metamorphism refers to the changes which occur in a rock below the earth’s surface with the exception of melting.
The agents of metamorphism are heat, pressure, and chemically active fluids.

2. Two different metamorphic rocks could form under exactly the same metamorphic conditions if they had different parent rocks, or they were subjected to these conditions for different lengths of time.

3. Compressive directed pressure causes the minerals that can be oriented in a rock to align themselves perpendicular to the direction of the maximum applied stress.

4. Foliation refers to any systematic arrangement of mineral grains produced by metamorphism. Generally this implies that the mineral grains are either aligned parallel to each other or grouped together into compositionally distinct layers. Bedding is not a foliation, however, because bedding is produced by sedimentation rather than metamorphism.

5. Confining (hydrostatic) pressure generally produces non-foliated rocks.

6. Before metamorphism occurs the clay particles in shale are aligned parallel to bedding planes. Shale becomes slate when clay particles change into microscopic mica grains. These mica grains will generally align themselves perpendicular to the direction of compressive stress and therefore will rarely be aligned parallel to bedding planes. Slate becomes schist as higher temperatures and pressures encourage mica grains to grow large enough to be visible without a lens. These large aligned mica grains typically give schist a scaly appearance. Schist becomes gneiss under the most intense metamorphic conditions. Under such high temperatures and pressures mica crystals separate from growing quartz and feldspar crystals forming compositional layers.

7. Rock cleavage occurs when a rock tends to separate or split between layers of minute mica grains - too small to be seen. Schistosity is a foliation characterized by a parallel arrangement of mica grains (sometimes amphibole) which are large enough to be seen.

8. Slate is dull, but phyllite has a sheen.

9. a. Non-foliated and typically formed from the metamorphism of limestone. MARBLE
    b. Compositionally banded and composed mainly of granular materials. GNEISS
    c. Represents a grade of metamorphism between slate and schist. PHYLLITE
    d. Very fine-grained with excellent rock cleavage. SLATE
    e. Not compositionally banded, but composed mostly of platy minerals - all of which are more or less parallel to each other. SCHIST
    f. A hard, dense, non-foliated rock that usually forms by contact metamorphism of fine-grained        rocks bearing little or no quartz or calcite (i.e. shale or any volcanic rock). HORNFELS
    g. A non-foliated rock formed from the metamorphism of a quartz-rich sandstone. QUARTZITE

10. Contact metamorphism affects only a small quantity of rock in close proximity to a magma body. Regional metamorphism affects the much larger quantity of rock at the base of the continental crust in the vicinity of a convergent plate boundary. Contact is associated with high temperatures and low pressures. Regional is associated with high temperatures and high pressures. Regional metamorphism creates the greatest quantity of metamorphic rock.

11. Schist and gneiss are foliated, quartzite and marble are not.

12. Slate contains parallel mica grains to small to be seen.
    Mica schist contains parallel mica grains large enough to be seen.
    Gneiss contains micas and other ferromagnesian minerals that have segregated into compositional bands.
    Gneiss represents the highest degree of metamorphism.
    Slate represents the lowest degree of metamorphism.

13. Migmatites are associated with high-grade metamorphism.