Answers to Chapter 10 Review Questions
- Heat and pressure are the main agents of metamorphism. Heat leads to mineralogical changes in the rock. Pressure also influences those mineralogical changes, while directed pressure (greater pressure in one direction) leads to foliation.
- Very low grade: slate; low grade: phyllite; medium grade: schist; high grade: gneiss.
- Granite remains largely unchanged at lower metamorphic grades because its minerals are still stable at those lower temperatures.
- Foliation develops in schist when new platy minerals grow with their longest dimension at a right angle to the direction of greatest pressure.
- At a spreading ridge the heat from volcanism leads to the development of a groundwater convection system in the rock of the oceanic crust. Heated water rises in the hot regions and is expelled into the ocean, while cold ocean water is drawn into the crust to replace it. The heated water leads to the conversion of olivine and pyroxene into chlorite and serpentine.
- The geothermal gradient varies as a function of tectonic setting, being greatest in volcanic regions and lowest along subduction zones. As a result the depth at which specific metamorphic grades is achieved will vary: the depth will be greater when the gradient is lower.
- The geothermal gradient is low within subduction zones because the cold subducting oceanic crust takes a long time to heat up. Pressure increases with depth at the normal rate, but temperature does not.
- Order of increasing metamorphic grade: chlorite, biotite, garnet, sillimanite.
- Water from any source facilitates metamorphism. Magmatic fluids typically contain dissolved ions at higher concentrations than in regular groundwater (especially copper, zinc, silver, gold, lithium, beryllium, boron and fluorine), leading to the formation of a unique set of minerals.
- Metasomatism involves fluids from magmatic or groundwater sources that play an important role in transporting ions into the system, and leading to the formation of new minerals. Regional metamorphism takes place over a larger area, depends more on plate tectonic conditions, and does not involve flushing the system with large amounts of fluid.
- A hot pluton heats the surrounding water, causing groundwater to convect. This can result a great deal of water, in some cases with elevated levels of specific ions, passing through the rock. Water from magma within the pluton also contributes to metasomatism.
- Limestone must be present to produce skarn.
- Parent rocks and metamorphic grades and types:
Metamorphic Rock Likely Parent Rock Grade and/or Type of Metamorphism Chlorite schist A rock enriched in ferromagnesian minerals, such as basalt Low-grade regional metamorphism Slate Mudrock (shale, mudstone) Very low grade regional metamorphism Mica-garnet schist A rock that is rich in aluminum, which includes most clay-bearing rocks Medium-grade regional metamorphism Amphibolite A rock enriched in ferromagnesian minerals, such as basalt Medium- to high-grade regional metamorphism Marble Limestone or dolomite Regional or contact metamorphism