Chapter 7 Flashcards
What are the two main agents of metamorphism, and what are their respective roles in producing metamorphic rocks?
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.
Into what metamorphic rocks will a mudrock be transformed at very low, low, medium, and high metamorphic grades?
At very low, low, medium, and high metamorphic grades, mudrock will be transformed into slate, phyllite, schist, and gneiss.
Why doesn’t granite change very much at lower metamorphic grades?
Granite remains largely unchanged at lower metamorphic grades because its minerals are still stable at those lower temperatures.
Describe the main process of foliation development in a metamorphic rock such as schist.
Foliation exists because as new minerals are forming in a situation of directed pressure they are forced to grow with their long axes perpendicular to the main pressure direction.
What process contributes to metamorphism of oceanic crust at a spreading ridge?
At a spreading ridge the heat from volcanism leads to the development of a groundwater convection system in the oceanic crustal rock. 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 ferromagnesian minerals (e.g., olivine and pyroxene) into chlorite and serpentine.
How do variations in the geothermal gradient affect the depth at which different metamorphic rocks form?
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 (greater depth where the gradient is least).
Blueschist metamorphism takes place within subduction zones. What are the unique temperature and pressure characteristics of this geological setting?
The geothermal gradient is low within subduction zones (because the cold subducting oceanic crust takes a long time to heat up), so while pressure increases at the normal rate the temperature does not.
Rearrange the following minerals in order of increasing metamorphic grade: biotite, garnet, sillimanite, chlorite.
In order of increasing metamorphic grade: chlorite biotite, garnet, sillimanite.
Why does contact metamorphism not normally take place at significant depth in the crust?
The rocks at significant depth in the crust are already hot and subject to regional metamorphism, so the additional heat from a pluton doesn’t make a large difference.
What is the role of magmatic fluids in metamorphism that takes place adjacent to a pluton?
Water from any source facilitates metamorphism. Magmatic fluids typically contain dissolved ions at higher levels than in regular groundwater (especially copper, zinc, silver, gold, lithium, beryllium, boron and fluorine) so can lead to formation of a unique set of minerals.
How does metasomatism differ from regional metamorphism?
Metasomatism involves fluids from magmatic or groundwater sources that play an important role in transporting ions and leading to the formation of new minerals.
How does the presence of a hot pluton contribute to the circulation of groundwater that facilitates metasomatism and hydrothermal processes?
A hot pluton heats the surrounding water and this contributes to the development of a convection system in the groundwater, which can result a great deal of water, in some cases with elevated levels of specific ions, passing through the rock. Magmatic water also contributes to metasomatism.
What must be present in the country rock to produce a skarn?
Limestone must be present in the country rock to produce a skarn.
Two things that a geologist first considers when looking at a metamorphic rock are what the parent rock might have been, and what type of metamorphism has taken place. This can be difficult to do, even if you have the actual rock in your hand, but give it a try for the following metamorphic rocks: Chlorite schist Slate Mica-garnet schist Amphibolite Marble
Two things that a geologist first considers when looking at a metamorphic rock are what the parent rock might have been, and what type of metamorphism has taken place.
Metamorphic Rock Likely Parent Rock Grade and/or Type of Metamorphism
1. Chlorite schist A rock enriched in ferromagnesian minerals, such as basalt. Low-grade regional metamorphism.
2. Slate Mudrock (shale, mudstone) Very low-grade regional metamorphism
3. Mica-garnet schist A rock that is rich in aluminum, which includes most clay-bearing rocks. Medium-grade regional metamorphism
4. Amphibolite A rock enriched in ferromagnesian minerals, such as basalt. Medium- to high-grade regional metamorphism.
5. Marble Limestone or dolomite Regional or contact metamorphism.
