Metamorphism Flashcards
Composition of the parent rock
Factors controlling the characteristics of metamorphic rocks
The mineral content of the metamorphic rock is controlled by the chemical composition of the parent rock.
Usually no new elements or chemical compounds are added to the rock during metamorphism, except possibly water.
Heat
Factors controlling the characteristics of metamorphic rocks
A mineral is said to be stable if, given enough time, it doesn’t react with or convert to a new mineral or substance. The upper limit on temperature overlaps with the temperature of partial melting.
Usually, the deeper a rock is beneath the surface, the hotter it will be (except along convergent plate boundaries).
Pressure
Factors controlling the characteristics of metamorphic rocks
Any new mineral that has crystallised under high-pressure conditions tends to occupy less space as pressure forces atoms closer together in a more closely packed crystal structure.
Differential stress
If forces on a rock are stronger or weaker in different directions, it is subjected to differential stress.
Differential stress tends to deform objects into oblong or flattened forms. It forces the constituents of the rock to become parallel to one another.
Shearing
Differential stress is also caused by shearing, which causes parts of a rock to move or slide relative to one another across a plane.
Some rocks can be attributed exclusively to shearing during faulting in a process sometimes called dynamic metamorphism.
Foliation
Near parallel orientation of platy (flat) minerals within a rock. If a platy mineral is crystallising within a rock undergoing differential stress, it remains parallel to the direction of shearing or perpendicular to the direction of compressive stress.
Has a planar texture.
Tectonic forces result in compressive stress.
Slaty/slaty cleavage
Variations of foliation
When the rock splits easily along nearly flat and parallel planes, indicating that preexisting, microscopic, platy minerals were realigned during metamorphism.
Low degree of metamorphism
Schistose
Variations of foliation
When visible minerals that are platy or needle-shaped have grown essentially parallel to a plane due to differential stress.
Intermediate degree of metamorphism
Gneissic
Variations of foliation
When the rock becomes very ductile and the new minerals separated into distinct (light and dark) layers or lenses.
High degree of metamorphism
Fluids
Factors controlling the characteristics of metamorphic rocks
Hot water (as vapour) is thought to help trigger metamorphic chemical reactions. Moving through fractures and along grain margins, it is a sort of transit for ions.
The water may have been:
- trapped in a parent sedimentary rock
- given off by a cooling proton
- given off from minerals that have water in their crystal structure
How does water help trigger metamorphic chemical reactions?
Under high pressure, water moves between grains, dissolves ions from one mineral, and then carries these ions elsewhere in the rock where they can react with the ions of a second mineral.
Marble (non-foliated)
Identify, define, and describe key index minerals
A coarse grained rock composed of interlocking calcite crystals, forms when limestone recrystallises during metamorphism. If the parent rock is dolomite, the recrystallised rock is a dolomite marble.
Predominant mineral is calcite (except in a dolomite marble), which effervesces in weak acid*.
Quartzite (non-foliated)
Identify, define, and describe key index minerals
Produced when grains of quartz in sandstone are welded together while the rock is subjected to high temperature. Has a sugary appearance and vitreous lustre.
Predominant mineral is quartz. The usual parent rocks are quartz and sandstone.
Slate (foliated)
Identify, define, and describe key index minerals
Very fine grained rock that splits easily along flat, parallel planes. Although some slate forms from volcanic ash, the usual parent rock is shale.
Lower grade rock. Under differential stress, the old and new platy minerals aligned, creating slaty cleavage in the rock. Index minerals are chlorite and mica.
Phyllite (foliated)
Identify, define, and describe key index minerals
A rock in which the newly formed micas are larger than the platy minerals in slate but still cannot be seen by the naked eye. The texture is between slaty and schistose.
Has a sheen. The slaty cleavage may be crinkled in the process of conversion of slate to phyllite.
Common minerals: Quartz, feldspar (sedimentary), muscovite, chlorite (new minerals)
Index minerals: Muscovite and chlorite
