Chapter 7: Metamorphism: A Process of Change Flashcards
What are metamorphic rocks?
Metamorphic rocks are those that have undergone solid-state alteration of preexisting rocks.
Meta = change. Morph = form
What are protoliths?
The preexisting rocks that are altered during metamorphism are called protoliths. Metamorphism can alter any protolith (igneous, sedimentary, or metamorphic)
- pre-existing rock, irrespective of its origin or nature
Metamorphism occurs when the protolith is subjected (w/o melting) to:
- Heat
- Pressure
- Differential stress (push, pull or shear)
- Bathing in hydrothermal fluids
Where does metamorphism occur and at what temperature?
Metamorphism occurs between 250oC and 850oC and the depth to this temperature varies with tectonic setting.
What are the sources of heat?
Sources of heat: the geothermal gradient, magmatic intrusions, and tectonic compression
How can you tell if a rock is a metamorphic rock?
Metamorphic mineral grains are typically large and interlock with each other rather than being cemented.
What is foliation?
parallel alignment of platy minerals and/or the presence of alternating light- and dark-colored layers
Explain how heat causes metamorphism
heat causes the atoms to vibrate faster, stretching and bending chemical bonds. If bonds are stretched too far and break, they detach from their original neighbors, move slightly, and form new bonds with other atoms “recrystallization”
What is recrystallization?
a rearrangement of atoms within grains or the migration of atoms into and out of grains. Mineral grains grow larger at the expense of smaller ones until they interlock
- modification of the shape and size of mineral grains without a change in the mineralogy.
Give an example to recrystallization?
Sandstone –> Quartzite
Limestone –> Marble
Snow flakes
How do minerals change size and shape?
Minerals change size and shape by dissolution and growth of crystals. The identity of the mineral does not necessarily change
Explain how pressure causes metamorphism
under the effect of pressure, mineral grains will deform depending on how fast and how the pressure is applied
As pressure increases, the space between mineral grains is eliminated as grains are pressed together. At higher pressures, grains dissolve where they contact and reprecipitate in the pore spaces forming larger grains. At even greater pressures, they may undergo a phase change to minerals that have a more compact atomic structure.
True/False
As pressure increases the density of the rock increases.
True
What is differential stress?
push, pull or shear
What is shear stress?
- Shear stress moves part of a material sideways causing it to be smeared out. It is like sliding out a deck of cards.
- Shear acts parallel to a surface
