chapter 8 (metamorphic rocks)

Amphibolite

nonfoliated metamorphic mafic rocks that consists of the minerals hornblende and plagioclase. These rocks were basalts and gabbros before they were deformed.

Agents of metamorphism

1) Heat 2) Pressure and stress 3) Chemically active fluids

Formation of a metamorphic rock take a very long time that involves several different processes

1) Recrystallization 2) Phase change 3) Metamorphic reaction 4) Plastic deformation

The settings of metamorphism

1. Contact or thermal metamorphism 2. Hydrothermal metamorphism 3. Regional metamorphism 4. Dynamic metamorphism 5. Burial metamorphism

Migmatite

A compositional banded foliated metamorphic rock formed when gneiss is heated high enough so that it begins to partially melt, creating layers, or lenses, of new igneous rock that mix with layers of the relict gneiss. Formed at the highest temperatures and pressures of any foliated metamorphic rock.

Gneiss

A compositionally banded compositional banded foliated metamorphic rock typically composed of alternating dark- and light-colored layers. Formed at higher temperatures and pressures than slate, phyllite, and schists. Usually formed from the melting of a protolith.

Phyllite

A fine-grained slaty foliated metamorphic rock. The foliation is caused by the preferred orientation of very fine-grained mica. Forms at higher temperatures and pressures than slate.

Schists

A medium-to-coarse-grained schistose foliated metamorphic rock that possesses schistosity. At higher temperatures and pressures than slate and phyllite.

mylonites

A metamorphic rock type that is produced as a result of dynamic metamorphism are _______________ (Fine-grained rock formed by the nonbrittle subdivision of larger grains).

Marble

A nonfoliated metamorphic rock composed of calcite and transformed from a protolith of limestone.

Anthracite coal

A nonfoliated metamorphic rock composed of carbon and transformed from a protolith of bituminous coal.

Quartzite

A nonfoliated metamorphic rock composed of quartz and transformed from a protolith of quartz sandstone.

metamorphic facies

A set of metamorphic mineral assemblages indicative of metamorphism under a specific range of pressures and temperatures. Different metamorphic minerals will tell a geologist how hot the rock became during metamorphism - can also tell pressure. This concept leads to these types of facies.

Shear stress

A stress that moves one part of a material sideways past another part.

Compositional banding

A type of metamorphic foliation, found in gneiss, defined by alternating bands of light and dark minerals.

burial metamorphism

As more rocks form on top of each other, rocks become buried deeper into the earth. Due to the geothermal gradient (~24°C/km) rocks will undergo _____________________ as a result of this process.

closer

At depth the deformation in fault zones tends to behave in a more plastic fashion (they are stretched); while ________ to the Earth's surface the rocks tend to break in a brittle fashion (they are smashed).

Chemically active fluids (Agents of metamorphism)

Dissolved elements in hot fluids will react chemically to change the minerals in a rock

Slate

Fine-grained, slaty foliated low-grade metamorphic rock, formed by the metamorphism of shale

Schistosity

Foliation caused by the preferred orientation of large mica flakes.

blueschist metamorphism

In subduction zones pressure become very high because of the collision of 2 tectonic plates; however, the temperature can stay relatively low because of all of the water.

foliated metamorphic rock

Layering formed as a consequence of the alignment of mineral grains, or of compositional banding in a metamorphic rock.

hydrothermal fluids

Metamorphic reactions usually take place in the presence of very hot fluids called ________________. These fluids react chemically with the rocks during metamorphism.

Regional metamorphism

Metamorphism that involves heat, pressure, and shearing. This type of metamorphism is the result of collisions of one tectonic plate with another tectonic plate.

dynamic metamorphism

Metamorphism that occurs along faults results from shearing forces applied to the rock. This results in _______________________________: Metamorphism that occurs as a consequence of shearing alone, with no change in temperature or pressure.

Non-Foliated Metamorphic Rock

Rock containing minerals that recrystallized during metamorphism, but which has no layering has appeared.

Slaty cleavage

The foliation typical of slate, and reflective of the preferred orientation of slate's clay minerals, that allows slate to be split into thin sheets. Forms perpendicular to differential stress applied to the rock.

metasomatism

The process by which a metamorphic rock's chemical composition changes due to the interaction with hydrothermal fluids

Metamorphism

The process by which one kind of rock transforms into a different kind of rock.

Normal stress

The push or pull that is perpendicular to a surface.

Aureole

The region around a pluton, stretching tens to hundreds of meters out, in which heat transferred into the country rock and metamorphosed it.

Rock cycle

The succession of events that results in the transformation of Earth materials from one rock type to another, then another, and so on.

Hydrothermal metamorphism

When cold sea water interacts with the hot rising magma at mid-ocean ridges, the sea water heats up and produces a metamorphism that is rich in Iron.

minerals

You can use the __________ in metamorphic rocks to estimate the grade, or level of metamorphism. Some rocks work better than other.

Differential stress (2 types)

a condition causing a material to experience a push or pull in one direction of a greater magnitude than the push or pull in another direction; in some cases, differential stress can result in shearing.

Metamorphic rocks

a rock that forms from the modification of another rock

Sedimentary rocks

can consists of fragments derived from older metamorphic, igneous and sedimentary rocks.

Igneous rocks

can form from the partial melting of older sedimentary, metamorphic and igneous rocks.

Metamorphic rocks

can form through the deformation of older sedimentary, igneous and metamorphic rocks.

Contact Metamorphism

caused by heat conducted into country rock from an igneous intrusion.

Plastic deformation

deformational process in which mineral grains behave like plastic and, when compressed or sheared, become flattened or elongate without cracking or breaking.

Metamorphic reaction

growth of new minerals out of the protolith due to metamorphic processes. This is a type of diffuse mineral growth.

heat (Agents of metamorphism)

increases with depth. Can cause a rock to recrystallize, grow new minerals that are not stable at lower temperatures, and can cause a rock to melt!

Pressure and stress (Agents of metamorphism)

increases with depth. Confining pressure can cause a rock to become more compact. Differential stress will deform a rock in one direction greater than another.

Blueschist

is a rock that forms by the metamorphism of basalt at high pressures and low temperatures within subduction zones. The blue color of the rock comes from blue colored minerals within the rock.

Greenstone

metamorphic rock produced by hydrothermal alteration. The green color come from the new metamorphic minerals (epidote, chlorite, etc..) formed as hot water interacted with the rock.

Hornfels

metamorphic rocks formed by contact metamorphism. The high heat of the magma "baked" the rock (not unlike clay in a kiln).

Protolith or parent rock

preexisting rock that was deformed and modified into a metamorphic rock. The addition of large amounts of heat and pressure will transform the protolith limestone into the metamorphic rock marble.

Phase change

to change one mineral into a different mineral occurs at depth in the earth when the mineral olivine converts into a different mineral structure called a spinel type structure.

Recrystallization

to change the shape and size of a mineral without changing its original composition.

Ecologite

very high-grade metamorphic rock, made up of mostly garnet and pyroxene, lacks water bearing minerals, and form deep within subduction zones. Form even deeper than blueschists. Basalt protolith.