Module 7 - Metamorphic Rocks

Metamorphic foliation

A fabric defined by parallel surfaces or layers that develop in a rock as a result of metamorphism; schistocity and gneissic layering are examples.

Shield

An older, interior region of a continent.

Contact metamorphism involves what agent of change?

Heat

Conglomerate-->

Metaconglomerate (w/foliation and flattened pebbles)

Polymorph

Minerals with the same chemical composition but different in atomic arrangement and crystal structure Ex: Al2SiO5 = Sillimanite and Kyanite

Protolith =

Parent rock, what is was before it was metamorphosed Ex: Limestone is the protolith to marble

low grade to high grade

Shale-Slate-Phyllite-Schist-Gneiss-Migmatite

Metamorphic texture

a distinctive arrangement of mineral grains produced by metamorphism

Sillimanite

hard, brittle, white to pink long fibrous crystals

Garnet

hard, red to red brown crystals with many sides, lacks cleavage.

Staurolite

hard, white to brown, hexagonal often in twinned crosses

Non-foliated texture

is interlocking crystals that are not in any particular orientation. This usually results from re-growth of minerals subjected to high temperatures. However, if there aren't many platy (flat) minerals rocks may appear non-foliated even if pressure has been applied. Non-foliated texture can form in both regional and contact metamorphic rocks. Rock Examples: Marble and Quartzite

Adding fluid to the equation...

new minerals brought in with fluid, potential of melting, will create different product in the end

Metamorphic Mineral

new minerals that grow in place within a solid rock under metamorphic temperatures and pressures

Foliated texture

onsists of platy minerals (like biotite/muscovite) that lie parallel to one another, creating a smooth, layered appearance. This texture is caused by stress during regional metamorphism. Minerals recrystallize and grow in the direction of least stress, so they tend to be aligned perpendicular to the direction of maximum stress. Rock Examples: Slate, Phyllite, Schist, and Gneiss.

Preferred orientation

parallelism of inequant grains in a metamorphic rock

Differential stress

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.

Phyllite

A fine-grained metamorphic rock with a foliation caused by the preferred orientation of very fine-grained mica.

Schist

A medium-to-coarse-grained metamorphic rock that possesses schistosity.

Marble

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

Metaconglomerate

A metamorphic rock produced by metamorphism of a conglomerate; typically, it contains flattened pebbles and cobbles.

Migmatite

A 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.

Metamorphic facies

A set of metamorphic mineral assemblages indicative of metamorphism under a specific range of pressures and temperatures.

Describe what metamorphism is

Alteration of the composition or structure of a rock by heat and/or pressure or other natural agent

What happens when you add volatile materials?

Anytime you add volatile materials (like water) that have very low boiling points relative to other materials, always have potential to melt the rock - like at a subduction zone - that's how you initiate melting in the overlying slab to create volcanism.

What is the lowest grade of foliation and what rock is it typically seen in?

Chlorite/Shale

What's the typical cause of regional metamorphism?

Continental collision/ formation of mountain belts

Slate

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

What happens to the texture of a rock when it is affected by both heat and pressure?

Foliation - minerals recrystallize and are squeezed so that they grow in alignment with new direction of stress and re-form into minerals stable at higher temperatures and pressures

Granite-->

Gneiss

Regional metamorphism or "mountain belt metamorphism" involves what agents of change?

Heat and pressure

What are the two main agents of change in metamorphic rock?

Heat and pressure

What would happen if heat and pressure continued to increase beyond the limits of the metamorphic grade graph?

If heat and pressure continued to increase, we would reach an area where the temp is high enough that melting witll occur and it will eventually be considered an igneous system - starting rock cycle all over again

What usually causes contact metamorphism?

Intrusion of hot magma into cool crust

Foliation

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

Limestone (pure-calcite only) is the protolith of?

Marble

What is one example of a contact metamorphic rock utilized by humans?

Marble

Conglomerate is the protolith of

Metaconglomerate

Dynamothermal metamorphism

Metamorphism that involves heat, pressure, and shearing.

New minerals can form (Granite to Gneiss)

Normal granite texture of igneous rock with large crystals interlocking, randomly distributed --> If we apply heat and pressure to granite, two things happen 1) In Gneiss, you have alignment of all the mineral grains horizontally, something has happened to align them = squeezing of the rock causes alignment through pressure 2) New minerals have shown up in gneiss, made from same elements present in original rock, atoms of elements that were freed up by heat applied to rock allowed them to re-form into new minerals

Common contact rocks

Quartz, marble

Sandstone is the protolith of?

Quartzite

Sandstone -->

Quartzite-->Gneiss

What happens to the texture of a rock when it is affected by heat from metamorphism?

Recrystallization - minerals can recrystallize, destroying the original texture by growing larger and becoming interlocking

Metamorphic rock

Rock that forms when preexisting rock changes into new rock as a result of an increase in pressure and temperature and/or shearing under elevated temperatures; metamorphism occurs without the rock first becoming a melt or a sediment.

Hornfels

Rock that undergoes metamorphism simply because of a change in temperature, without being subjected to differential stress.

How does gneiss end up with bands or layers of separate minerals?

Shearing - causes dark minerals to line up and band, and light colors to do the same, results in a zebra stripe pattern

Limestone (impure-calcite+silicates) is the protolith of?

Skarn

Shale is the protolith of?

Slate, phyllite, schist (hornfels)

Shale -->

Slate-->phyllie-->schist-->Gneiss

Exhumation

The process (involving uplift and erosion) that returns deeply buried rocks to the surface.

Metamorphic aureole

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

Metamorphic zone

The region between two metamorphic isograds, typically named after an index mineral found within the region.

Heat destroying original texture of a rock through recrystallization (Transition from limestone to marble through heat alone)

When we apply heat, ions of calcium and carbonate start to be freed from original crystal structure by application of heat, find each other again in very hot rock to create and grow new crystals that destroy original very fine texture that created outline of the shells, all of calcite crystals grown larger and become interlocking like puzzle pieces

Foliation

With heat and pressure, minerals recrystallize and are squeezed so that they grow in alignment with the new direction of stress and re-form into minerals stable at higher temperatures and pressures

Gneiss

a compositionally banded metamorphic rock typically composed of alternating dark and light colored layers

Quartzite

a metamorphic rock composed of quartz and transformed from a protolith of quartz sandstone

Metamorphic grade

a representation of the intensity of metamorphism, meaning the amount or degree of metamorphic change

What is foliation?

alignment of mineral grains/growth

What do we mean by metamorphic grade?

amount of temp and pressure required to cause metamorphic change

Metamorphic rocks...

are those that result from solid-state alteration of preexisting rocks, result of changes while rock is still very hot, but not liquid

Stress and pressure are...

both squeezing but stress = unequal squeezing

Kyanite

brittle, elongate, bladed crystals with a blue hue

Tectonic stress

could be agent of metamorphism, different than geostatic gradient b/c directed stress (stresses are not equal in all direction)

Biotite

dark brown platy minerals

Two ways grain shapes change

flatter and larger

Pressure =

geostatic experiences stress equally in all directions

Flattened, streched fossils from a squeezed sedimentary rock indicates...

how lithified materials can deform over time. rocks can be squeezed and stretched down in earth

Marble is white when

it is pure CaCO3

Diagenesis

low temperature change the phys. and chem. changes occurring during the conversion of sediment to sedimentary rock Diagenesis zone = relatively low pressure and temperature

Contact metamorphic rocks often have a...

massive or coarse crystalline structure and exhibit no alignment of mineral grains

Thermal metamorphism

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

Burial metamorphism

metamorphism due only to the consequences of very deep burial

Regional metamorphism (mountain belt)

metamorphism of a broad regions, usually the result of deep burial during an orogeny. Heat and pressure

Dynamic metamorphism

metamorphism that occurs as a consequence of shearing alone, with no change in temperature or pressure

A variety of colors produced in contact rocks are produced by...

mineral impurities such as clay, silt, sand, iron oxides, or chert

Regional can turn...

previously massive (unlayered) rocks into layered rocks

Contact metamorphism

same as thermal

Chlorite

soft, green platy mineral with small crystal size

Contact

taking tiny clasts, and in the process of applying heat only, we get new large grains, crystals become large and interlocking

Shock metamorphism

the changes that can occur in a rock due to the passage of a shock wave, generally resulting from a meteorite impact

Protolith

the original rock from which a metamorphic rock formed

Metasomatism

the process by which a rock's overall chemical composition changes during metamorphism because of reactions with hot water that bring in or remove elements

Metamorphism

the process by which one kind of rock transforms into a different kind of rock

Fossils and sed. structures are...

usually destroyed in the process of recrystallization

Geothermal Gradient

w/depth in Earth, temperature increases

Regional causes...

we can take the proto, take the spherical grains and through application of pressure we squeeze them and change their shapes - change spherical grains to elliptical grains, stretched them out, can cause foliation

Hydrothermal metamorphism

when very hot water passes through the crust and causes metamorphism of rock

Geostatic/Lithostatic gradient

with depth in earth, pressure increases due to overlying rock

Recrystallization

with heat only, minerals can recrystallize, destroying the original texture by growing larger and becoming interlocking