Earth - Metamorphic Rocks

What are metamorphic rocks?

Form through solid-state transformation of preexisting rocks (protoliths) due to heat, pressure, or chemically active fluids

What is recrystallization in metamorphism?

Metamorphism changes the texture of rocks. The minerals change size and shape through dissolution and growth of new crystals, but the composition of the mineral does not change.

What is neocrystallization in metamorphism?

Metamorphism changes mineralogy and texture. Protolith minerals become unstable and undergo chemical reactions that recycle elements to form new mineral assemblage. The rock rearranges its minerals to form new compounds.

What is phase change in metamorphism?

This transforms a grain of one mineral into a grain of another mineral with the same composition but a different crystalline structure (polymorphs!)

What are the agents of metamorphism?

Temperature, pressure, differential stress, hydrothermal fluids

How does temperature contribute to metamorphism?

Heat energy breaks the bonds and reforms them into more stable atomic bonds. The sources of heat are geothermal gradients, magmatic intrusions, hydrothermal vents, compression and shear deformation.

How does pressure contribute to metamorphism?

Pressure increases as you go deeper into the Earth. Through pressure, atoms are packed more densely, and the pressure also drives phase changes.

What is differential stress and how does it contribute to metamorphism?

Stress that is greater in one direction than another. It differs from pressure in that pressure is constant in all directions. It is a result of tectonic forces. There are two kinds: Normal (perpendicular to a surface) and shear (moves one part of the material sideways, causes it to be smeared out)

What is foliation?

A layered texture formed by the alignment of platy or plastically flattened minerals during metamorphism. It creates planar surfaces and gives the rock a streaked or stripy appearance. Foliated rocks often break along their planes, and these rocks often develop under differential stress. Non-foliated rocks are formed with an absence of differential stress.

Discuss the different grades of metamorphic rocks

• Low: Clay aligns to slate, neocrystallizes into mica in phyllite

• Intermediate: Micas grow larger and form schist

• High: Mica decomposes to form new minerals, creates gneiss

What are porphyroblasts?

Large non-mica minerals in schists and phyllites

What is compositional banding?

Can develop during metamorphic differentiation, chemical elements segregate into light and dark layers.

What is migmatite?

A hybrid rock that forms when gneiss partially melts, displaying both igneous and metamorphic textures. Mineralogy controls melting behavior, so felsic minerals melt before mafic minerals, so the felsic bands migrate and recrystallize with a combination of different structures.

What are the compositional classes of metamorphic rocks?

• Pelitic (derived from shales, Al-rich)

• Mafic (Fe & Mg rich)

• Calcareous (Ca rich)

• Quartzo-feldspathic (granitic)

What is metamorphic grade?

The measure of intensity of T and P conditions that lead to alteration. Low T and P is low grade and vice versa.

What are metamorphic facies?

A mineral assemblage that forms together under specific temperature and pressure conditions. They include different rock types formed under the same temperature and pressure due to the mineral assemblage.

What is prograde metamorphism?

When a rock is buried deep in an orogenic belt and experiences progressive changes due to increasing pressure and temperature.

What is retrograde metamorphism?

Occurs to deep-seated rocks that are brought back to the surface and experience changes due to decreasing temperature and pressure. Cannot occur without water, so many metamorphic rocks preserve their prograde conditions if exhumed fast enough or if there is no water available.

What are the types and settings of metamorphism?

• Burial (Increase in P and T by deep burial)

• Thermal (heating by plutonic intrusion)

• Dynamic (shearing in fault zone)

• Regional (P and T alteration due to mountain building / orogenesis)

• Hydrothermal (hot water leaching)

• Subduction (high P, low T)

• Shock (extreme high P attending bolide impact)

What is the process in which metamorphic rocks return to the surface?

Exhumation. It occurs due to:

• Rock uplift (compression squeezes deep rocks up)

• Extensional collapse (uplifted range spreads outward)

• Erosional unroofing (weathering and erosion removes vast amount of rock)