Lesson 4.1 Metamorphic Processes

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Clemson University *

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1010

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Geology

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Oct 30, 2023

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docx

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Uploaded by DeaconFangKingfisher36

Metamorphic Rocks - Metamorphism occurs when solid rock changes in composition and/or texture without the mineral crystals melting - Metamorphic source rocks (the rocks that experience the metamorphism) are called the parent rock or protolith - Most metamorphic processes take place deep underground Metamorphic processes (agents of metamorphism): - Heat - Pressure - Fluids Role of HEAT in Metamorphism - Elevated temperature can cause chemical reactions between minerals to occur, recrystallizing minerals o i.e. recrystallization can happen while the rock is still solid - Temperature of Metamorphism: 200-800 Celsius - Temp increases with depth within the Earth o This is called the geothermal gradient  The average geothermal gradient is 30 Celsius/km o Metamorphic rocks record these temp changes Role of PRESSURE in Metamorphism - Pressure is the force exerted over a unit area on a material - Two types of pressure cause metamorphic changes in rocks o Confining pressure o Directed pressure Confining Pressure - Equal pressure on all sides - Generated by burial - Responsible for causing chemical reactions to occur, recrystallizing minerals (just like heat can do) - Because forces on all sides are balanced, it does not deform rock (change its shape) Directed Pressure - Different amounts of pressure in each direction o Aka. Differential or tectonic stress - Generated by movement of tectonic plates - Responsible for deformation (change in shape) of rock o Directed stress modifies the parent rock at a mechanical level, changing the arrangement, size, and/or shape of the mineral crystals

o This creates unique TEXTURES Role of FLUIDS in Metamorphism - Chemically reactive fluids are expelled by crystallizing magma and created by metamorphic reactions o These reactive fluids are made of mostly water and carbon dioxide - The fluids cause chemical reactions that recrystallize minerals (just like heat and pressure can do) o When fluids are the driver of metamorphic changes, we call it hydrothermal metamorphism or hydrothermal alteration o Water actively participates in chemical reactions and allows extra mobility of elements Two Most Important Types of Metamorphism 1. Contact metamorphism : high T only rocks around igneous intrusions are subjected to high T but not high P a. This is also commonly associated with hydrothermal metamorphism b. Relatively small-scale events; only rocks close to the magma will be metamorphosed 2. Regional metamorphism : high T and P due to deep burial—all rocks in a large region are metamorphosed a. These are large-scale events effecting large bodies of rock i. Ex. a convergent plate boundary. Lots of pressure is generated when plates collide Metamorphic Rocks - The character of the rock is produced by metamorphism depends on: o The original rock type prior to metamorphism (aka. The protolith ) o The conditions of metamorphism (P and T) o Character: Texture (foliated vs. non-foliated) and mineral assemblage (types of coexisting minerals) Texture (foliation) - Foliation : a through-going planar fabric (parallel surfaces or layers) in a metamorphic rock o Metamorphic foliation is due to aligned platy minerals (e.g. micas) or compositional bands - It is NOT sedimentary layering

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