THE LANGUAGE OF THE EARTH PART IIIMETAMORPHICROCKS

Rock Cycle

DefinitionMetamorphism- changes in the mineral assemblage and textures of igneous, sedimentary or other metamorphic rocks due to prolonged exposure to elevated temperatures and pressures (or increased grades of metamorphism)

Types of Metamorphism1) Burial metamorphism (also known as Diagenesis or Lithification) - Low T, low-mod confining to differential P; original sedimentary/igneous features generally preserved; forms in deeper parts of thick sedimentary sequences in non-tectonic area (e.g., on passive continental margins)

Types of Metamorphism2) Contact (or Thermal) metamorphism - High T, low-mod. confining P; forms adjacent to igneous intrusions.

Types of Metamorphism3) High-pressure metamorphism - High P (strongly differential), low-mod T; forms at plate boundaries convergent boundary zones. Also know as blueschist metamorphism.

Types of Metamorphism4) Regional metamorphism -Variable T, mod-high differential P; characteristic of over-thickened plates (i.e., mountain belts) above convergent boundaries; affects large areas

Types of Metamorphism5) Cataclasis metamorphism -Variable T, very high directed P; typically localized to narrow zones of intense mechanical deformation (shear zones).

Types of Metamorphism6) Metasomatism -Variable T&P; distinguished from other forms of metamorphism by the loss and/or gain of material (usually transported by a fluid). Other types of metamorphism are thought to occur as nearly closed processes (except for water loss).

Controls on Rates of Metamorphic ChangeFluid content (particularly water) of the rock. Water helps to catalyze the mineral transformations. Water gets baked out of rocks during prograde metamorphism.Temperature - chemical reactions occur faster at higher temperatures.Time - For a rock to develop a new metamorphic mineral assemblage corresponding to a particular P & T, it must exist under those condition for a sufficiently long period of time (generally tens of thousands to millions of years).

Why do we see high grade metamorphic rocks at the earths surface?Isostatic ReboundRapid erosion of mountain belts of dry rocks

Mineralogic Response to MetamorphismMinerals that form depend on: T and P conditionsBulk composition of the source rockProgressivemetamorphismof a graywacke(dirty sandstone)

Textural Response to MetamorphismReflects the intensity and directionality of pressure (or stress).Increased grain size - During prograde metamorphism or at a particular grade that is maintained for a long period of time, minerals will tend to increase in size. Foliation - As new platy minerals grow, they will align themselves perpendicular to the maximum stress direction. For clay mineral and fine-grained micas, the planar fabric that results is referred to as a slaty cleavage. In higher grade rocks, coarser grained mica minerals are said to impart a schistosity to the rock. Gneissic Banding - In very high grade rocks, the dark minerals tend to segregate from the lighter colored minerals (feldspar and quartz) resulting in banded rock..

Development of Foliation in Bedded Sedimentary Rocks

Summary of Metamorphic Rock Types

STRUCTURAL DEFORMATIONLarge-scale Response to Directed PressureDependent on Temperature and Rate of StressHigh Temperature, Low Rate of Stress FOLDING

STRUCTURAL DEFORMATIONLarge-scale Response to Directed PressureLow Temperature, Rapid Rate of Stress FAULTING

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