Metamorphism and Metamorphic Rocks

Metamorphism The transformation of rock by temperature and pressure Metamorphic rocks are produced by transformation of: Igneous, sedimentary and igneous rxs

Metamorphism in Hollywood Heat and Stress Increasing 2007

Why are the changes in Michael Jackson’s facial features such a perfect (albeit warped) analog for rock metamorphism? A)The physical composition of his face is unchanged (in metam. rock, bulk composition remains constant) B) The physical appearance of his face has changed (in metam. rock, mineral assemblages change) C)He will continue to change with new external conditions (increased heat and pressure will cause further change) D) All of the above

Hollywood’s analogue to metamorphism- Comparison of timescales Given Michaels rate of change over 25 years, how much faster is this than the time it takes to metamorphose rocks under added heat and pressure A)1000 times faster B)10,000 times faster C)100,000 times faster D)1000,000 times faster

Metamorphism Metamorphism progresses from low to high grades Rocks remain solid during metamorphism

Agents of metamorphism… Heat Most important agent Heat drives recrystallization - creates new, stable minerals Pressure (stress) Increases with depth Pressure can be applied equally in all directions or differentially

Why does temperature increase with depth beneath the surface? A) heat given off from radioactive decay B) pressure greater beneath surface, heats things up C) remnant heat from planetary formation D) A & C E) A, B, & C

Origin of pressure in metamorphism

Main factor affecting metamorphism Parent rock Metamorphic rocks typically have the same chemical composition as the rock they were formed from Different minerals, but made of the same stuff.

Metamorphism Three types of metamorphic settings: Contact metamorphism – from a rise in temperature within host rock Hydrothermal metamorphism – chemical alterations from hot, ion-rich water Regional metamorphism -- Occurs in the cores of mountain belts and makes great volumes of metamorphic rock

Contact metamorphism Produced mostly by local heat source

Hydrothermal metamorphism Requires hot circulation of fluids

Regional metamorphism in Mountain Belts Requires crustal thickening and shortening

Metamorphism and plate tectonics Most regional metamorphism occurs along convergent plate boundaries Compressional stresses deform plate edge Occurs in major mountain belts: Alps, Himalayas, and Appalachians

Location of metamorphic zones in a subduction zone

Metamorphism and plate tectonics Metamorphism at subduction zones Cores of subduction zones contain linear belts of metamorphic rocks –High-P, low-T zones near trench –High-T, low-P zones in region of igneous activity (arc)

Location of metamorphic zones in a subduction zone

Metamorphic Environments Index minerals and metamorphic grade Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form

Index Minerals in metamorphic rocks

Metamorphic Environments Metamorphic grade A group of minerals that form in a particular P-T environment Zeolite (really low T,P; <200C) Greenschist (low T, P; C, km) Blueschist (low T, high P - subduction zones) Amphibolite (high T, P; C, km) Granulite (super high T, P; >700C, >25km)

A)2 km B)3 km C)4 km D)5 km E)None of the above A certain type of metamorphic rock forms when the temperature equals 200 deg C. At what depth do these rocks form, if the geothermal gradient is 45 degC per km, and temperature at surface is 20 degC?

A)2 km B)3 km C)4 km D)5 km E)None of the above A certain type of metamorphic rock forms when the temperature equals 200 deg C. At what depth do these rocks form, if the geothermal gradient is 45 degC per km, and temperature at surface is 20 degC?

Metamorphic Environments in Subduction Zones

Greenschist Hand Sample Greenschist Thin Section

Mica Schist

Blueschist Amphibolite

Common metamorphic rocks Nonfoliated rocks Quartzite –Formed from a parent rock of quartz-rich sandstone –Quartz grains are fused together –Forms in intermediate T, P conditions

Sample of quartzite Thin section of quartzite

Flattening of quartz grains in quartzite

Sandstone to Quartzite Morph

Common metamorphic rocks Nonfoliated rocks Marble –Coarse, crystalline –Parent rock usually limestone –Composed of calcite crystals –Fabric can be random or oriented

Marble

Marble (Random fabric = annealing; nonfoliated)

Change in metamorphic grade with depth

Shale to Slate to Schist Morph

Common metamorphic rocks Foliated rocks Slate –Very fine-grained –Excellent rock cleavage –Made by low-grade metamorphism of shale

Example of slate

Slate roof

Common metamorphic rocks Foliated rocks Phyllite –Grade of metamorphism between slate and schist –Made of small platy minerals –Glossy sheen with rock cleavage –Composed mainly of muscovite and/or chlorite

Phyllite (left) and Slate (right) lack visible mineral grains

Common metamorphic rocks Foliated rocks Schist –Medium- to coarse-grained –Comprised of platy minerals (micas) –The term schist describes the texture –To indicate composition, mineral names are used (such as mica schist)

Mica Schist - note well developed foliation

A mica garnet schist

Common metamorphic rocks Foliated rocks Gneiss –Medium- to coarse-grained –Banded appearance –High-grade metamorphism –Composed of light-colored feldspar layers with bands of dark mafic minerals

Gneiss displays bands of light and dark minerals

Diorite to Gneiss Morph (orthogneiss - from igneous protolith)

Question: How is the compressive stress oriented relative the rock below… A)Horizontally B)Vertically C)Diagonally D)None of above

What are metamorphic textures? Texture refers to the size, shape, and arrangement of mineral grains within a rock Foliation – planar arrangement of mineral grains within a rock

Outcrop of foliated gneiss

Metamorphic textures Foliation Foliation can form in various ways: –Rotation of platy or elongated minerals –Recrystallization of minerals in a preferred orientation –Changing the shape of equidimensional grains into elongated and aligned shapes

Flattened Pebble Conglomerate = flattening

Development of foliation due to directed pressure

End of Chapter 7