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Earth Materials: Metamorphic Rocks

• What is metamorphism

• Agents of metamorphism

• Types of metamorphism

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• Foliation

• Classification of metamorphic rocks

• Metamorphic grade

• The rock cycle

Metamorphism

Metamorphic rocks are formed from igneous rocks,

sedimentary rocks, or other metamorphic rocks through by

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changes in pressure, changes in temperature, and/or the action

of fluids.

Causes/Agents of Metamorphism

Pressureconfining pressure (lithostatic pressure)

-equal pressure in all directions- increased confining pressure can cause minerals with more compact (denser) structures to form

directed pressure- pressure which is greater in a specific direction- may cause folding or cause mineral to grow with a specific orientation

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Temperature- different minerals are stable at different temperatures- changing the temperature can cause the types of minerals present in a rock to change

Hydrothermal fluids (mostly water and/or carbon dioxide plus dissolved material)- can change the overall chemical composition of a rock by adding or removing material- many important ore deposits have been formed by this metamorphic process

Major Types of Metamorphism

Regional metamorphism- the most wide-spread- occurs over very large areas- changes in pressure and temperature are the most important

factors - commonly results from:

- mountain building - with directed pressure

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- deep burial - lithostatic pressure only

Contact metamorphism- occurs near the edges of igneous intrusions- the intensity of metamorphism is greatest close to the intrusion

and less farther away- increased temperatures and hydrothermal fluids are the most

important factors- the zone of contact metamorphism is called a contact aureole

Two of the major types of metamorphism

Regional MetamorphismExample with directed pressure

(foliated rocks)

Contact Metamorphism

(non-foliated rocks)

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Fig 3.5 Understanding Earth

Metamorphic textures

Foliated metamorphic rocks

- foliation refers to a preferred orientation of mineral grains found in some metamorphic rocks

- metamorphic rocks which have a preferred orientation f i l i ll d f li t d t hi k

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of mineral grains are called foliated metamorphic rocks

- the preferred orientation is the result of directed pressure

Non-foliated metamorphic rocks

- lack a preferred orientation of mineral grains

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Foliation in metamorphic rocksNote how some minerals are oriented perpendicular to the direction of maximum pressureFig 8.5 Understanding Earth

Common types of foliated metamorphic rocks

slate• has slaty cleavage• dull• crystals too small to see

phyllite• similar to slate but is shiny

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• crystals too small to see

schist• has visible crystals• often composed primarily of micas

gneiss• has visible crystals• has bands of different colors

Slate

Fig 8.8 Understanding Earth

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Geology - Chernicoff

Schist composed of micas(mica schist)

Schist with micas and large garnet crystals

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Fig 8.10 Understanding Earth

S. Kuehn 2005

Geology - Chernicoff

Gneiss

Note the lighter and darker colored bands

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Fig 8.8 Understanding Earth

Other metamorphic rocks(composition of major importance for classification)

Typically non-foliated or weakly foliated:

quartzite - composed primarily of quartz

marble composed mostly of calcite and/or dolomite

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marble - composed mostly of calcite and/or dolomite

Typically non-foliated:

hornfels - variable composition

skarn - contains mostly calcite and/or dolomite with garnet

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Quartzite, a type of non-foliated metamorphic rockThe parent rock is sandstoneFig 8.9 Understanding Earth

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Marble, a type of non-foliated metamorphic rockThe parent rock is limestoneFig 8.9 Understanding Earth

Other metamorphic rocks

mylonite

- formed in the deepest parts of fault zones

- a type of gneiss (so always foliated)

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migmatite

- formed when a rock begins to partially melt

- represents the transition between metamorphic and igneousrock

- usually foliated

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Mylonite Produced by shearing in the deepest parts of fault zones. Arrows indicate the shear direction in this sample Understanding Earth

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MigmatiteThe lighter-colored part of this rock represents a granite-like liquid produced by partial melting Geology - Chernicoff

Metamorphic grade

- refers to how much a rock has been changed by metamorphism

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- greater pressures and greater temperaturescorrespond to highergrades of metamorphism

Fig 8.2 Understanding Earth

Different degrees of metamorphism

Low grade(relatively little change)

High grade (much change)

Original rock

Mod

ifie

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om:

Fig

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Und

erst

andi

ng E

arth

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In contact metamorphism, the intensity (grade) decreases with distance from the source of heat (intrusion).

At different

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temperatures, different minerals form in the metamorphic auriole around the intrusion.

A geologist studying the metamorphism could use the minerals like a thermometer.

Fig 8.15 Understanding Earth

Rock Cycle exercise

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The rock cycleFig 3.10 Understanding Earth

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Rocks, Minerals, and the Rock Cycle

Rock cycle; Formation of igneous, sedimentary, and metamorphic rockshttp://serc.carleton.edu/NAGTWorkshops/visualization/collections/rock_cycle.html

Rock cycle http://www.rocksandminerals.com/rockcycle.htm

Rocks, minerals, rock cycle, etc. http://www earth2class org/er/teachers/lessonplans/index php

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http://www.earth2class.org/er/teachers/lessonplans/index.php

Rocks & minerals at Volcano Worldhttp://volcano.oregonstate.edu/education/vwlessons/lessons/lesson4.html

Rock identification (ppt)http://www.gc.maricopa.edu/earthsci/imagearchive/rock_identification.htm

Resources for teachershttp://www.dlese.org/dds/query.do?q=rock&s=0