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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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y , p g y
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
d fr
om:
Fig
8.7
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
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