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Earth Materials: Metamorphic Rocks
• What is metamorphism
Metamorphism
• Agents of metamorphism
Metamorphic rocks are formed from igneous rocks,
p rocks through
g byy
sedimentaryy rocks,, or other metamorphic
changes in pressure, changes in temperature, and/or the action
of fluids.
• Types of metamorphism
• Foliation
• Classification of metamorphic rocks
• Metamorphic grade
• The rock cycle
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Causes/Agents of Metamorphism
Pressure
confining 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
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
Two of the major types of metamorphism
Regional Metamorphism
Example with directed pressure
(foliated rocks)
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
- 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
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Contact Metamorphism
(non-foliated rocks)
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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
off mineral
i
l grains
i are called
ll d foliated
f li t d metamorphic
t
hi rocks
k
- the preferred orientation is the result of directed
pressure
Non-foliated metamorphic rocks
- lack a preferred orientation of mineral grains
Fig 3.5 Understanding Earth
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Common types of foliated metamorphic rocks
slate
• has slaty cleavage
• dull
• crystals too small to see
phyllite
• similar to slate but is shiny
• crystals too small to see
schist
• has visible crystals
• often composed primarily of micas
Foliation in metamorphic rocks
Note how some minerals are oriented perpendicular to the
direction of maximum pressure
gneiss
• has visible crystals
• has bands of different colors
Fig 8.5 Understanding Earth
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Slate
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Schist with micas and
large garnet crystals
Schist composed of micas
(mica schist)
Fig 8.8 Understanding Earth
Fig 8.10 Understanding Earth
Geology - Chernicoff
S. Kuehn 2005
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Other metamorphic rocks
Gneiss
(composition of major importance for classification)
Note the lighter and
darker colored bands
Typically non-foliated or weakly foliated:
quartzite - composed primarily of quartz
Geology - Chernicoff
marble - composed mostly of calcite and/or dolomite
Fig 8.8 Understanding Earth
Typically non-foliated:
hornfels - variable composition
skarn - contains mostly calcite and/or dolomite with garnet
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Quartzite, a type of nonfoliated metamorphic rock
The parent rock is sandstone
Other metamorphic rocks
Fig 8.9 Understanding Earth
mylonite
- formed in the deepest parts of fault zones
- a type of gneiss (so always foliated)
migmatite
- formed when a rock begins to partially melt
- represents the transition between metamorphic and igneous
rock
Marble, a type of nonfoliated metamorphic rock
The parent rock is limestone
- usually foliated
Fig 8.9 Understanding Earth
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Migmatite
The lighter-colored part of this rock represents a granite-like
liquid produced by partial melting Geology - Chernicoff
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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Different degrees of metamorphism
Original
rock
Metamorphic grade
Low grade
High grade
(relatively little change)
(much change)
- greater pressures and
greater temperatures
correspond to higher
grades of
metamorphism
Fig 8.2 Understanding Earth
Modified from: Fig 8.7 Understanding Earth
- refers to how much a
rock has been
changed by
metamorphism
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In contact
metamorphism, the
intensity (grade)
decreases with
distance from the
source of heat
(intrusion).
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Rock Cycle exercise
At different
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
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The rock cycle
Fig 3.10 Understanding Earth
Rocks, Minerals, and the Rock Cycle
Rock cycle; Formation of igneous, sedimentary, and metamorphic rocks
http://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
http://www.earth2class.org/er/teachers/lessonplans/index.php
Rocks & minerals at Volcano World
http://volcano.oregonstate.edu/education/vwlessons/lessons/lesson4.html
Rock identification (ppt)
http://www.gc.maricopa.edu/earthsci/imagearchive/rock_identification.htm
Resources for teachers
http://www.dlese.org/dds/query.do?q=rock&s=0
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