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Metamorphism / Metamorphic Rocks
Metamorphism: occurs when rocks are subjected to heat,
pressure, and/or other environmental conditions
- The rock remains a solid during this time period
- Why Should You Study Metamorphic Rocks?
- Exposed metamorphic rocks make up large parts of
continents
- Certain minerals in metamorphic rocks give clues
about the conditions existing when they formed
- Used widely as building materials and manufacturing
!
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- Why does Metamorphism occur?
- Minerals are only stable only within certain
temperature and pressure ranges.
- Minerals will decompose and rearrange to form
different minerals that are stable at the new
temperature and pressure
!
!2
- Factors that Cause Metamorphism
* Temperature and Pressure!*
!
- Temperature
- The deeper you go, the hotter it gets
- Geothermal gradient = 20-300 C/Km
- Depth Temp
- Increases the rate of chemical reactions that
produce different minerals
- Heat sources
- intrusive magma
- deep burial along subduction zones
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- Pressure
- The deeper you go the more pressure there is
- lithostatic pressure results from the weight of
overlying rocks
- Depth Temp
- Expressed in kilobars
- 1 kilobar = 1000 atmospheres
- 1 atmosphere = 14.7 lb/in2
- 1 kilobar = 14700 lb/in2
- Pressure increases .3 kilobar/km
- Mineral grains are more closely packed
- Recrystallization may occur
- Differential pressure results from unequal forces
applied to the rock
!
Lithostatic pressure is applied equally in all directions in Earth’s
crust due to the weight of overlying rocks. Thus, pressure increases
with depth, as indicated by the sloping black line.
!4
!
A similar situation occurs when 200-ml cups composed of
Styrofoam are lowered to ocean depths of approximately 750 m
and 1,500 m. Increased water pressure is exerted equally in all
directions on the cups, and they consequently decrease in volume
while maintaining their general shape.
- Deformation: Caused by Differential
Pressure. Changes in the shape of the rock
due to mechanical forces. Results in foliation
(layering). Foliation is perpendicular to pressure.
Platy minerals
!
*The layers are said to have Slaty Cleavage*
!5
- Fluid Activity
- water and carbon dioxide are almost always
present in some amount in metamorphic regions
- these fluids enhance metamorphism by
increasing the rate of chemical reactions
- Fluid Source
- water trapped in the pore space of
sedimentary rocks (groundwater)
- from cooling magma (juvenile water)
- dehydration of water-bearing minerals that
are subjected to heat and pressure; water
released due to metamorphism
(metamorphic water)
- Textural Changes
- grain size increases
- ex.
limestone
(fine grained)
marble
(coarse grain)
!
Fossiliferous Limestone
Marble
- Mineralogical Changes
- 2 types of metamorphic reactions
1) No change in mineral type, just bigger grains
- limestone to marble
!6
2) Original mineral decompose
new mineral
- atoms rearrange to gain stability
- ex. shales to hornfels
!
Shale
Hornfels
How are Metamorphic Rocks Classified?
1) Foliated: minerals arranged in a parallel fashion
- subjected to differential pressure
- examples: slate, phyllite, schist, gneiss
2) Nonfoliated: minerals that are not layered
- most form from contact or regional metamorphism
from non-platy or non-elongated minerals.
- examples: marble, quartzite, greenstones, hornfels,
anthracite
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Three Main Types of Metamorphism
1) Contact Metamorphism: produced when a body of
magma alters the surrounding country rock
- causes old minerals to decompose and new ones
to form, causes metamorphism to occur
- Factors in contact metamorphism
- initial temperature and size of the intrusion
- presence and chemistry of fluids
- Metamorphic aureoles (halo)
- zones of mineral assemblages surrounding
intrusion
!
Contact metamorphic aureole or halo surrounding a pluton
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!
Metamorphic Aureole: A metamorphic aureole, the area
surrounding an intrusion, consists of zones that reflect the degree
of metamorphism.
!
Contact Metamorphism from a Lava Flow A highly weathered
basaltic lava flow near Susanville, California, has altered an
underlying rhyolitic volcanic ash by contact metamorphism. The
!9
red zone below the lava flow has been baked by the heat of the
lava when it flowed over the ash layer. The lava flow displays
spheroidal weathering, a type of weathering common in fractured
rocks (see Chapter 6).
- Rock Characteristics
- Nonfoliated
- Little to no deformation
- Typical Contact Metamorphism Rocks
- Hornfels: dark to black
- Skarn (tactite): Contact metamorphism of
carbonate rocks: limestone or dolomite
- usually large grained or crystals
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2) Dynamic Metamorphism
- occurs along fault zones
- fault: fracture along which movement has
occurred
- rocks are subjected to differential shear pressure.
!
!
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Mylonite: An outcrop of mylonite from the Adirondack Highlands,
New York. Mylonites result from dynamic metamorphism, where
rocks are subjected to high levels of differential pressure. Note the
thin laminations that are characteristic of many mylonites.
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3) Regional Metamorphism
- occurs when 2 lithospheric plates collide and squeeze
and deform rocks as a result
- highly deformed
- associated with mountain building
- produces most metamorphic rocks; makes up the
shields
- results from extreme heat and pressure generated by
the processes at convergent plate boundaries
- covers large geographic areas, and usually shows a
gradation of deformation corresponding to areas of the
most intense heat/pressure
- certain minerals are known to form under specific
conditions of temperature and pressure: index
minerals
- low, intermediate, and high grades of metamorphism
are recognized, based on conditions and the resulting
mineral assemblages
!12
- Foliated Metamorphic Rocks
- Heat and differential pressure cause minerals to
arrange themselves in a parallel fashion called
foliation
- size and shape of crystals determines texture;
fine to coarse
- with increasing grain size, common foliated
metamorphic rocks include:
- Slate
!
- Phyllite
!
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- Schist
!
- Gneiss
!
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Metamorphic Grade Change in mineral assemblage and rock type
with increasing metamorphism of shale. When a clay-rich rock
such as shale is subjected to increasing metamorphism, new
minerals form, as shown by the colored bars. The progressive
appearance of certain minerals, known as index minerals, allows
geologists to recognize low-, intermediate-, and high-grade
metamorphism.
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!
Metamorphic Zones and Facies
- metamorphic zone: area that has the same metamorphic
minerals
- occur because metamorphic minerals are only stable at
certain pressures and temperatures
- metamorphic facie: assemblages that were formed under
similar pressures and temperatures
- rocks that have the same minerals, have same geologic
history
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!
Metamorphic Zones in the Upper Peninsula of Michigan The zones
in this region are based on the presence of distinctive silicate
mineral assemblages resulting from the metamorphism of
sedimentary rocks during an interval of mountain building and
minor granitic intrusion during the Proterozoic Eon, about 1.5
billion years ago. The lines separating the different metamorphic
zones are isograds.
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!
Metamorphic Facies and Their Associated Temperature–Pressure
Conditions. A temperature–pressure diagram showing under what
conditions various metamorphic facies occur. A metamorphic
facies is characterized by a particular mineral assemblage that
formed under the same broad temperature–pressure conditions.
Each facies is named after its most characteristic rock or mineral.
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- Regional Burial Metamorphism (subset of regional)
- occurs when rock is buried deeper than 2 Km
- Lots of pressure; 1 cubic foot of granite equals
168 lbs
- Non-foliated
- Common rocks
- Marble: parent rock is limestone
!
!
Limestone
Marble
- Quartzite: parent rock is quartz sandstone
!
!
Quartz sandstone
Quartzite
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- In sandstone (A) fractures run between the
sand grains. In quartzite (B) fractures run
through the grains
!
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