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Transcript
6.3 Metamorphic Rocks
Metamorphic Rock
• Meta means change
• Morphe means form
• A metamorphic rock forms when high
temperature and pressure combine and
change the texture, mineral composition or
chemical composition of a rock without
melting it
AGENTS OF
METAMORPHISM
heat
pressure
chemically-active fluids
Figure 4.18
Metamorphism
occurs at high
temperature
within Earth
although not
high enough
temperatures
(at least 600oC
for some rocks)
to cause
melting.
Earth’s
Geothermal
Gradient
HEAT is the most essential
agent of metamorphism
Increasing temperatures
-break chemical bonds,
-increase reaction rates
-and lead to formation of:
-New Minerals stable at
higher temperatures
-Larger mineral grains
Figure 8.4A
Increasing confining pressures produce denser minerals
and new minerals with more “compact” crystal structures.
Figure 8.4B
DIRECTED PRESSURE
Much of the pressure exerted on rocks does
not push in on the rocks evenly from all directions.
Figure 8.3
Directed Pressures develop at Convergent Plate Boundaries
DIRECTED
PRESSURE
Development
of Foliation
due to
DIRECTED
PRESSURE
Metamorphic Rock Foliation
Foliation is different from original bedding planes
Figure 8.8
Figure 8.12
Metamorphic Textures
•
Metamorphic Rocks are classified into two
textural groups
1) Foliated - layers and bands (left)
2) Nonfoliated - blocky crystal shapes (right)
Grades of Metamorphism
• Low-grade metamorphism is associated
with low temperatures and pressures
• High-grade metamorphism is associated
with high temperatures and pressures
Types of Metamorphic Rock
1) Regional Metamorphism
2) Contact Metamorphism
3) Hydrothermal Metamorphism
Regional Metamorphism
• When a large belt regionally has
metamorphed
• Results include changes in mineral and
rock types, plus folding and deformation of
rock layers
• Mountains can be an example of regional
metamorphism
Regional metamorphism occurs where
rocks are squeezed between two converging
tectonic plates during mountain building
Contact Metamorphism
• When molten material (like
an igneous intrusion)
comes into contact with
solid rock contact
metamorphism has
occurred
• Lava cools so contact
metamorphism from lava
occurs most near the lava
• As the distance from an
intrusion increases it is
harder for contact
metamorphism to occur
since the temperature has
dropped
Figure 8.19
Contact
Metamorphism
along
Mid-ocean
Ridge
around pluton
Hydrothermal Metamorphism
• When hot water
reacts with rock
and alters its
chemical and
mineral
composition
hydrothermal
metamorphism
has occurred
• Ore deposits of
gold, copper, zinc,
tungsten and lead
are formed in this
manner
Economic Importance of
Metamorphic Rocks and Minerals
• Metamorphism of ultrabasic igneous rocks
produce the minerals talc and asbestos
• Graphite can be formed by the
metamorphism of coal
The Rock Cycle
• The three types of rocks: igneous,
sedimentary, and metamorphic are
grouped based on how they form
• Igneous = crystallize from magma
• Sedimentary = cemented or precipitated
sediments
• Metamorphic = changes in temperature
and pressure
The Rock Cycle
• Once a rock forms does it remain the
same? Maybe but quite possibly it
changes into a different rock.
• Heat and pressure can change an igneous
rock into a metamorphic rock
• A metamorphic rock can break into
sediments that become cemented into a
sedimentary rock