Volcanoes and Igneous Activity Earth

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Transcript Volcanoes and Igneous Activity Earth

Metamorphism and Metamorphic Rocks
Metamorphism
The transformation of rock by
temperature and pressure
Metamorphic rocks are produced by
transformation of:
• Igneous, sedimentary and igneous rxs
Metamorphism in Hollywood
2007
Heat and Stress
Increasing
Why are the changes in Michael Jackson’s facial
features such a perfect (albeit warped) analog for
rock metamorphism?
A) The physical composition of his face is unchanged
(in metam. rock, bulk composition remains constant)
B) The physical appearance of his face has changed
(in metam. rock, mineral assemblages change)
C) He will continue to change with new external conditions
(increased heat and pressure will cause further change)
D) All of the above
Hollywood’s analogue to metamorphismComparison of timescales
Given Michaels rate of change over 25 years,
how much faster is this than the time it takes to
metamorphose rocks under added heat and pressure
A) 1000 times faster
B) 10,000 times faster
C) 100,000 times faster
D) 1000,000 times faster
Metamorphism
Metamorphism progresses from low to
high grades
Rocks remain solid during metamorphism
Agents of
metamorphism…
Heat
• Most important agent
• Heat drives recrystallization - creates new, stable
minerals
Pressure (stress)
• Increases with depth
• Pressure can be applied equally in all directions
or differentially
Why does temperature increase with
depth beneath the surface?
A) heat given off from radioactive decay
B) pressure greater beneath surface, heats things up
C) remnant heat from planetary formation
D) A & C
E) A, B, & C
Origin of pressure in metamorphism
Main factor affecting
metamorphism
Parent rock
• Metamorphic rocks typically have the
same chemical composition as the rock
they were formed from
• Different minerals, but made of the same
stuff.
Metamorphism
Three types of metamorphic settings:
• Contact metamorphism – from a rise in
temperature within host rock
• Hydrothermal metamorphism – chemical
alterations from hot, ion-rich water
• Regional metamorphism -- Occurs in the
cores of mountain belts and makes great
volumes of metamorphic rock
Contact metamorphism
Produced mostly by local heat source
Hydrothermal metamorphism
Requires hot circulation of fluids
Regional metamorphism
in Mountain Belts
Requires crustal thickening and shortening
Metamorphism and plate tectonics
Most regional metamorphism occurs
along convergent plate boundaries
• Compressional stresses deform plate edge
• Occurs in major mountain belts: Alps,
Himalayas, and Appalachians
Location of metamorphic zones
in a subduction zone
Metamorphism and plate tectonics
Metamorphism at subduction zones
• Cores of subduction zones contain linear
belts of metamorphic rocks
– High-P, low-T zones near trench
– High-T, low-P zones in region of igneous
activity (arc)
Location of metamorphic zones
in a subduction zone
Metamorphic Environments
Index minerals and metamorphic grade
• Certain minerals, called index minerals,
are good indicators of the metamorphic
conditions in which they form
Index Minerals in metamorphic rocks
Metamorphic Environments
Metamorphic grade
• A group of minerals that form in a
particular P-T environment
Zeolite (really low T,P; <200C)
Greenschist (low T, P; 200-450C, 10-15 km)
Blueschist (low T, high P - subduction zones)
Amphibolite (high T, P; 450-650C, 15-20 km)
Granulite (super high T, P; >700C, >25km)
A certain type of metamorphic rock forms when the
temperature equals 200 deg C. At what depth do
these rocks form, if the geothermal gradient is 45
degC per km, and temperature at surface is 20
degC?
A)
B)
C)
D)
E)
2 km
3 km
4 km
5 km
None of the above
A certain type of metamorphic rock forms when the
temperature equals 200 deg C. At what depth do
these rocks form, if the geothermal gradient is 45
degC per km, and temperature at surface is 20
degC?
A)
B)
C)
D)
E)
2 km
3 km
4 km
5 km
None of the above
Metamorphic Environments in Subduction Zones
Greenschist Hand Sample
Greenschist Thin Section
Mica
Schist
Blueschist
Amphibolite
Common metamorphic rocks
Nonfoliated rocks
• Quartzite
– Formed from a parent rock of quartz-rich
sandstone
– Quartz grains are fused together
– Forms in intermediate T, P conditions
Sample of
quartzite
Thin section
of quartzite
Flattening of quartz grains in quartzite
Sandstone to Quartzite Morph
Common metamorphic rocks
Nonfoliated rocks
• Marble
– Coarse, crystalline
– Parent rock usually limestone
– Composed of calcite crystals
– Fabric can be random or oriented
Marble
Marble
Marble (Random fabric = annealing; nonfoliated)
Change in metamorphic grade with depth
Shale to Slate to Schist Morph
Common metamorphic rocks
Foliated rocks
• Slate
– Very fine-grained
– Excellent rock cleavage
– Made by low-grade metamorphism of shale
Example of slate
Slate roof
Common metamorphic rocks
Foliated rocks
• Phyllite
– Grade of metamorphism between slate and schist
– Made of small platy minerals
– Glossy sheen with rock cleavage
– Composed mainly of muscovite and/or chlorite
Phyllite (left) and Slate (right)
lack visible mineral grains
Common metamorphic rocks
Foliated rocks
• Schist
– Medium- to coarse-grained
– Comprised of platy minerals (micas)
– The term schist describes the texture
– To indicate composition, mineral names are
used (such as mica schist)
Mica Schist - note well developed foliation
A mica garnet schist
Common metamorphic rocks
Foliated rocks
• Gneiss
– Medium- to coarse-grained
– Banded appearance
– High-grade metamorphism
– Composed of light-colored feldspar layers
with bands of dark mafic minerals
Gneiss displays bands of light and
dark minerals
Diorite to Gneiss Morph
(orthogneiss - from igneous protolith)
Question:
How is the compressive stress oriented relative the rock below…
A) Horizontally
B) Vertically
C) Diagonally
D) None of above
What are metamorphic textures?
Texture refers to the size, shape, and
arrangement of mineral grains within a
rock
Foliation – planar arrangement of
mineral grains within a rock
Outcrop of foliated gneiss
Metamorphic textures
Foliation
• Foliation can form in various ways:
– Rotation of platy or elongated minerals
– Recrystallization of minerals in a preferred
orientation
– Changing the shape of equidimensional
grains into elongated and aligned shapes
Flattened Pebble Conglomerate = flattening
Development of foliation due
to directed pressure
End of Chapter 7