metamorphism, metamorphic rocks, and hydrothermal rocks chapter 7 metamorphic gneiss from greenland,...
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Metamorphism, Metamorphic Rocks, and Hydrothermal
Rocks
Chapter 7
Metamorphic gneiss from Greenland, 3.7 Ba
© 2008, John Wiley and Sons, Inc.
Michaelangelocarving of Pieta
Looked far and widefor a large pristinemarble block
He watched the rays of the rising sun strike the block and make it transparent as pink alabaster, with not a hole or hollow or crack or knot in all its massive white weight. It tested out perfect against the hammer, against water, its crystals soft and compacted with fine graining. His Pieta had come home. -Irving Stone (The Agony and Ecstasy)
Metamorphic Rocks• Metamorphism refers to solid-state
changes to rocks in Earth’s interior– Produced by increased heat, pressure, or
the action of hot, reactive fluids
– Old minerals, unstable under new conditions, recrystallize into stable ones
• Rocks produced from pre-existing or parent rocks in this way are called metamorphic rocks
bread toast
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Metamorphic Rocks – are like Cooking
* Ingredients added together are cooked at high temperatures to produce a new and different substance with different textures and form.* Rocks exposed to high temps and pressures react chemically, change mineralogy, and textures.
© 2008, John Wiley and Sons, Inc.
Metamorphic Rocks
Metamorphism is the mineralogical, textural, chemical, and structural changes that occur in rocks as a result of exposure to elevated temperatures and/or pressures.
Metamorphism occurs in solid state, without any melting processes.
So deformation process is preserved in the metamorphic rocksand can be “tracked” in time and through tectonic cycles.
© 2008, John Wiley and Sons, Inc.
Metamorphic Rocks
Factors effecting metamorphism:
TemperaturePressureRock compositionFluids presentStress environment
• Metamorphic rocks common in the old, stable cores of continents, known as cratons
Metamorphic Rocks
*Oldest stable continental core*Seismically fast*Geothermally cold*Buoyant – low density (chemically depleted)
Factors Controlling Metamorphic Rock Characteristics
• Temperature during metamorphism– Heat for metamorphism comes from Earth’s deep interior
– If temperature gets high enough, melting will occur
• Pressure during metamorphism– Confining pressure applied equally in all directions
– Pressure proportional to depth within the Earth• increases ~1 kilobar per 3.3 km of burial within the crust
1 kb = 14,500 psi and VW bug = 2000 lbsPressure at 100 km depth = 239 VW Bugs on 1 square inch area (A bigger area of 1 sq foot would require 34,452 VW bugs!)
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Temperature and pressure conditions for metamorphism
Figure 10.2
© 2008, John Wiley and Sons, Inc.
Temperature and pressure conditions for metamorphism
Figure 10.2
Temperature and pressure conditions for metamorphism
Figure 10.2
Low grade metamorphism: Rocks that are metamorphosed under temperature and pressures upto 400C and 400 MPa
High grade metamorphism: Rocks that are metamorphosed under temperature and pressures higher than 400C and 400 MPa
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From shale to gneiss Figure 10.3
Shale
Slate
Phyllite
Gneiss
Diagenesis
Low-Grade
Medium-Grade
High-Grade
* Sillmanite and KyaniteAl2O3 with different
crystal structures
Pore Fluids and Metamorphic Veins
The presence of pore space and aqueous fulids can facilitate metamorphic reactions. Porous fluid flow transports mineral chemicals and speeds up mineral growth and recrystallization.
Metamorphic Rocks Under Stress
Granite consists of quartz, feldspar, biotite which recrystallized from magma under conditions of uniform stress. Grains are randomly oriented.
This gneiss, a high-grade metamorphic rock, contains the same minerals as the rock on the left but deformed in solid state under differential stress. Biotite grains are aligned in a layered texture.
Foliation is the planar arrangement of mineral grains giving a layered texture
Slaty cleavage
Figure 10.7
Horizontal slaty cleavage
Compression
Compression
Vertical slaty cleavage
Bedding Planes
Bedding planes are perpendicular to the maximum stress direction.
Bedding and cleavage planes change if applied stress changes.
Metamorphism and Billiards
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Wood in early pool tables vibrated and warped. Slate used in 1825 – better table and game. Slaty (low grade met.) cleavage surface, strong, still.
Rocks with Foliation
Rocks without Foliation
Metamorphic Rocks
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Metamorphism:New Rocks from Old
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Metamophism of Sedimentary Rocks
Shale Slate Phyllite
SedimentaryRock
Fine grained, low grade metamorphic rock with slaty cleavage.
Fine grained, high grade metamorphic rock with foliation.
Metamophism of Sedimentary Rocks
Limestone Marble
Travertine, Getty Museum
Metamophism of Sedimentary Rocks
Sandstone Quartzite
Dominantly quartz minerals Very little foliation due to low diversity of minerals.
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Metamorphism of Volcanic Rocks
Granite Gneiss
Mineral content includesquartz, feldspar, biotite micas
Metamorphism of complex mineralogy induces foliation.
Figure 10.9
Greenschist Amphibolite
Blueschist Ecologite
Increasing grade of metamorphism
Increasing pressure(w
ithout increasing tem
perature)
Metamorphism of Basalt
Tectonics and Basalt Metamorphism
Figure 10.13 Blueschist and eclogite metamorphism
Basalt
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Metamorphic facies
Figure 10.18
Types of Metamorphism
Metamsomatism
Metamorphic Processes
Metamorphism:New Rocks from Old
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Mechanical deformation
Conglomerate pebbles Deformed pebbles due to differential pressure and shear.
Types of Metamorphism
Metamorphic Processes
Metamorphism:New Rocks from Old
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Types of Metamorphism
-Contact metamorphism: Occurs when rocks are heated and chemically changed adjacent to an introduced body of hot magma
-Burial: Occurs as a result of burial of sediments in deep sedimentary basins
-Regional: Extensive area of crust associated with plate convergence, collision, and subduction.
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Contact metamorphism
Figure 10.12
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Tectonics and Metamorphism
Figure 10.13
Zone of burial metamorphism
Blueschist and eclogite metamorphism
Regional metamorphism
Granitic magma rises and causes contact metamorphism
© 2008, John Wiley and Sons, Inc.
Nordlingen Cathedral(in Germany)
Constructed within an impact crater. Rock construction include local rocks that were metamorphosed by high heat and pressure during impact
Shocked quartz from impact