Metamorphism, Metamorphic Rocks, and Hydrothermal

Rocks

Chapter 7

Metamorphic gneiss from Greenland, 3.7 Ba

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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

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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