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Metamorphism, Metamorphic Rocks, and Hydrothermal Rocks Chapter 7 Metamorphic gneiss from Greenland, 3.7 Ba.

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Presentation on theme: "Metamorphism, Metamorphic Rocks, and Hydrothermal Rocks Chapter 7 Metamorphic gneiss from Greenland, 3.7 Ba."— Presentation transcript:

1 Metamorphism, Metamorphic Rocks, and Hydrothermal Rocks Chapter 7 Metamorphic gneiss from Greenland, 3.7 Ba

2 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 breadtoast

3 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)

4 Factors Controlling Metamorphic Rock Characteristics Texture and mineral content of metamorphic rocks depend on: –Parent rock composition –Temperature and pressure –Effects of tectonic forces –Effects of fluids, such as water

5 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 lbs Pressure 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!)

6 Characteristics of Metamorphic Rock Tectonic forces –Often lead to forces that are not equal in all directions (differential stress) –Compressive stress causes flattening perpendicular to stress –Shearing causes flattening by sliding parallel to stress –Planar rock texture of aligned minerals produced by differential stress is known as foliation

7 Deformation of Metamorphic Rock Fluids –Hot water (as vapor) is most important, –Rising temperature causes water to be released from unstable minerals Time –Metamorphism, particularly from high pressures, may take millions of years –Longer times allow newly stable minerals to grow larger and increase foliation

8 Metamorphic Rock Classification Classification based on rock texture –Foliated (layered) vs. non-foliated (non-layered) –Foliated rocks named based on type of foliation (slaty, schistose, gneissic) –Non-foliated rocks named based on composition marble Foliated gneiss limestone granite

9 Types of Metamorphism Contact metamorphism –High temperature –Produces non-foliated rocks –Rocks come in contact with magma bodies intruding cooler country rock quartzite marble

10 Types of Metamorphism Regional metamorphism –High pressure –Results in rocks with foliated textures –Can deform in mountain ranges –May occur over wide temperature range

11 Types of Metamorphism Regional metamorphism –Higher pressure and temperature will produce increased metamorphic grade –Prograde metamorphism of shale produces: slate phyllite schist gneiss slate phyllite schist gneiss

12 Types of Metamorphism Partial melting during metamorphism produces migmatites –Migmatites exhibit both intrusive igneous and foliated metamorphic textures Shock metamorphism is produced by rapid application of extreme pressure –Meteor impacts produce this –Shocked rocks are found around and beneath impact craters

13 Plate Tectonics and Metamorphism Regional metamorphism associated with convergent plate boundaries –Pressure proportional to depth –Temperature varies laterally at convergent boundaries Isotherms bow down in sinking oceanic plate and bow up where magma rises

14 Hydrothermal Processes Rocks precipitated from or altered by hot water are referred to as hydrothermal –Common at spreading centers (under water) Hydrothermal processes add water for metamorphic reactions Formation of hydrothermal rocks Water passes through rocks and precipitates new minerals on walls of cracks and in pore spaces Metallic ore deposits often form this way (veins)


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