Unusual mantle Poisson's ratio, subduction, and crustal structure in central Alaska
METAMORPHIC ROCKS. TERMS Subduction – Descent of one crustal plate beneath another – Creates...
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Transcript of METAMORPHIC ROCKS. TERMS Subduction – Descent of one crustal plate beneath another – Creates...
METAMORPHIC ROCKS
TERMS
• Subduction– Descent of one crustal plate beneath another– Creates intense horizontal pressure
• Preferred orientation– Parallel alignment of elongated mineral crystals
TERMS
• Recrystallization– Replacement of original mineral crystals with new
ones
• Equant– Having the same measurements in all directions– E.g. a sphere or a cube
METAMORPHIC ROCKS
• What is Metamorphism?
• What Drives Metamorphism?
• Metamorphic Textures
• Common Metamorphic Rocks
Metamorphic Rocks
What Is Metamorphism?
WHAT IS METAMORPHISM?• “meta” = change; “morph” = shape
• Change of one rock type into another– Heat and (usually) pressure
• Parent rock– Rock that gets metamorphosed into something else
– ANY rock type can be a parent rock, including a metamorphic rock
GEOL 131: Metamorphic Rocks
Metamorphic Rocks
What Drives Metamorphism?
Three Metamorphic Agents
• Heat: most critical
• Pressure
• Chemically active fluids
GEOL 131: Metamorphic Rocks: What Drives Metamorphism?
Heat• A form of energy• Vibration of atoms and molecules• Hotter = faster vibration
GEOL 131: Metamorphic Rocks: What Drives Metamorphism?
Role of Heat in Metamorphism
• Provides energy for chemical reactions
• Weakens atomic bonds in mineral crystals
GEOL 131: Metamorphic Rocks: What Drives Metamorphism?
Sources of Heat
• Geothermal– Remnant heat of formation– Radioactive decay
• Magmas
GEOL 131: Metamorphic Rocks: What Drives Metamorphism?
Pressure
• Confining pressure
• Differential stress
GEOL 131: Metamorphic Rocks: What Drives Metamorphism?
Confining Pressure
• Equal in all directions
• Deep burial
GEOL 131: Metamorphic Rocks: What Drives Metamorphism?
Differential Stress
• Higher pressure from sides• Converging tectonic plates
GEOL 131: Metamorphic Rocks: What Drives Metamorphism?
Zone of intense differential stress
Chemically Active Fluids
• Some minerals are hydrated– E.g. gypsum CaSO4 + 2H20
• Water driven out by heat
• Circulating water promotes transport of atoms
GEOL 131: Metamorphic Rocks: What Drives Metamorphism?
Metamorphic Rocks
Metamorphic Textures
Foliation
• Created by differential stress
• Preferred orientation of elongated minerals
GEOL 131: Metamorphic Rocks: Textures
Foliated Textures
Slaty cleavage
Schistosity Gneissic texture
GEOL 131: Metamorphic Rocks: Textures
How Foliation Develops: Three Ways
• Rotation
• Recrystallization
• Elongation
GEOL 131: Metamorphic Rocks: Textures
How Foliation Develops: Rotation
• Elongated crystals rotate into alignment
GEOL 131: Metamorphic Rocks: Textures
How Foliation Develops: Recrystallization
• Original grains replaced by more stable ones
• New grains are elongated and aligned
GEOL 131: Metamorphic Rocks: Textures
How Foliation Develops: Elongation
• Original equant grains are “squashed” into elongated ones
GEOL 131: Metamorphic Rocks: Textures
Nonfoliated Textures
• Confining pressure
• Absence of pressure
• Equant grains, usually interlocking
GEOL 131: Metamorphic Rocks: Textures
Nonfoliated Textures
Marble, made of interlocking equant calcite grains.
Limestone metamorphoses into the nonfoliated rock marble.
GEOL 131: Metamorphic Rocks: Textures
Metamorphic Rocks
Common Metamorphic
Rocks
Common Foliated Rocks
• Slate• Phyllite• Schist• Gneiss
GEOL 131: Metamorphic Rocks: Common Types
From: radford.edu
Common Nonfoliated RocksGEOL 131: Metamorphic Rocks: Common Types
MarbleParent: limestone
QuartziteParent: sandstone
AnthraciteParent: coal
End of Metamorphic Rocks