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