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 CaSO 4 + 2H 2 0 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 Rocks GEOL 131: Metamorphic Rocks: Common Types Marble Parent: limestone Quartzite Parent: sandstone Anthracite Parent: coal

End of Metamorphic Rocks