METAMORPHIC ROCKS

METAMORPHIC ROCKS What is Metamorphism? What Drives Metamorphism? Metamorphic Textures Common Metamorphic Rocks Metamorphic Environments

Metamorphic Rocks What Is Metamorphism?

WHAT IS METAMORPHISM? “meta” = change; “morph” = shape GEOL 131: Metamorphic Rocks 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

Metamorphic Grade Intensity levels of metamorphic conditions GEOL 131: Metamorphic Rocks: What Is Metamorphism? Metamorphic Grade Intensity levels of metamorphic conditions Higher grade = hotter and (usually) greater pressure

GEOL 131: Metamorphic Rocks: What Is Metamorphism? Metamorphic Settings Geologic environments where metamorphism can occur Enough heat to drive chemical changes, but not enough to cause melting

What Drives Metamorphism? Metamorphic Rocks What Drives Metamorphism?

Four Metamorphic Agents GEOL 131: Metamorphic Rocks: What Drives Metamorphism? Four Metamorphic Agents Heat: most critical Confining Pressure Differential Stress Chemically active fluids

Heat A form of energy Vibration of atoms and molecules GEOL 131: Metamorphic Rocks: What Drives Metamorphism? Heat A form of energy Vibration of atoms and molecules Hotter = faster vibration

Role of Heat in Metamorphism GEOL 131: Metamorphic Rocks: What Drives Metamorphism? Role of Heat in Metamorphism Provides energy for chemical reactions Weakens atomic bonds in mineral crystals

Role of Heat in Metamorphism GEOL 131: Metamorphic Rocks: What Drives Metamorphism? Role of Heat in Metamorphism Compositional changes Minerals become unstable at higher temps Change into new, stable minerals From: facweb.bhc.edu Clay minerals in shale (above) change into micas when shale metamorphoses into schist (right). From: itc.gsw.edu

Role of Heat in Metamorphism GEOL 131: Metamorphic Rocks: What Drives Metamorphism? Role of Heat in Metamorphism Atomic structure changes Weakened atomic bonds allow atoms to be rearranged From: pengellytrust.org Calcite and aragonite have the same composition (calcium carbonate) but different atomic structures.

Sources of Heat Geothermal Magmas Remnant heat of formation GEOL 131: Metamorphic Rocks: What Drives Metamorphism? Sources of Heat Geothermal Remnant heat of formation Radioactive decay Magmas

Confining Pressure Equal in all directions Deep burial GEOL 131: Metamorphic Rocks: What Drives Metamorphism? Confining Pressure Equal in all directions Deep burial

Differential Stress Higher pressure from sides GEOL 131: Metamorphic Rocks: What Drives Metamorphism? Differential Stress Higher pressure from sides Converging tectonic plates Zone of intense differential stress

Chemically Active Fluids GEOL 131: Metamorphic Rocks: What Drives Metamorphism? Chemically Active Fluids Some minerals are hydrated E.g. gypsum CaSO4 + 2H20 Water driven out by heat Circulating water promotes transport of atoms

Metamorphic Rocks Metamorphic Textures

Foliation Created by differential stress GEOL 131: Metamorphic Rocks: Textures Foliation Created by differential stress Preferred orientation of elongated minerals

Foliated Textures Slaty cleavage (low-grade) Gneissic texture GEOL 131: Metamorphic Rocks: Textures Foliated Textures Slaty cleavage (low-grade) Gneissic texture (high-grade) Schistosity (intermediate-grade)

How Foliation Develops: Three Ways GEOL 131: Metamorphic Rocks: Textures How Foliation Develops: Three Ways Rotation Recrystallization Flattening

How Foliation Develops: Rotation GEOL 131: Metamorphic Rocks: Textures How Foliation Develops: Rotation Elongated crystals rotate into alignment

How Foliation Develops: Recrystallization GEOL 131: Metamorphic Rocks: Textures How Foliation Develops: Recrystallization Original grains replaced by more stable ones New grains are elongated and aligned

How Foliation Develops: Elongation GEOL 131: Metamorphic Rocks: Textures How Foliation Develops: Elongation Original equant grains are “squashed” into elongated ones

Nonfoliated Textures Confining pressure Absence of pressure GEOL 131: Metamorphic Rocks: Textures Nonfoliated Textures Confining pressure Absence of pressure Equant grains, usually interlocking

Nonfoliated Textures GEOL 131: Metamorphic Rocks: Textures Limestone metamorphoses into the nonfoliated rock marble. Marble, made of interlocking equant calcite grains.

Nonfoliated Textures Porphyroblastic texture GEOL 131: Metamorphic Rocks: Textures Nonfoliated Textures Porphyroblastic texture Unusually large crystals (porphyroblasts) Schist with garnet porphyroblasts

Common Metamorphic Rocks

Common Foliated Rocks Slate Phyllite Schist Gneiss GEOL 131: Metamorphic Rocks: Common Types Common Foliated Rocks Slate Phyllite Schist Gneiss From: radford.edu

Common Nonfoliated Rocks GEOL 131: Metamorphic Rocks: Common Types Common Nonfoliated Rocks Quartzite Parent: sandstone Marble Parent: limestone Anthracite Parent: coal

Metamorphic Environments Metamorphic Rocks Metamorphic Environments

What is a Metamorphic Environment? GEOL 131: Metamorphic Rocks: Environments What is a Metamorphic Environment? Defined by a given temperature and pressure “P/T” conditions

Contact Low pressure, high temperature Contact with magma GEOL 131: Metamorphic Rocks: Environments Contact Low pressure, high temperature Contact with magma Nonfoliated rocks

Hydrothermal Low pressure, low temperature Contact with hot water GEOL 131: Metamorphic Rocks: Environments Hydrothermal Low pressure, low temperature Contact with hot water Common at undersea volcanoes Nonfoliated rocks

Burial & Subduction Zone GEOL 131: Metamorphic Rocks: Environments Burial & Subduction Zone Burial Under sediment or volcanic deposits High confining pressure, low temp 100s to 1000s of feet deep

Burial & Subduction Zone GEOL 131: Metamorphic Rocks: Environments Burial & Subduction Zone Subduction zone High differential stress, high temp 10s of miles deep

Regional Continental collision zones Very large-scale GEOL 131: Metamorphic Rocks: Environments Regional Continental collision zones Very large-scale High confining pressure and differential stress Low to high temp

Regional GEOL 131: Metamorphic Rocks: Environments Collision of India with Asia created a large zone of regional metamorphism.

End of Metamorphic Rocks Chapter