Chapter 7 Metamorphism and Metamorphic Rocks

METAMORPHISM & METAMORPHIC ROCKS

NORMAL DEPTHS FOR METAMORPHISM: >10-KM FROM SURFACE (BELOW SEDIMENTARY DEPTHS & ABOVE IGNEOUS DEPTHS)
METAMORPHISM: MINERAL CONTENT & STRUCTURE OF SOLID ROCK ALTERED BY HEAT, PRESSURE & CHEMICALLY ACTIVE FLUIDS – DOESN’T MELT
CONDITIONS PROMOTING METAMORPHISM:
ROCK STABILITY IS RELATED TO FORMATION ENVIRONMENT
METAMORPHISM BREAKS SOME BONDS, BUT DOES NOT INCLUDE MELTING
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Sedi., meta., igne. rocks

Metamorphic Rocks in North America

CONDITIONS-CONTD

l  HEAT – MOST IMPORTANT FACTOR

GEOTHERMAL GRADIENT (20-30 DEG C/KM-200 DEG C TO METAMORPHOSE ROCKS)

l  SOURCES OF HEAT FOR METAMORPHISM:

–  CONDUCTION FROM DEEP INTERIOR

–  RISING MAGMA

–  RADIOACTIVE ISOTOPES

–  FRICTION BETWEEN ROCK BODIES

PRESSURE

Pressure-contd.

l  Lithostatic: Confining pressure in all directions, rock becomes compressed

l  Directed Pressure: Acts in one principal plane

l  Results of Pressure:

–  Compaction – Smaller & Denser

–  Ion migration of unbonded ions (recrystallization)

–  Foliation – Preferred alignment of minerals -Subjected to direct pressure

–  Units of Pressure: 1 Bar = 1 Atm. Pressure = 1.02 kg/cm2 = 14.7 lbs/inch2 = 105 Pascal

Circulating Fluids

l  Often necessary for chemical reactions (Facilitates movement of ions and atoms)

l  Sources of Water

–  Surface Infiltration

–  Pores in Sedimentary Rocks

–  Cracks on Subducting Plates

–  Magmas

–  Decomposing Minerals

ALL METAMORPHIC ROCKS ARE CATEGORIZED AS ‘FOLIATED’ OR ‘NON FOLIATED’

PARALLEL CLEAVAGE

MINERAL GRAINS

MINERAL GRAINS-CONTD.

Deep burial of rocks

What Drives Metamorphism

l  Heat

u Accelerate pace of chemical reactions

l  Pressure

u Lithostatic (confining)- rock becomes smaller and denser

u Directed- minerals become aligned- Foliation

l  Circulating Fluids

u Ions in water- change mineral composition

l  Parent Rocks

u Original rock’s composition will affect the outcome of metamorphism

CONFINING PRESSURE

DIRECTED PRESSURE

TYPES OF METAMORPHISM

l  Contact

u  Heat is the dominant factor

u  Area affected generally smaller than regional metarmorphism

l  Regional are two types with extensive coverage

t  Burial- occurs in deep sedimentary basins- no plate tectonics involved
t  Dynamothermal- occurs where converging plates squeeze a rock caught between them

l  Others

u  Hydrothermal- involves hot water from magma

u  Fault-zone- rocks grinding past one another

u  Shock- meteorites strike

u  Pyrometamorphism- lightning

CONTACT METAMORPHISM

DYNAMOTHERMAL METAM.

HYDROTHER. METAM-CONTD

Hydrothermal Metamorphism

METAMORPHIC ROCK TYPES

l  Foliated- based on type of foliation

u Slate- fine grain

u Phyllite- fine grain with sheen

u Schist- has ‘split’ appearance

u Gneiss- layers/bands of minerals

l  Non-foliated- based on mineral composition

u Marble

u Quartzite

u Hornsfel

l  Mixed Rock

u Migmatite- indicates partial melting

Quartz-Quartzite

Quartz-Sandstone

Metamorphism Temperature & Pressure

Information about degree to which a metamorphic rock differs from its parent material

l  Metamorphic Grade-

u low (200-400) slate

u high (500-800) gneiss

l  Index minerals/metamorphic Zones are used to determine metamorphic condition of temperature and pressure

u Chlorite, muscovite-low grade (low P/T)

u Garnet, staurolite- intermediate

u Sillianite- high grade (high P/T)

Common Metamorphic Rocks

l  FOLIATED ROCKS DERIVED FROM SHALE OR MUDSTONES:

–  SLATE (FINE GRAINED)

l  SLATY CLEVAGE
l  SLATE COLORS (RED: IRON OXIDE; GREEN: CHLORITE; PURPLE: MANGANESE OXIDES; BLACK: CARBON)
l  PHYLLITE: FINE-GRAINED
l  SCHIST: COARSE GRAINED
l  GNEISS: COARSE GRAINED

Relationships that produce crystallization

Three main metamorphic environments

Relationships that produce metamorphic rocks

Metamorphic Surface Rocks

Rock Slide of 1959

St. Francis Dam

CHAPTER SUMMARY - 7

l  What rocks can become metamorphic rocks

l  Conditions under which rocks & Minerals are stable

l  Geothermal gradient

l  Temperature and depth – Metamorphism

l  Lithostatic, direct pressure

l  Foliation in metamorphic rocks

l  Types of metamorphism – contact, regional, shock metamorphism

l  Criterion to classify metamorphic rocks

l  Characteristics of Schist, Gneiss

l  Metamorphosis of Limestone, Sandstone, Granite, Basalt