Chapter 7 Metamorphism and Metamorphic Rocks
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
l
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