Transcript MYSTERIES OF PLANET EARTH

Chapter 7 Metamorphism and
Metamorphic Rocks
Metamorphic
rock is formed when existing rocks
change due to subjection to pressure and or
temperature
Any
rock can undergo metamorphism
Metamorphism
is the process by which heat,
pressure, and chemical reactions deep within the
earth alter the mineral content and or structure of
existing rock without melting it down
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
Sedi., meta., igne. rocks
Metamorphic Rocks in North America
CONDITIONS-CONTD
1)
HEAT – MOST IMPORTANT FACTOR
GEOTHERMAL GRADIENT (20-30 DEG C/KM-200
DEG C TO METAMORPHOSE ROCKS)

SOURCES OF HEAT FOR METAMORPHISM:
–
–
–
–
CONDUCTION FROM DEEP INTERIOR
RISING MAGMA
RADIOACTIVE ISOTOPES
FRICTION BETWEEN ROCK BODIES
PRESSURE
Pressure-contd.

Lithostatic: Confining pressure in all directions,
rock becomes compressed
 Directed Pressure: Acts in one principal plane
 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

Often necessary for chemical reactions (Facilitates
movement of ions and atoms)
 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

Heat
 Accelerate pace of chemical reactions

Pressure
 Lithostatic (confining)- rock becomes smaller and
denser
 Directed- minerals become aligned- Foliation

Circulating Fluids
 Ions in water- change mineral composition

Parent Rocks
 Original rock’s composition will affect the outcome of
metamorphism

CONFINING PRESSURE
DIRECTED PRESSURE
TYPES OF METAMORPHISM

Contact
 Heat is the dominant factor
 Area affected generally smaller than regional metarmorphism

Regional are two types with extensive coverage



Burial- occurs in deep sedimentary basins- no plate tectonics
involved
Dynamothermal- occurs where converging plates squeeze a
rock caught between them
Others
 Hydrothermal- involves hot water from magma
 Fault-zone- rocks grinding past one another
 Shock- meteorites strike
 Pyrometamorphism- lightning
CONTACT METAMORPHISM
DYNAMOTHERMAL METAM.
HYDROTHER. METAM-CONTD
Hydrothermal Metamorphism
METAMORPHIC ROCK TYPES

Foliated- based on type of foliation
 Slate- fine grain
 Phyllite- fine grain with sheen
 Schist- has ‘split’ appearance
 Gneiss- layers/bands of minerals

Non-foliated- based on mineral composition
 Marble
 Quartzite
 Hornsfel

Mixed Rock
 Migmatite- indicates partial melting
Quartz-Quartzite
Quartz-Sandstone
Metamorphism
Temperature & Pressure
Information about degree to which a metamorphic
rock differs from its parent material
 Metamorphic Grade low (200-400) slate
 high (500-800) gneiss

Index minerals/metamorphic Zones are used to
determine metamorphic condition of temperature
and pressure
 Chlorite, muscovite-low grade (low P/T)
 Garnet, staurolite- intermediate
 Sillianite- high grade (high P/T)
Common Metamorphic Rocks

FOLIATED ROCKS DERIVED FROM
SHALE OR MUDSTONES:
– SLATE (FINE GRAINED)
 SLATY CLEVAGE
 SLATE COLORS (RED: IRON OXIDE; GREEN:
CHLORITE; PURPLE: MANGANESE OXIDES;
BLACK: CARBON)
 PHYLLITE: FINE-GRAINED
 SCHIST: COARSE GRAINED
 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