Transcript Metamorphism and Metamorphic Rocks

Chapter 7
Deformation and Metamorphism
• Three parts
– Metamorphic settings and processes
– Deformation of rock units
– Metamorphic rocks and facies
Deformation
• Deformation is a general term that
refers to all changes in the original
form and/or size of a rock body
» Deformation involves
• Force – that which tends to put stationary
objects in motion or changes the motions of
moving objects
• Most crustal deformation occurs along
plate margins
Deformation involves
• Stress or force applied to a given area
• Stress = Force/Area (psi, tsf, kg/m2)
• Geologic Stress is in kbar (1,000 X surface
pressure)
Types of stress
• Compressional stress – shortens a rock body
• Tensional stress – tends to elongate or pull apart a
rock unit
• Lithostatic or hydrostatic stress is equal in all
directions and produces a chance in size (dilation).
• Differential stress is excess of lithostatic stress and
produces a change in shape (distortion).
• Shear stress – produces a motion similar to slippage
that occurs between individual playing cards when the
top of the stack is moved relative to the bottom
What type of stress is involved
in stretching?
30
1. Compressional
2. Tensional
3. Shearing
0
Direction of Stress
Horizontal - Buckle
Vertical - Bend
Sheep Mountain Anticline
Bighorn Basin, Wyoming
Beds dip away form core
Oldest beds in core
Large Structures
Black Hills, S.D
Michigan Basin
Faults are classified
• Attitude of fault
• Vertical-near vertical
• High angle > 45o
• Low angle < 45o generally less than 20 o
• Relative motion along fault
– Dip slip
• Normal
• Reverse (thrust if dip is less than 20o)
– Strike slip
• Right lateral
• Left lateral
Velocity of plates
San Andreas Fault
BEMT
SBCC
Velocity of plates
• SBCC
Relative Strike Slip Motion
– 26.8 N
– -26.8 E
– -3.9 V
= ~38 mm/yr
to the
northwest
BEMT
SBCC
• BEMT
– 5.0 N
– -5.4 E
– -2.1 V
= ~7.3
SBCC is moving ~5 times more quickly than BEMT
mm/yr to
the
northwest
Pressure reduces strength
Metamorphism
Rock Cycle
Meta
1.
2.
3.
Changed in position or form
After
About
Metamorphism
• The transition of one rock into another by
temperatures and/or pressures unlike those
in which it formed
• Metamorphic rocks are produced from
• Igneous rocks
• Sedimentary rocks
• Other metamorphic rocks
Agents of Metamorphism
• Heat
• Pressure
• Chemically Active Fluids
Agents of Metamorphism
• Heat
• Most important agent
• Recrystallization results in new, stable
minerals
• Two major sources of heat
– Contact metamorphism – heat from magma
– An increase in temperature with depth due to the
geothermal gradient
• Minor heat source – local burning
Powder River Basin
Wyodak Coal
Powder River Basin, Wyoming
Wyodak Mine, Gillete, Wyoming
Agents of Metamorphism
• Pressure and differential stress
• Increases with depth
• Confining pressure applies forces equally in
all directions
• Rocks may also be subjected to differential
stress which is unequal in different directions
Pressure
• Confining pressure applies forces
equally in all directions
• Rocks may also be subjected to
differential stress which is unequal in
different directions
Agents of Metamorphism
• Chemically active fluids
• Mainly water with other volatile components
• Enhances migration of ions
• Aids in recrystalization of existing minerals
Agents of Metamorphism
• Chemically Active Fluids
– Water
– CO2
Normal Water
• Solid
resists shape change, resists volume change
very low capacity for dissolved chemicals
can penetrate only relatively large openings (immobile in the
ground)
• Liquid
easy shape change, resists volume change, surface tension
very high capacity for dissolved chemicals
can penetrate relatively small openings (mobile in the ground)
• Gas
easy shape change, easy volume change, no surface tension
low capacity to carry dissolved chemicals
easily penetrate the smallest openings (very mobile in the
ground)
Critical Water
• T>273oC and 10 bars pressure
• No surface tension, can be driven through the
most minute openings, can transport lots of
dissolved chemicals through relatively hot rocks
• Critical water transports ions to reshape
minerals or create new minerals with elongation
perpendicular to maximum compressive stress
Metamorphism and Metamorphic
Rocks
Protolith
• The original rock type
• Sets rock chemistry
– Quartz sandstone – Quartzite
– Muds – Al, Ca, Fe, Mg,
– Carbonates – Ca, CO3, Mg, Fe
• Influences fabric
– Basalts
– Muds
Quartzite
Rochford, S.D.
• Metamorphism progresses incrementally
from low-grade to high-grade
• During metamorphism the rock must
remain essentially solid
• Metamorphic settings
• Contact or thermal metamorphism – driven
by a rise in temperature within the host rock
Barrovian Sequence (muds)
(George Barrows Metamorphic Mineral
Sequence)
Foliated Rocks
• Slate
• Phylite
• Schist
• Gneiss
Nonfoliated Rocks
•
•
•
•
Quartzite
Marble
Anthracite
Metabasalt
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