Deformation of the Crust Download

Transcript
Deformation of the Crust
How Rocks Deform
• Deformation
= bending, tilting, or breaking Earth’s crust.
Isostasy
Deformation can be due to two opposing forces:
• gravity, or weight, of the lithosphere pressing down on the
asthenosphere.
• And the buoyant force of the asthenosphere pressing up on the
lithosphere.
• When these two forces are in balance = isostasy
• As the earth changes, isostatic adjustments occur until isostasy
(balance) is reached again.
• Isostatic adjustments cause rock to deform.
Isostastic Adjustments
• As the Lithosphere
thickens.
• Becomes heavier
• Sinks deeper into
asthenosphere
• Mountain building,
glaciation, and
deposition of sediments
by rivers adds weight =
Subsidence
(sinking).
• As the Lithosphere
becomes thinner.
• Becomes lighter
• Rises higher in the
asthenosphere
• Erosion off mountains and
glacial retreat can cause
the crust to become
lighter = Uplift
Sinking, uplift, sinking, uplift, etc.
Stress
• The amount of force that is exerted on rock.
• Occurs when crust is squeezed, stretched, and twisted when the
lithosphere moves.
• Compression
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–
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–
Squeezes and shortens
Reduces the amount of space a rock occupies
Reduces the volume of rock
Pushes rock higher up, uplift
Near convergent boundaries
• Tension
– Stretches and pulls rock
– Rock becomes thinner
– Occurs near divergent boundaries
• Shear
– Distorts rock by pushing parts of the rock in opposite directions.
– Rocks bend, twist, or break as they slide past each other.
– Common at transform boundaries
Strain
• Any change in the shape or volume of rock that
results from stress.
• If stress is applied slowly, the deformed rock may
regain its original shape when the stress is
removed.
• Some stress leads to permanent deformation of the rock.
• Type of strain depends on composition of rock,
temperature, and pressure.
– Brittle strain appears as cracks or fractures.
• Occurs mostly at the surface, lower temperature/pressure
• Also occurs when stress is applied more quickly.
– Ductile materials bend or deform without breaking.
• Occur at higher temperature/pressure
Ductile: bend without breaking
Folding
• A form of ductile strain
• A fold is a bend in a rock layer.
• Occurs when rock is compressed
and squeezed.
• Can also occur from shear stress.
• MONOCLINE fold
– Both limbs are horizontal
– Form when one side moves up or
down
• ANTICLINE fold
– Oldest layers are in the center,
turns downwards
• SYNCLINE fold
– Youngest layers are in the center,
turns upwards
Faults
• Stress (brittle strain) may cause rocks to break.
• If no movement occurs along the break = fracture.
• If movement occurs along the break = fault
• Normal fault
– Hanging wall (which is above fault) moves down compared to footwall (below fault).
– Occur at divergent boundaries
– Great Rift Valley, Africa
• Reverse fault
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Hanging wall moves up compared to footwall
Occur at convergent boundaries (compression)
Thrust fault (type of reverse fault) – hanging wall pushed up over the footwall
Rockies and Alps
• Strike-slip fault
– Rocks slide horizontally to each other
– Due to shear stress at transform boundaries
– San Andreas fault
Normal Fault (tensional stress)
Reverse Fault or Thrust Fault
(compressional stress)
Strike-Slip Fault (shear stress)
How Mountains form (orogeny)
• A mountain is the most extreme type of
deformation.
– Mt. Everest… 8 km and still rising
– Part of the Great Himalaya range
– Mountain ranges: Great Smokey, Blue Ridge,
Cumberland, Green, Appalachian.
– Mountain belts: Circum-Pacific, Eurasian-Melanesian.
Plate Tectonics and Mountains
• Collisions: continental and oceanic crust
• Melting may also form volcanic mountains
• Cascade range, N. America
• Andes, S. America
Cascade Mountains
Mt. Hood
Mt. Jefferson
Three Sisters
Plate Tectonics and Mountains
• Collisions: oceanic and oceanic crust
• Melting may form an arc of volcanic mountains.
– Mariana islands
Aleutian Islands
Plate Tectonics and Mountains
• Collisions: continental and continental crust
• Forms uplift mountains
– Himalayas
Himalayas
Folded Mountains
• Occur when two
continents collide
• Form high mountains
– Alps, Himalayas,
Appalachians, Urals.
Plateau
• Occur when large, flat,
areas of rock are slowly
uplifted and remain flat.
• Located near mountain
ranges.
– Tibetan plateau (Himalaya)
– Colorado plateau (Rockies)
• Can also form when
layers of molten rock
accumulate.
• Or when large areas of
rock are eroded.
Fault-Block
Mountains
• Occur where parts of
Earth’s crust have been
stretched and broken
into large blocks.
• Some blocks tilt or drop
relative to other blocks.
– Sierra Nevada Range, CA
Grabens
• Also forms long narrow valleys
• Form when steep faults break the
crust into blocks and one block
slips downward relative to the
surrounding blocks.
• Occur with Fault-Block Mountains.
– Basin and Range Province,
Western U.S.
The Grand Tetons
(Fault-Block Mountains)
Dome Mountains
Volcanic Mountains
• Occur when magma rises through •
the crust and pushes up the rock
•
layers above the magma.
– Black Hills, S. Dakota
– Adirondack, NY
•
•
Occur when magma erupts onto Earth’s
surface.
Common along convergent boundaries
Cascades (Washington, Oregon, N. CA)
Mid-Ocean Ridges form volcanic islands
– Azores, N. Atlantic Ocean
•
Some also form at hot spots (volcanically
active areas that do not lie near tectonic
plate boundaries).
• Hawaiian Islands