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Definitions |
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Divergent
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Tectonic plates move apart
-Spreading boundary
-Mid-Ocean Ridge
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PLate Velocities
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Absolute velocities may be mapped by:
-Plotting plate motion relative to fixed position
-Measuring volcano ages/distance along hot spot track.....cm/year
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Tsunami Train
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-Multiple Tusnamis, no order of which is biggest
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Oceanic Crustal Age
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New crust spreads away from ridge axis
New closer to the ridge, old further away
Oldest at far edges of basin
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Plate Boundaries
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20 Tectonic Plates
Move- 1-15 cm/year (fast for geologic time)
Interact at borders
Locations where they meet:
-concentrations of earthquakes
-many other dynamic phenomena
Interiors almost quake free
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Collion May Include
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Two Continents
A continent and an island arc
Collision "sutures" the convergent boundary plate
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Convergent Boundaries
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Lithospheric plates go toward each other
One dives back to mantle (SUBDUCTION)
-recycles lithosphere
-balanced by sea-floor spreading
-Earth maintaining constant circumference
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Tsunami Video
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Thailand, Hawaii, Indo
Dawn, earthquake tears ocean floor
SUBDUCTION in Indian Ocean
-Plate pulls down, snaps back causing Tsunami
-Perpendicular to subduction zone
Picked up by the Pacific Tsunami Warning Ctr
-Most powerful of five years combined
-No water-level guages in area
-ENERGY dissipates as EARTHQUAKE or TSUNAMI
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Modern
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2 other forces drive plate motions
Ridge-push - elevated MOR pushes adjacent lithosphere
Slab pull- gravity pulls a subducting plate downward
-convection in asthenosphere adds or subtracts
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Tsunami Initiation
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Tons of Hiroshima like engery
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Passive Margin
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Continental crust things sea-ward
Transitions into ocean crust
Traps eroded sediments
Becomes continental shelf
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Convergent
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Plates move together
-Called convergent margin, subduction zones, consuming boundary at trench
3 types: OCean to Ocean, Crust to Crust, Ocean to Crust
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Wobbled Earth
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Shifts and caused the 5th largest of all time
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Continental Crust
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550 km
Granitic Crust
Less Dense
More Buoyant (float higher)
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"Black Smokers"
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Found at some MOR's
Water heated by magma in fractures
Hot water dissolves minerals and cycles them back out of rock
Mineral precipitate quickly when reach ocean
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Volcanic Arcs
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Belts on the over-riding plate
Plate partially melts, magma burns upward fueling eruptions
Arc depends on over-riding plate
Continental Arc- continental crust
Oceanic Arc- oceanic crust
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Global Positioning System
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Determines plate velocities
Knowledge now accurate and precise
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Back-arc Basins
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...
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Plate Collisions
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Subduction consumes ocean basins
Ocean closure ends in continental collision (subduct)
-buoyant continental subduction
-subduction ends, mountains smashed upward
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Four Tsunami Causes
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Tectonic movement
Volcano
Asteroids
Explsoive Eruption
Spontaneous Landslides
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Oceanic Transformations
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MOR axis offset by transform- necessary in sphere
Strong evidence of sea-floor spreading
-abundant quakes between ridge segments
Quakes vanish past ridge segment overlaps
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Plate Tectonics
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-Separate plate pieces that move with respect to one another
-20 outer shell plates that move (rigid)
-Evolved in 1960's - Hess/Dietz (1960)
Wegener (1915)
Powerful Theory
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Late Stage
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Mature, wide ocean basin
Edge of basin-oldest ridge
Proximal, youngest
Increase of age with distance (linear)
Ex: Atlantic Ocean
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Mid-Ocean Ridges
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-Mid-Atlantic Ridge
-Ranges in ocean basins (linear)
-Goes North to South, 2 km above abyssal plains
-Spreads apart at axial rift valley
-rising asthenosphere melts forming MAFIC MAGMA
-Pooled magma cools to oceanic crustal floor
-Pillow Basalt- magma quenced at seafloor
-Dikes- preserved magma conduits
-Gabbro- deeper magma
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Continental Rifting
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Continental lithosphere can break apart
Stretches and thins
Brittle upper crust faults
Ductile lower crust flows
Asthenosphere melts
EX: Sea floor spreading
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Driving Mechanisms
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Plates dragged atop a convecting mantle
Too complex motions
Convection does occur, not prime driving mech
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Amplification
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Growing of a wave
-Back catches the front, chases
-Surge, wall, keeps coming constantly, doesnt roll
-Morphology of crust determines how far inland they go
-Steep of slope of coast determines damage and how far inland
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Dynamic Planet
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Tectonics- understanding geology
-Mantle transferred to surface and back down
-Interior and surface in constant motion
-explosive earthquakess, volcanos, continental drift
-changes occur to us
-geologically, change is rapid
-Diff in past, diff in future
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Oceanic Crust
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7-100 km thick
Basaltic Crust
Heavier (More dense)
Less buoyant - sinks lower
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Unified Mechanism Explains
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-Igneous, Sedimentary, Metamorphic rock
-Earthquake & volcano
-Origins of contients and ocean basins
-Fossil distribution/plants
-Continental Drift
Genesis/Destruction of mountain chains (belts)
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Triple Junction
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3 plate boundaries coincide
Multiple combinations occur
Migrate and change across time
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Buoyancy
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Floating solids displace water equal to mass
Continental - floats higher
Oceanic- Sinks lower
Archimedes described buoyancy
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Accretionary Prisms
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Sediment zone scraped off by over-riding plates (welded)
Deformed sediment wedges
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Mid-Stage
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-Widening, new floor added
-Further pushed continents apart
Ex: Greenland
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Plates
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-Discrete slates of lithosphere (crust & rigid mantle)
-3 boundaries trigger earthquakes, delineated belts of historical quake activity
-volcanism associated with convergent and divergent
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Continental Margins
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Where lands meet oceans
-Margins near plate boundaries are "active"
-Margins far from boundaries "passive"
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Hot Spots
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Volcanic plumes independent of plate boundaries
-Mafic magma derived from lower mantle
-tattoos over-riding plates with volcanoes
Perforate over-riding plate
-Drift off the plate (magma underneath)
-goes extinct & erodes
-subsidence creates a GUYOT
-supports sea-floor spreading
Age change marks direction of over-riding plates
MANTLE PLUME- underground magma
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East African Rift
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Arabian plate drifting from African plate
Progressed to sea-floor spreading in
Red Sea, Gulf of Aden
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Oceanic Lithosphere
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Asthenosphere at base of MOR
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Africa Rift Ongoing
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Thinned crust
elongated trough
volcanoes
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Lithosphere
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Tectonic plates are fragments of this
-Crust and upper rigid mantle
-In motion of asthenosphere
Bends elastically when loaded (litho)
Bends plastically when loaded (asthenosphere)
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Tectonic Boundaries Evolve
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Change over geologic time
Oceanic Plates- created at MOR spreading centers, destroyed at subduction zone
Continental Plates- torn apart at rifts, joined during collisions
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Transform Boundary
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Litho slides past, not made or destroyed
Offset spreading of ridge segments
Some cut through continental crust
Cause earthquakes, abscence of volcanism
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Cascadic Subduction Zone
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Oregon, Washington
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Transform
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Plates slide sideways
-Called Transform Fault
-ex: San Andreas Fault
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Divergent
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Spreading of the sea-floor moves plates apart
Magma fills gap, cools, adding to plate
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Wadati-Beniff Earthquakes
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Descent of subducting plates revealed by this
Quakes deeper away from trench deepest
Mark frictional contact and transformations
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Subduction
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Old oceanic lithosphere more dense than mantle
Flat plate wont subduct
Bent down, leading edge sinks like an anchor
Plates subduct at 45 degrees
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Early Stage
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-Progressed to Mid Ocean Ridge
-Before widening of ocean
-Makes lengthening basin with young crust (oceanic)
Ex: Red Sea
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