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-large supercontinent (Pangea) existed and then split into pieces
-fossil & glacial deposit evidence
Wegener not able to provide MECHANISM for his theory
Major mechanism later found in the OCEANS
Correlation of mountain belts
Diversity of species
Glaciers
It took ocean floor data to solidify ideas and convince the scientific community!
lithosphere riding on asthenosphere
world's most explosive volcanos are formed over subduction zones
not to be outdone, our Cascade mountains were formed over the Cascadia subduction zone
"CRUNCH" = collision of continental plates
Transform faults are active
Fracture zones are inactive extensions of transforms ("fossil
transforms")
ocean floors - 200 million years
Why is seafloor so young relative to continents?
Slab Pull - gravity pulls cooled, dense plates back down into mantle at subduction zone
Ridge Push - rising, hot rock pushes plates apart at spreading center
VIDEO
Evidence for Continental Drift
Fit of the continents
-- correlation of dinosaur species (from when Pangea existed)
-- isolation of mammal species (after breakup of Pangea)Important Quote in Video
"These relationships known since early in the century, yet the validity of
continental drift was not generally recognized until the late 1960's"One Mechanism for Continental Drift
seafloor-spreadingAnother Mechanism
subduction
-- Juan de Fuca plate is sliding underneath the North American plate"Munch and Crunch"
MUNCH = subduction of oceanic plate under continental plate OR oceanic plate
-- oceanic crust is thin and dense
-- dives into mantle very easily
-- India into Tibet and China
-- continental crust is thick and light
Sliding By
Transform faults - plates are neither created nor destroyed
Young & Old
Driving Forces
Proof for Seafloor Spreading
What made people believe in seafloor spreading? (poor Wegener!)
changes in inclinations of magnetic field
Earths magnetic field a mystery
currents in liquid core one hypothesis
Earths magnetic field flips back & forth
magma freezes magnetic minerals
minerals lines themselves up w/ prevailing field of earth
anomaly in field is the key - normal (positive) or reversed (negative)
Vine and Matthews noted this in the 60s and flagged this as PROOF for seafloor spreading
Divergent - seafloor spreading centers
Mid-Atlantic RIDGE, East Pacific RISE
transform faults (active) - San Andreas Fault
fracture zones (inactive) - Mendocino FZ
lithosphere on either side of transform slips in opposite directions
lithosphere on either side of FZ travels in same direction
transform has earthquakes, FZ no quakes
transform connects offset portions of spreading center, FZ does not
How do we know where these boundaries are?
deepest earthquakes at CONVERGENT boundaries (subduction zones/trenches)
slab breakage causes earthquake
like potato chip breaking off in bean dip
Last update: April 7, 2002
http://dusk.geo.orst.edu/oceans/lec05.html