6. Continental Drift
Alfred Wegener -1912
– 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
7. Seafloor Spreading &
Plate Tectonics
Harry Hess - 1960s
– new ocean basins form from volcanism
– ocean floor forms IN BETWEEN pieces
that have split
– SEAFLOOR SPREADING
8. Seafloor Spreading &
Plate Tectonics
Theoretical breakthrough
– PLATE TECTONICS
– surface of earth composed of “plates”
(LITHOSPHERE) that move on a “conveyor
belt” (ASTHENOSPHERE)
9.
10. Evidence for Continental
Drift
Fit of the continents
Correlation of mountain belts
Diversity of species
– correlation of dinosaur species (Pangea)
– isolation of mammal species (after
breakup)
Glaciers
11. Important Quote
“Relationships known since early in the
century yet the validity of continental
drift was not generally recognized until
the late 1960’s”
It took ocean floor data to solidify
ideas and convince scientific
community!
The oceans rule!
15. Another Mechanism
subduction
world’s most explosive volcanos are
formed over subduction zones
Oregon’s Cascades Mtns. over
Cascadia Subduction Zone
– Juan de Fuca plate under N. American
plate
16. “Munch and Crunch”
“MUNCH” - subduction of oceanic plate
under continent or ocean
– oceanic crust is thin and dense
– dives
“CRUNCH” - collision of continental
plates
– India into Tibet and China
– continental crust is thick and light
17.
18. Sliding By
Transform faults
–plates neither created nor destroyed
Transform faults are active
Fracture zones are inactive extensions
of transforms
–“fossil transforms”
19.
20.
21. Young & Old
Oldest seafloor - 200 million years
Oldest land - billions of years
With seafloor spreading, is the earth
expanding?
Why is seafloor so young relative to
continents?
SUBDUCTION
“law of conservation of ocean floor”
25. Plate Boundaries
How do we know where these
boundaries are?
bathymetry
earthquakes
deepest earthquakes at CONVERGENT
boundaries (subduction
zones/trenches)
downgoing plate breakage causes
earthquake
like potato chip breaking off in bean dip
26.
27.
28. Extra Slides not covered
in lecture, but cover
valuable info that can
help in lab too
29. Proof for Seafloor
Spreading
What made people believe in seafloor
spreading? (poor Wegener!)
changes in inclinations of magnetic
field
Earth’s magnetic field a mystery
currents in liquid core one hypothesis
30.
31. SF Spreading Proof - 2
Earth’s 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
37. Driving Forces
“Slab pull” versus “ridge push”
Ridge Push - rising, hot rock pushes
plates apart at spreading center
Slab Pull - gravity pulls cooled,
dense plates back down into mantle
at subduction zone
