The Shifting Crust
Chapter 3
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
Seafloor Spreading &
Plate Tectonics
 Harry
Hess - 1960s
– new ocean basins form from volcanism
– ocean floor forms IN BETWEEN pieces that
have split
– SEAFLOOR SPREADING
Seafloor Spreading &
Plate Tectonics
 Theoretical
breakthrough
– PLATE TECTONICS
– surface of earth composed of “plates”
(LITHOSPHERE) that move on a “conveyor
belt” (ASTHENOSPHERE)
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
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!
One Mechanism
 seafloor-spreading
 lithosphere
 asthenosphere
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
“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
Sliding By
 Tranforms
faults
– plates neither created nor destroyed
 Transform faults are active
 Fracture zones are inactive extensions of
transforms
– “fossil transforms”
Young & Old
 ocean
floors - 200 million years
 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”
Driving Forces
 “Slab
 Slab
pull” versus “ridge push”
Pull - gravity pulls cooled, dense
plates back down into mantle at
subduction zone
 Ridge Push - rising, hot rock pushes plates
apart at spreading center
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
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
Grocery
bar code
Calibrated
by agedating of
rocks
Last
reversal
200 Ka
Divergent Plate Boundaries
 Divergent
- seafloor spreading
– mid-ocean ridges or rises
– Mid-Atlantic RIDGE, East Pacific RISE
Convergent
 Convergent
- subduction
– trenches
– Tonga Trench, Cascadia Subduction Zone
Translational
 Translational
- strike-slip faulting
– transform faults (active) - San Andreas Fault
– fracture zones (inactive) - Mendocino FZ
Plate Boundaries
 How
do we know where these boundaries
are?
 bathymetry
 earthquakes
 deepest earthquakes at CONVERGENT
boundaries (subduction zones/trenches)
 slab breakage causes earthquake
 like potato chip breaking off in bean dip