Today’s Tunes
 Fathoms

Below / Under the Sea
From soundtrack of Disney’s The Little Mermaid
Seafloor Features:
Continental Margins
Submarine canyons
(cut into the c. slope)
Abyssal
plain
Continental shelf
Continental rise
Abyssal
plain
Continental
slope
Seafloor spreading center
(e.g., East Pacific RISE or
Mid-Atlantic RIDGE)
Deep Ocean
Seamounts
Abyssal Hills
(linear hills)
Seafloor Features: Deep Ocean
 Plate
Tectonic Boundaries
Ridges or Rises
Trenches
Transform Faults, Fracture Zones
The Shifting Crust
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
 Transform
faults
– plates neither created nor destroyed
 Transform faults are active
 Fracture zones are inactive extensions of
transforms
– “fossil transforms”
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”
Convergent
 Convergent
- subduction
– trenches
– Tonga Trench, Cascadia Subduction Zone
Divergent Plate Boundaries
 Divergent
- seafloor spreading
– mid-ocean ridges or rises
– Mid-Atlantic RIDGE, East Pacific RISE
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)
 downgoing plate breakage causes
earthquake
 like potato chip breaking off in bean dip
Extra Slides not covered in
lecture, but cover valuable
info that can help in lab too
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
Driving Forces

“Slab pull” versus “ridge push”
 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