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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


Evidence for Continental Drift

Fit of the continents

Correlation of mountain belts

Diversity of species
-- 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"

It took ocean floor data to solidify ideas and convince the scientific community!

One Mechanism for Continental Drift


lithosphere riding on asthenosphere

Another Mechanism


world's most explosive volcanos are formed over subduction zones

not to be outdone, our Cascade mountains were formed over the Cascadia subduction zone
-- 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

"CRUNCH" = collision of continental plates
-- India into Tibet and China
-- continental crust is thick and light

Sliding By

Transform faults - plates are 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

Why is seafloor so young relative to continents?

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

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

SF Spreading Proof - 2

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

Magnetic Anomaly Images

Seafloor Spreading in Action
(requires an MPEG or QuickTime player in your web browser)

Plate Boundaries

Divergent - seafloor spreading centers

mid-ocean ridges or rises

Mid-Atlantic RIDGE, East Pacific RISE

Convergent - subduction zone


Tonga Trench

Lateral - strike-slip fault

transform faults (active) - San Andreas Fault

fracture zones (inactive) - Mendocino FZ

Transform versus Fracture Zone

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

Plate Boundaries

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

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Last update: April 7, 2002