The Shifting Crust – How Solid is the “Solid

Earth”?

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 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

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

Grocerybar code

Calibratedby age-dating ofrocks

Last reversal200 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