The Shifting Crust – How Solid is the “Solid Earth”?

Continental Drift  Alfred Wegener –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 s –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 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

Grocery bar code Calibrated by age- dating 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