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