Fathoms Below / Under the Sea From soundtrack of Disney’s The Little Mermaid
Plate Tectonic Boundaries Plate Tectonic Boundaries
Alfred Wegener -1912 Alfred Wegener -1912 Wegener not able to provide MECHANISM for his theory Major mechanism later found in the OCEANS
Harry Hess - 1960s Harry Hess - 1960s - new ocean basins form from volcanism
- ocean floor forms IN BETWEEN pieces that have split
- SEAFLOOR SPREADING
Theoretical breakthrough - PLATE TECTONICS
- surface of earth composed of “plates” (LITHOSPHERE) that move on a “conveyor belt” (ASTHENOSPHERE)
Fit of the continents Fit of the continents Correlation of mountain belts - correlation of dinosaur species (Pangea)
- isolation of mammal species (after breakup)
Glaciers
“Relationships known since early in the century yet the validity of continental drift was not generally recognized until the late 1960’s” “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!
seafloor-spreading lithosphere asthenosphere
subduction 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” - subduction of oceanic plate under continent or ocean “MUNCH” - subduction of oceanic plate under continent or ocean “CRUNCH” - collision of continental plates - India into Tibet and China
- continental crust is thick and light
Transform faults Transform faults - plates neither created nor destroyed
Transform faults are active
Oldest seafloor - 200 million years 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 - subduction Convergent - subduction - trenches
- Tonga Trench, Cascadia Subduction Zone
Divergent - seafloor spreading Divergent - seafloor spreading - mid-ocean ridges or rises
- Mid-Atlantic RIDGE, East Pacific RISE
Translational - strike-slip faulting Translational - strike-slip faulting
How do we know where these boundaries are? 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
What made people believe in seafloor spreading? (poor Wegener!) What made people believe in seafloor spreading? (poor Wegener!) changes in inclinations of magnetic field Earth’s magnetic field a mystery
Earth’s magnetic field flips back & forth 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
“Slab pull” versus “ridge push” “Slab pull” versus “ridge push”
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