Geography and Oceanography - Chapter 4 ppsx

slope Abyssal plain Continental rise Continental slope Seafloor Features: Continental Margins Abyssal plain... Abyssal Hills linear hills Seafloor spreading center e.g., East Pacific R

Today’s Tunes

❥ Fathoms Below / Under the Sea

❥ From soundtrack of Disney’s The Little Mermaid

Continental shelf

Submarine canyons (cut into the c slope)

Abyssal

plain

Continental rise

Continental slope

Seafloor Features: Continental Margins

Abyssal

plain

Abyssal Hills (linear hills)

Seafloor spreading center (e.g., East Pacific RISE or Mid-Atlantic RIDGE)

Deep Ocean

Seafloor Features: Deep Ocean

❥ Plate Tectonic Boundaries

Transform Faults, Fracture Zones

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

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

❥ Transform faults

– plates neither created nor destroyed

❥ Transform faults are active

❥ Fracture zones are inactive extensions of transforms

– “fossil transforms”

Young & Old

❥ 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”

Divergent Plate Boundaries

❥ Divergent - seafloor spreading

– mid-ocean ridges or rises

– Mid-Atlantic RIDGE, East Pacific RISE

❥ Translational - strike-slip faulting

– transform faults (active) - San Andreas Fault – fracture zones (inactive) - Mendocino FZ

❥ downgoing plate breakage causes earthquake

❥ like potato chip breaking off in bean dip

Extra Slides not covered in lecture, but cover valuable info that can help in lab too

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

200 Ka

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