Lecture6

Lecture #6 Continental Margins and Ocean Basins 22 August 2007

The ocean floor is mapped by bathymetry http://www.the-planet-mars.com/spacecraft/Mars-global-surveyor.html Mars Global Surveyor Mars

http://www.the-planet-mars.com/map-of-mars.html

[object Object],[object Object],Mars http://www.modern.tsukuba.ac.jp/earth.jpg Earth ,[object Object]

[object Object],[object Object],[object Object],http://www-groups.mcs.st-and.ac.uk/history/BigPictures/Posidonius.jpeg 2 km Bathymetry = study of ocean floor contours The early, simplest methods involved lowering a weight on a line.

http://www.nefsc.noaa.gov/history/ships/albatross1/sigsbee-sounding.jpg ,[object Object],[object Object],[object Object],[object Object],http://woodshole.er.usgs.gov/operations/sfmapping/images/theb0914_small.jpg Sometimes the weight was tipped with wax to retrieve a sample of bottom sediment.

[object Object],[object Object],[object Object],[object Object],confirmed the discovery of the Mid-Atlantic Ridge

Scientists now use beams of sound to measure depth.

Scientists now use beams of sound to measure depth. ,[object Object],[object Object]

http://en.wikipedia.org/wiki/Reginald_Fessenden ,[object Object],[object Object],[object Object],“ Iceberg Detector and Echo Depth Sounder”

V = speed of sound in water (about 1.5 km/sec) T = time Echo sounders sense the contour of the seafloor by beaming sound waves to the bottom and measuring the time required for the sound waves to bounce back to the ship.

[object Object],[object Object],http://www.eastlanddisaster.org/uc97.jpg ,[object Object],[object Object],http://oceanworld.tamu.edu/resources/ocng_textbook/chapter02/Images/Fig2-2s.jpg

http://www.panorama-map.com/Europeans/Berann/berannpacificocean500.html Pacific Ocean

http://www.berann.com/panorama/Everest.jpg Mt. Everest (1962)

Two new techniques improved studies of the seafloor: 1) multibeam echo sounders 2) satellite altimetry

Multibeam systems combine many echo sounders. ,[object Object],[object Object],<200 research vessels are equipped with multibeam systems

Seabed contours can be mapped using satellites . ,[object Object],[object Object]

Gravitational attraction “pulls” water Over a 2000 m seamount, water rises about 2 m Seafloor Sea surface

http://www.ngdc.noaa.gov/mgg/image/global_grav_large.gif Mapped by: Geosat , TOPEX/Poseidon , and Jason-1 Seafloor topography inferred from sea surface height measurements

Ocean-floor topography varies with location

>50% of Earth’s surface is >3,000 m below sea level

Oceans can be divided into two major provinces: 1) continental margin 2) ocean basin

[object Object],[object Object],Continental margins are “ active ” or “ passive ”. ,[object Object],[object Object],Face the edges of diverging plates Near converging plates

Three main parts of the continental margin:

Continental shelf: shallow submerged extension of a continent

http://media.allrefer.com/s1/l/c0601400-continental-shelf.jpg ,[object Object],[object Object]

http://www.cryingvoice.com/Evolution/gifs/hydroNA.jpg Atlantic

active margin – often very narrow passive margin – broad The shelf width is usually determined by its proximity to a plate boundary.

Continental shelves are greatly influenced by changes in sea level Sea level rise

http://en.wikipedia.org/wiki/Image:Sea_level_temp_140ky.gif Sea level is now high and rising as the ocean warms. Wisconsin glaciation

ice age http://en.wikipedia.org/wiki/Image:Northern_icesheet_hg.png ,[object Object],[object Object],[object Object],[object Object],[object Object]

http://whyfiles.org/shorties/202mass_extinct/images/land_bridge.gif Bering Strait

http://whyfiles.org/shorties/202mass_extinct/images/land_bridge.gif Allowed human migration 12,000 years ago Bering Strait

http://www.morien-institute.org/images/korea_2_sing.jpg Ice age sea level in Asia

http://cocos.arecaceae.com/indo19.gif Ice age sea level in Asia

Continental slopes connect continental shelves to the deep-ocean floor shelf break

Submarine canyons form at the junction between continental shelf and continental slope.

http://www.tahoemaps.com/files/Monterey_large.jpg Monterey Bay canyon

http://www.marine-geo.org/gallery/images/MontereyBay3D.jpg 2000 m Monterey Bay canyon

How do submarine canyons form? Submarine canyons cut into the continental shelf and slope, often terminating on the deep-sea floor in a fan-shaped wedge of sediment.

Submarine cables near Nova Scotia, Canada http://www.geol.lsu.edu/Faculty/Juan/PhysicalGeology_F2004/images/Turbidity2.gif

turbidity current an underwater “avalanche” of sediment http://unit.aist.go.jp/igg/rg/igi-rg/beta/sl-support/R-formation/TurbidityCurrent.jpg Most geologists believe that submarine canyons have been formed by abrasive turbidity currents plunging down the canyons.

Continental rises form as sediments accumulate at the base of the continental slope continental rise ,[object Object],[object Object]

The topology of deep-ocean basins differs from that of the continental margin ,[object Object],[object Object],[object Object]

Oceanic ridges circle the world ,[object Object],[object Object],[object Object]

http://www.berann.com/panorama/archive/image/PN_W_10.jpg Mid-Atlantic Ridge

Iceland fracture 15 cm/year http://library.thinkquest.org/C003124/images/diverg.jpg

http://www.berann.com/panorama/archive/image/PN_W_10.jpg Mid-Atlantic Ridge transform faults fracture zones

Hydrothermal vents are hot springs on active oceanic ridges ,[object Object],Alvin http://www.mbari.org/molecular/images/EPR%20mussel-map.jpg

Alvin ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]

http://www.csa.com/discoveryguides/vent/images/smoker.jpg http://whyfiles.org/coolimages/images/csi/nur04506.jpg “ black smokers” 20 m 350 o C 2,800 m depth ,[object Object],[object Object]

http://www.divediscover.whoi.edu/images/vent_chemistry.jpg “ black smokers” seawater is heated and reacts chemically with the surrounding basalt

hydrothermal vent community http://www.sciencedaily.com/images/2005/02/050223124700.jpg ,[object Object],[object Object],chemosynthesis Tube worms deep-sea vent mussels

Hydrothermal vents are common on oceanic ridges http://www.nature.com/nature/journal/v439/n7079/images/hydro_vents/index.html AND in freshwater (Lake Baikal)

Abyssal plains and abyssal hills cover most of Earth’s surface. ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Flat

Volcanic seamounts and guyots project above the seabed ,[object Object],[object Object],http://upload.wikimedia.org/wikipedia/en/thumb/2/25/Seamount_Locations.png/350px-Seamount_Locations.png ,[object Object],[object Object],Emperor Seamounts seamount

blobfish orange roughy southern spineback southern whiptail

Guyot : f lat-topped seamount that once reached the surface http://en.wikipedia.org/wiki/Image:Guyot.jpg

Trench : arc-shaped depression on the deep-ocean floor ,[object Object],[object Object],[object Object],http://geology.com/records/ocean-trench.gif

Peru-Chile trench http://en.wikipedia.org/wiki/Image:Peru-Chile_trench.jpg Puerto Rico trench

Japan Trench 10,595 m Mariana Trench 11,022 m

Kaiko ,[object Object],[object Object]

Sampling of the world's deepest sea sediment by " Kaiko " at the Mariana Trench, Challenger Deep Bacteria collected from the Mariana Trench

Key Points 1. The ocean floor is mapped by bathymetry . 2. Ocean-floor topography varies with location . 3. Continental margins are “active” or “passive”. 4. The topology of deep-ocean basins differs from that of the continental margin.

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