key points 1. the ocean floor is mapped by bathymetry. 2. ocean-floor topography varies with...

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 topography of deep-ocean basins differs from that of the continental margin.

Posidonius • conducted the first bathymetric studies• 85 B.C.

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.

HMS Challenger

(1872-1876)• made the first

systematic attempt to chart the basins of the world ocean

• made 492 bottom

soundings confirmed the discovery of the Mid-Atlantic Ridge

http://en.wikipedia.org/wiki/Reginald_Fessenden

Reginald Fessenden (1866-1932)• Canadian inventor• in 1914, developed a

type of sonar system for locating icebergs

“Iceberg Detector and Echo Depth Sounder”

http://en.wikipedia.org/wiki/Reginald_Fessenden

Reginald Fessenden (1866-1932)• Canadian inventor• in 1914, developed a

type of sonar system for locating icebergs

“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.

During World War I (1914-1918)• used to detect enemy

submarines

http://www.eastlanddisaster.org/uc97.jpg

Meteor expedition (1925-1927)• used to study the seabed

http://oceanworld.tamu.edu/resources/ocng_textbook/chapter02/Images/Fig2-2s.jpg

World Ocean Floor

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

Pacific Ocean

Two new techniques improved studies of the seafloor:

1) multibeam echo sounders

2) satellite altimetry

Multibeam systems combine many echo sounders.

• up to 121 beams• signal sent every 10 secs <200 research vessels are

equipped with multibeam systems

 Seabed contours can be mapped using satellites.

Satellites cannot measure ocean depths directly• but, they can measure sea surface height

Sea surface

Seafloor

？Sea surface

Seafloor

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

Oceans can be divided into two major provinces: 1) continental margin2) ocean basin (deep ocean floor – Basalt)

• earthquakes• volcanic activity

Continental margins are “active” or “passive”.

Pacific-Type Atlantic-type

• no earthquakes• no volcanic activity

Three main parts of the continental margin:1. Shelf 2. Slope 3. Rise

Continental shelf: shallow submerged extension of a continent

• Average width: 1280 km (800 miles)

• Granitic rock covered by sediments

• Methane compounds

http://media.allrefer.com/s1/l/c0601400-continental-shelf.jpg

• up to 350 km

• most material comes from erosion of continent

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 (Ice Ages)

Sea level rise

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

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

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

Allowed human migration 12,000 years ago

Bering Strait

Continental slopes connect continental shelves to the deep-ocean floor

shelf break

Submarine canyons form at the junction between continental shelf and continental slope. (Edge of ocean basins)

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.

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

• much sediment

• usually along passive margins

The topology of deep-ocean basins differs from that of the continental margin

Deep-ocean basins comprise mainly:

1) oceanic ridge systems

2) sediment-covered plains

Oceanic ridges circle the world

• underwater mountain ranges• stretch 65,000 km• often covered with little

sediment

http://www.berann.com/panorama/archive/image/PN_W_10.jpg

Mid-Atlantic Ridge

http://www.berann.com/panorama/archive/image/PN_W_10.jpg

Mid-Atlantic Ridge

transform faultsfracture zones

Hydrothermal vents are hot springs on active oceanic ridges•350 degrees Celsius• discovered in 1977 by Robert Ballard and J. F. Grassle

Alvin

http://www.mbari.org/molecular/images/EPR%20mussel-map.jpg

Alvin• can carry 3 people• can dive to 4000 m• 1964 – 2007• >4000 dives

manned submersible• 6,500 m

unmanned submersible• 11,000 m

http://www.csa.com/discoveryguides/vent/images/smoker.jpghttp://whyfiles.org/coolimages/images/csi/nur04506.jpg

“black smokers”

20 m

350oC

2,800 m depth

solutions exiting vents are acidic (pH = ~3.5) and contain up to 300 ppm hydrogen sulfide (H2S)

• a highly reduced molecule, so much energy can be obtained when it is oxidized

hydrothermal vent community

http://www.sciencedaily.com/images/2005/02/050223124700.jpg

• includes snails, shrimps, crabs, tube worms, fishes

• depends on chemosynthetic bacteria for food

chemosynthesis Tube worms

deep-sea vent mussels

Abyssal plains and abyssal hills cover most of Earth’s surface.

Abyssal hills• small sediment-covered

extinct volcanoes or rock

• > 200 m (650 ft)

Abyssal plains• 40% of the ocean floor• common in the Atlantic• rare in the Pacific• covered by sediment• cold

Flat

 Volcanic seamounts and guyots project above the seabed

• about 30,000• about 10,000 in the Pacific

http://upload.wikimedia.org/wikipedia/en/thumb/2/25/Seamount_Locations.png/350px-Seamount_Locations.png

• >1 km in height• important

fishing areas

Emperor Seamounts

seamount

Guyot: flat-topped seamount that once reached the surface

http://en.wikipedia.org/wiki/Image:Guyot.jpg

Trench: arc-shaped depression on the deep-ocean floor

• occur near subduction zones

• deepest places in the ocean

• most in the Pacifichttp://geology.com/records/ocean-trench.gif

Peru-Chile trench

http://en.wikipedia.org/wiki/Image:Peru-Chile_trench.jpg

Puerto Rico trench

Japan Trench10,595 m

Mariana Trench11,022 m

Trieste

• reached the bottom of the Mariana Trench in 1960 – Challenger Deep

Kaiko

• Japanese deep-sea submarine

• sampled bacteria from the bottom (10,897 m) of the Mariana Trench in 1996

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

Bacteria collected from the Mariana Trench

Nereus

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 topography of deep-ocean basins differs from that of the continental margin.