[ home port | lectures | labs | surf | email prof. ]

Lecture 3
Shape of the Seafloor

Techniques of Bathymetry

Challenger expedition (1872-1876) -1st systematic bathymetric survey

--ocean floor not flat - significant topographic relief

German ship Meteor (1920s) - 1st echosounding survey

--light does not travel very far in water

--sounds travels through water much better

--velocity = distance/time


Image courtesy of D. Reed, San Jose State Geology

--SONAR (Sound Navigation and Ranging)

WWII - U.S. Navy further developed SONAR technology

--knowledge of the enemy

--knowledge of the ocean

1950s - 1960s - single, focused high-frequency, short wavelength sound beam

-- wide-beam bathymetry

--sound beam spreads out as it reaches bottom

--range of depths - fuzzy estimate

1970s - revolution in bathymetric mapping with Sea Beam

multiple, focused, high-frequency, short wavelength sound beams

--narrow-beam or multibeam bathymetry

--sound beam stays narrow and focused all the way to the bottom

--depths much more precise

--Sea Beam has 16 beams, Sea Beam 2000 has 121
Image courtesy of D. Reed, San Jose State Geology



with Sea Beam can see things on seafloor the size of this room

--swath width of 6 km or 3.7 miles

good, high-resolution maps possible only since 1980s

other instruments needed to see things smaller than size of room

--remotely-operated vehicles (ROVs)


See More at This Awesome Web Site!

Sonar Also Used as...

a catscan of oceans to see water structure ABOVE seafloor

an x-ray of seafloor to see structure BENEATH seafloor

--seismic reflection & seismic refraction

--low frequency, long-wavelength sound

sidescan sonar to get pictures of seafloor in addition to depth

--backscatter strength as opposed to traveltime


Seafloor Features: Continental Margins

continental shelf - extends from shore to a point marked by great increase in slope

continental slope - steep slope beyond the continental shelf break

slopes often cut by submarine canyons

--turbidity currents - dense flows of sediment-laden water

--deepsea fans

continental rise

abyssal plain - extensive, flat
Image courtesy of L. Pratson and B. Haxby, Lamont-Doherty Earth Observatory

Seafloor Features: Deep Ocean

seamounts - underwater volcanoes 500 m to 1000 m high

--flat-topped ones are called guyots

--volcanic features (buoyed up by hot rock, lava)

abyssal hills - features around 200 m high

--pervasive on seafloor

--volcanic AND tectonic in origin - still debated

Plate Boundaries - Ridges (Rises), Trenches, Transform Faults, Fracture Zones

[ home port | lectures | labs | discussion | surf | email prof. ]

Last update: March 31, 2000