mapping ocean depths - clark science center · mapping ocean depths matthew maury (the “father of...
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Take away the water and what do you see?
Physiography of the Ocean Floor
Earth’s dominant topographic features lie beneath the ocean’s surface.
Distribution of topography and bathymetry
Bimodal distribution
Average land elevation 0.84 km
Average ocean depth 3.8 km
Deepest depth (Mariana Trench) more extreme than highest height (Mt. Everest)
Ocean provinces
The submarine “landscape,” or sea floor, can be subdivided into three distinct provinces:
continental margins~22% of total area
deep ocean basins~42% of total area
mid-ocean ridges~31% of total area
(remaining ~5% of total area occupied by deep-sea trenches)
Continental margins
submerged edges of the continents massive wedges of sediment eroded from the land and deposited along continental edgewide, flat in Atlanticthin, steep in Pacificcan be divided into three parts:
continental shelf – continuation of land below sea surface (slope < 1o)continental slope – where margin breaks (slope ~ 4o); often contain huge submarine canyons where sediment cascades downcontinental rise – sedimentary wedge at base of slope (slope 1o or less)
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Continental margins
Grand Canyon
- 6,672 ft.
Monterey Canyon
Submarine canyon off Barcelona, Spain
submarine canyons
Deep ocean basins
between the continental margins and the mid-oceanic ridge
includes a variety of features from mountainous to flat plains:
abyssal plains – flattest parts of the worldabyssal hills – elongated dome-shaped hills of oceanic crustseamounts – abyssal mountains, largely volcanic (active and extinct); includes flat-topped guyots formed by wave erosion deep sea trenches – deepest regions on Earth, found close to land
Mid-ocean ridges
continuous submarine mountain rangeextends for about 60,000 km around the Earthcontains rift valley in center of ridgesite of new oceanic crust formationflanked by transform faults and fracture zones
Prior to the early 20th century, “soundings” were the only means to determine water depth
weighted lines lowered from ships
time-consuming, relatively few, doubtful accuracy
HMS Challenger one of the first to do this
Mapping ocean depths
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Mapping ocean depths
Matthew Maury(the “father of oceanography”)constructed the first ocean-wide bathymetric map(N. Atlantic, 1855).
Bathymetric maps
depict the topography of the seafloor;
isobathsconnect points of equal elevation.
Echo Sounder
Seismic ReflectionProfiler (sound sourceand hydrophone)
Mapping ocean depths
Mapping ocean depths
echo sounders
sound source and receiver (hydrophone) on hull of shiphigh frequency sound waves travel through the water, reflectoff the seafloor (and off sub-bottom sediment layers), and are recordedby the hydrophone based on travel time (t) of sound waves with known velocity (v), get depth [d = v x (t/2)]provide continuous depth profilesalong a ship’s cruise trackbut only 2-dimensional profile (horizontal, vertical)
sound source and hydrophone towed by ship
continuous but also only 2-dimensional profile
lower frequencyenergy source (stronger sound source with fewer sound waves per second)
deeper penetration into sedimentary layers and ocean crust
seismic reflection profilers
vs.
Mapping ocean depths
seismicreflection survey
guyotseamount
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like echo sounder, but images a 60 km swath of seaflooroverlapping swaths = complete coverage (3-dimensional)produces 3-dimensional bathymetric mapof seafloor
Mapping ocean depths
Multi-beam sonarand
Side scan sonar
Mapping ocean depths
Seabeam Multibeam bathymetry
Sea ice keel scour
bathymetric map based on aseabeam surveyfor Deep Sea Drilling Project sites off northwest Africa(contour interval = 50 m); note the very steep Mazagan Escarpmentdropping off to the deep-sea
isobaths
Mapping ocean depths
Precise altimeters (using microwaves) can map the relief of the ocean surfacewith centimeter-scale resolution.
Altimetry uses satellites to determine bathymetry based upon slight changes in the elevation of the sea surface
Gravitational attraction of large rock masses on the seafloor distorts the ocean surface, very slightly but enough to be measured from space!
Mapping ocean depths
Satellites
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Mapping ocean depths
North Atlantic Ocean relief from
satellite altimetry