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Transcript
Physiography of the Ocean Floor Take away the water and what do you see? 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) Earth’s dominant topographic features lie beneath the ocean’s surface. 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 edge wide, flat in Atlantic thin, steep in Pacific can 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 down continental rise – sedimentary wedge at base of slope (slope 1o or less) 1 Continental margins Deep ocean basins submarine canyons Monterey Canyon Grand Canyon - 6,672 ft. Submarine canyon off Barcelona, Spain 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 world abyssal hills – elongated dome-shaped hills of oceanic crust seamounts – 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 range extends for about 60,000 km around the Earth contains rift valley in center of ridge site of new oceanic crust formation flanked by transform faults and fracture zones Mapping ocean depths 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 2 Mapping ocean depths Mapping ocean depths Matthew Maury (the “father of oceanography”) constructed the first ocean-wide bathymetric map (N. Atlantic, 1855). Echo Sounder Seismic Reflection Profiler (sound source and hydrophone) Bathymetric maps depict the topography of the seafloor; isobaths connect points of equal elevation. Mapping ocean depths echo sounders sound source and receiver (hydrophone) on hull of ship high frequency sound waves travel through the water, reflect off the seafloor (and off subbottom sediment layers), and are recorded by the hydrophone based on travel time (t) of sound waves with known velocity (v), get depth [d = v x (t/2)] provide continuous depth profiles along a ship’s cruise track but only 2-dimensional profile (horizontal, vertical) vs. Mapping ocean depths seismic reflection profilers sound source and hydrophone towed by ship continuous but also only 2-dimensional profile lower frequency energy source (stronger sound source with fewer sound waves per second) deeper penetration into sedimentary layers and ocean crust seismic reflection survey guyot seamount 3 Mapping ocean depths Mapping ocean depths Multi-beam sonar and Side scan sonar Seabeam Multibeam bathymetry like echo sounder, but images a 60 km swath of seafloor overlapping swaths = complete coverage (3-dimensional) produces 3-dimensional bathymetric map of seafloor Sea ice keel scour Mapping ocean depths Mapping ocean depths Satellites bathymetric map based on a seabeam survey for Deep Sea Drilling Project sites off northwest Africa (contour interval = 50 m); note the very steep Mazagan Escarpment dropping off to the deep-sea Precise altimeters (using microwaves) can map the relief of the ocean surface with centimeter-scale resolution. isobaths 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! 4 Mapping ocean depths North Atlantic Ocean relief from satellite altimetry 5