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3 What does the bottom of the ocean look like? OR What is the topography or bathymetry of the ocean floor? Topography of the Ocean Floor 3 • echo soundings (1920’s) • ocean was not deepest in the center • deepest part of the ocean lie near its edges 3 Fig 4-2a, p.67 3 Fig 4-3a, p.67 An echo sounder trace. A sound pulse from a ship is reflected off the seabed and returns to the ship. Transit time provides a measure of depth. For example, it takes about 2 seconds for a sound pulse to strike the bottom and return to the ship when the water depth is 1,500 meters (4,900 feet). Bottom contours are revealed as the ship sails a steady course. In this trace, the horizontal axis represents the course of the ship, and the vertical axis represents the water depth. The ship has sailed over a small submarine canyon. 3 Fig. 4-2b, p. 67 Ridges/trenches in South Atlantic Sea Floor 3 Fig. 4-4c, p. 68 3 Bathymetry can tell you where things are: Oceanic ridge system Fig nft BATHYMETRY – OCEAN FLOOR CONTOURS 3 Fig 4-5, p.69 3 Fnft: The major plates of the earth's crusts Courtesy of Reto Stockli, NASA Earth Observatory Atlantic Ocean 3 Fig. 4-17a, p. 78 3 Fnft: Some large-scale features of the North Atlantic seafloor Courtesy National Geophysical Data Center/NOAA Folded ridges of sediment cover the ocean floor west of Oregon 3 Fig. 4-11, p. 74 Topography of the Ocean Floor 3 REMEMBER: • deepest part of the ocean lie near its edges 3 fnft 3 Topography of the Ocean Floor • Submerged outer edge of the continents are called continental margins • Deep-sea floor beyond these is called the ocean basin 3 Fig 4-6, p.70 3 Fig. 4-25, p. 84 3 Fig 4-9, p.72 3 Florida coast Fig 4-10, p.73 but 3 •There are 2 sides to every…OCEAN! 3 Fig 4-8, p.71 2 types of “margins” 3 • PASSIVE MARGIN: Continental margins that face diverging plates. These do not coincide with plate boundaries. Little or no activity. Typically associated with the Atlantic. • ACTIVE MARGIN: Continental margins that face converging plates. These coincide w/plate boundaries. A lot of activity (earthquake/volcano). Typically associated with the Pacific. Continental Margins 3 •Passive margins – – continental margins not located on plate boundaries – Atlantic-type margins Continental Margins 3 •Active margins– continental margins on the edge of convergent or transform plate boundaries – Pacific-type margins 3 Fig 4-8, p.71 3 Fig 4-9, p.72 Continental Margins (Shelves) 3 • Width of Continental Shelf is determined by : • proximity to a plate boundary (active margins have narrow shelves while passive margins have broad shelves) Continental Margins (Shelves) 3 • Continental Shelves – Shallow, submerged extension of a continent – broad, gently sloping – 7.4% of earths Ocean area 3 Fig nft Continental (Passive) Margins 3 • Shelf break – – transition between the continental shelf and the continental slope There are also changes from the continental slope (edge of shelf) to the continental rise (ends at edge of ocean floor). Continental Margins 3 • Continental Slopes – Steeper than the shelf – end at the deep ocean 3 Fig 4-9, p.72 3 Fig 4-9, p.72 Continental Margins 3 • Continental rises – at the base of continental slope – covered by a blanket of accumulated sediment – gradual slope Continental Margins 3 • Submarine Canyons – cut into the continental shelf and slope – formed by turbidity currents (avalanche-like sediment movements) 3 F 4.12 Submarine Canyon Off of The coast Of New Jersey 3 Fig 4-13, p.75 3 Mid-Atlantic Ridge between Florida and western Africa 3 Canyon in middle of ridge Fig. 4-17b, p. 78 Ocean Basin 3 • thick layer of sediment (up to 5 km or 3mi thick) covering basaltic rocks • Make up more than ½ of the earth’s surface Ocean Basin 3 • Oceanic ridges – Underwater mountain chain – an active spreading center – offset at regular intervals by transform faults 3 WHERE THE RIDGES ARE! Fig nft • Midoceanic Ridge Province consists of a continuous submarine mountain range. • It covers about one third of the ocean floor. • It extends for about 60,000 km around the Earth. 3 3 Examples of what “exists” on the Ocean Floor • Hydrothermal Vents • SeaMounts • Guyots • Abyssal Hills • Abysall Plains • Trenches • Island Arcs (seen above “land”) Ocean Basin 3 • Seamounts – Inactive volcanoes that do not rise above the surface of the ocean – They are tall with steep slopes Ocean Basin 3 • Guyots – Flat-topped seamounts that were eroded by wave action • Abyssal Hills – abundant, small sediment-covered extinct volcanoes 3 guyots (G) and seamounts Fig. 4-23a, p. 82 Ocean Basin 3 • Island Arcs – Curving chains of volcanic islands and seamounts found paralleling the edge of trenches 3 Fig 4-23, p.76 Coral Reefs? 3 • All of these different land/(under)water formations can yield MANY different types of coral reefs (a very diverse, valuable, marine community) too! 3 Fnft 3 Fnft 3 Fnft 3 Fnft Ocean Basin 3 • Abyssal Plains – Flat, featureless, sediment-covered ocean floor • Trenches – Arc-shaped depression in the deep seafloor – a converging oceanic plate is subducted • Deep Ocean Province is between the continental margins and the midoceanic ridge . • It includes a variety of features from mountainous to flat plains: 3 – – – – Abyssal plains Abyssal hills Seamounts Deep sea trenches Ocean Basin 3 • Hydrothermal vents – average temp is about 8-16oC (46-61oF) much warmer than the typical 3-4oC (3739oF) – support a unique community of organisms that depend on bacteria 3 Cross-section of a ridge axis and the plumbing connected to a vent chimney 3 Fig 4-20, p.80 3 Sidescan sonar image overlaid onto multibeam bathymetry 3 Approximate locations of confirmed hydrothermal vents and cold seeps 3 Red-plumed tube worms Courtesy of Monika Bright, University of Vienna, hydrothermalvent.com 3 A black smoker on the Galápagos Rift Zone. Courtesy of UCSB, University S. Carolina, WHOI/NOAA 3 Fig 4-19, p.79