Ocean Basins, Bathymetry and Sea Levels

Transcription

1 Ocean Basins, Bathymetry and Sea Levels Chapter 4 Please read chapter 5: sediments for next class and start chapter 6 on seawater for Thursday

2 Basic concepts in Chapter 4 Bathymetry the measurement of water depths and mapping of sea floor features uses several types of technology. The sea floor has two distinct regions: continental margins and deep ocean basins. Continental margins are the relatively shallow areas of the ocean floor near shore. Geologically they are part of the adjacent continent. Passive vs. active continental margins are distinguished by their plate tectonic setting. Major features: continental shelf, shelf break, continental slope, continental rise, submarine canyons, submarine fans. Deep-ocean basins differ from the continental margins in tectonic origin, history, and composition. Most important features of the deep ocean basins were formed by plate tectonic processes. Major features: mid-ocean ridge, transform faults & fracture zones, hydrothermal vents, abyssal plain, abyssal hills, seamounts & guyots, oceanic trench & volcanic arc.

3 Evolution of the oceans (and continents) over the last 650 million years

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21 Present Vail Curve of Sea Level History Transgression Regression 540 million years BP

22 Blue = No deposition

23 Interglacial Glacial

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25 What about changes in sea level? glacial/interglacial timescale Increase of ~120 meters

26 What about changes in sea level? Over Figure the last 5.13 century Increase of ~15 centimeters

27 Bathymetry: Measuring Ocean Depths As late as 1870 bathymetric studies were often performed using a weighted line dropped to measure depth. Advances in Bathymetry Echo sounding (1922) Multibeam systems Satellite altimetry

28 Echo sounding is a method of measuring depth using powerful sound pulses. The time it takes for the sound pulse to travel to the sea bed and bounce back is a measure of the depth. Distance = Rate x Time

29 Trace of an echo sounder is just a line

30 Multibeam systems can provide more accurate measurements than echo sounders. Multibeam systems collect data from as many as 121 beams to measure the contours of the ocean floor.

31 Fig. 4-3b, p. 79

32 Fig. 4-4 (a-b), p. 80

33 Satellite altimetry is an indirect way of measuring depth and detecting sea floor features pulses a second while moving 4 miles a second over head. Height accuracy to within 1 inch!

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36 Size comparison of various volcanic features

37 Continental Margins Components: 1. Continental shelf = the shallow, submerged edge of the continent 2. Shelf break = the abrupt transition from continental shelf to the continental slope 3. Continental slope = the transition between the continental shelf and the deep-ocean floor 4. Continental rise = thick accumulations of sediment found at the base of the continental slope

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40 Continental margin = the submerged outer edge of a continent; really just an extension of the continental crust, which has an average composition of granite. vs. Deep-ocean basin = the deep sea floor beyond the continental margin; made up of oceanic crust, which is composed mostly of volcanic basalt.

41 Types of continental margins Passive margins (= Atlantic-type margins) " They face the edges of diverging tectonic plates " Very little volcanic or earthquake activity Active margins (= Pacific-type margins) " Located near the edges of converging plates, where one plate subducts beneath another at an oceanic trench " Extensive volcanic and earthquake activity

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44 Fig. 4-10, p. 85

45 Fig. 4-11, p. 86

46 Submarine Canyons Underwater landslides or avalanches called turbidity currents commonly flow down submarine canyons. The debris settles out to build up a submarine fan at the base of the canyon.

47 Continental slope Continental rise

48 An underwater debris flow, i.e. a turbidite.

49 Fig. 4-14, p. 87

50 Deep Ocean Basins Major features: Mid-ocean ridges Fracture zones and transform faults Hydrothermal vents Shallow earthquakes Abyssal plains Abyssal hills Seamounts and guyots Oceanic trenches Volcanic arcs Deep earthquakes

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54 Fig. 4-17a, p. 90

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56 JuandeFucaSmoker.avi

57 Abyssal Plains Abyssal plains = broad flat areas of sediment-covered ocean floor found between the continental margins and the mid-ocean ridges Typically 4-6 km below sea level The flattest surface on Earth Sedimentation rates are very slow -- millimeters/1000 years! Mostly very fine clay, windblown dust, and shells of microscopic organisms Carbonate sediments are rare, as most of the plain is below the Carbonate Compensation Depth (CCD)

58 Thick layers of sediment on top of submerged mountains

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60 Trenches Oceanic trenches are the deepest parts of the sea floor. They are formed by subduction. Notice that most trenches (and therefore most subduction) occur in the Pacific ocean basin.

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63 Subduction at oceanic trenches causes the descending plate to generate melting of the overlying mantle, forming volcanic island arcs = lines of volcanic islands and seamounts running parallel to the edges of trenches. Subduction also causes many earthquakes.

64 Seismic activity in the Aleutian Islands

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66 The Aleutian Island Chain

67 Formation of Guyots and submerged mountain chains Fig. 4-23a, p. 95

68 Oceanic-Oceanic Subduction Zone Oceanic-Continental Subduction Zone

69 Oceanic-Oceanic and Oceanic-Continental Subduction Zones

70 Passive Continental Margin

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