Unit 6: The Sea Floor

Transcription

1 Unit 6: The Sea Floor

2 Turn to Page 62 in Introduction to the World s Oceans Study the bathymetric chart What features do you see? How do the Atlantic and Pacific Oceans differ? What do you think accounts for those differences?

3 Ocean Bathymetry: What do you see?

4 What is Bathymetry? Bathymetry is from the Greek: bathos = deep, and metry = measure is the study of ocean contours A bathymetric chart shows what the ocean floor would look like if all the water drained out

5 Bathymetry continued More is known about the surface of Venus, Mars, and the dark side of the moon than the ocean floor Only 5% of the oceans have been mapped with the same precision as the moon there are areas the size of Kansas where no ship soundings have been made and even well surveyed areas are based on widely separated survey tracks.

6 Early bathymetric studies were performed with a sounding line or lead line to study the depth of the ocean floor Sailors measured depth in fathoms (6 ft) by stretching the rope between two outstretched arms

7 Precision Depth Recorder (PDR) developed in the 1950s used a focused beam to measure depth to 1 m in accuracy

8 Multi-beam echo sounders and side-scan sonar allows mapping of swaths up to 60 km wide

9 Bottom Coverage & Data Density by Survey Method Leadline Single Beam Multibeam 1-2 K soundings per survey K soundings per survey 400,000 1,000,000 K soundings per survey Image courtesy of NOAA & UNH

10 National Geographic Video layer/news/environment-news/us-oceanfloor-mapping-vin.html

11 Satellite Altimetry measures differences in sea surface elevation A sea surface anomaly is caused by the different gravitational pull of ocean mounts and trenches

12 Satellite Bathymetry continued

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14 Sea Floor Bathymetry as Measured by Satellite over 4.5 year period

15 NOAA Satellite Bathymetry animation bxguo4 QMyQ&feature=related

16 USS San Francisco In 2005 the nuclear attack sub USS San Francisco collided with an uncharted sea mount at flank (maximum) speed The sub was sailing at a depth of 500 ft. in the South Pacific about 350 south of Guam The sub was almost lost They repaired it by cutting the bow off the retiring USS Honolulu and installing it on the San Francisco.

17 Why Study Bathymetry?

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20 Ocean-Floor Topography Varies with Location Hypsographic Curve

21 What are the three classifications of ocean floor? Continental Margins the submerged outer edge of a continent Deep Ocean Basin the deep seafloor beyond the continental margin Mid-Ocean Ridges The submarine mountain range in the middle

22 Deepest not in the middle, but off to either side of the shallower mid-ocean ridge

23 Mid-Atlantic Ridge

24 Passive margins Active margins Continental Margins

25 active margin passive margin

26 Continental Margins Continental Shelf - shallow, submerged edge of the continent. Shelf break - abrupt transition from continental shelf to the continental slope. Continental Slope - transition between the continental shelf & the deep-ocean floor. Continental Rise - accumulated sediment found at the base of the continental slope.

27 Continental Shelves Are Seaward Extensions of the Continents The features of a passive continental margin: (a) Vertical exaggeration 50:1 (b) No vertical exaggeration.

28 Continental Margins Submarine Canyons

29 California s canyons

30 Submarine Canyons Form at the Junction between Continental Shelf & Continental Slope turbidity currents

31 How do canyons form? river erosion during previous glaciation periods turbidity currents: rapid movement of a mass of sediment down a slope ~ avalanche often caused by earthquakes

32 Topology of Deep-Ocean Basins Differs from That of the Continental Margin A mid-ocean ridge is a mountainous chain of young, basaltic rock at an active spreading center of an ocean.

33 Hydrothermal Vents hydrothermal vents: spring of hot, mineral- and gas-rich seawater near the spreading centers of mid-ocean ridges In 1977, Robert Ballard & J. Grassle discovered new life forms in Alvin, at 3000 m near the Galápagos Islands

34 Hydrothermal vents are sites where superheated water containing dissolved minerals & gases escapes through fissures, or vents.

35 Mid-Oceanic ridges & Hydrothermal Vents Warm-water vents are below 30⁰ C and emit clear water. White smokers emit water between 30 ⁰ and 350 ⁰. Water is white because of light-colored dissolved minerals. Black smokers are above 350⁰ C (662 ⁰F) and emit black water because of presence of dark metal sulfides. Question: Why don t they emit steam instead of water?

36 Volcanic Seamounts & Guyots Project Above the Seabed Seamounts are volcanic projections from the ocean floor that do not rise above sea level. Flat-topped seamounts eroded by wave action are called guyots. Abyssal hills are small, extinct volcanoes or rock intrusions near the oceanic ridges.

37 Deep-Ocean Basins Trenches are depressions in the ocean floor caused by the subduction of a converging ocean plate.

38 Deep-Ocean Basins Island Arcs, chains of volcanic islands & seamounts, are usually found parallel to the edges of ocean trenches. (left) As two oceanic plates converge, an island arc is formed by volcanic activity.

39 Trenches & Island Arcs Form in Subduction Zones

40 Video Marianas Trench 3F4

41 Coral atolls form when volcanic islands sink leaving behind the coral reef surrounding a lagoon where the volcano used to be.

42 Atolls Fanning Island

43 The Abyss abyssal plains: flat, featureless expanses of seafloor covered w/ sediment between continental margins & oceanic ridges m Why is the abyssal plain so flat? It is flatter than any similar plain on land

44 Sea Floor Spreading Evidence In the 1950s, scientists discovered parallel patterns of magnetic stripes on the sea floor. This provides evidence of seafloor spreading and a record of magnetic pole reversals.

45 Global Distribution of Earthquakes Global distribution of earthquakes also provides evidence for theory of plate tectonics and seafloor movement.

46 Age of the Seafloor Late 1960s deep-sea drilling began on the Glomar Explorer Radiometric dating of ocean rocks to determine seafloor age Oldest seafloor is about 200 million years old compared to 3.5 billion years old for oldest rocks on land.

47 Age of Seafloor Symmetric pattern of age distribution - youngest at the ridges and oldest at the subduction zones

48 Plate Tectonics Theory Lithosphere tectonic plates that float on ductile asthenosphere Large scale geologic features occur at plate boundaries 2011 Pearson Education, Inc.

49 Plate Tectonic Processes

50 Global Plate Boundaries

51 Driving Forces of Plate Motion Slab Pull Theory: Older dense plates at the subduction zones sink into the underlying asthenosphere providing most of the driving force for plate motion.

52 Types of Plate Boundaries 2011 Pearson Education, Inc.

53 Divergent Boundary Features Plates move apart at the mid-ocean ridge forming a rift valley New ocean floor is created Shallow focus earthquakes happen here

54 Generation of a Divergent Boundary

55 Convergent Boundary Features Plates move toward each other Oceanic crust destroyed Trenches and volcanic arcs are found here Deep focus earthquakes

56 Three Types of Convergent Boundaries

57 Oceanic Continental Convergence Ocean plate is subducted Continental arcs generated Explosive andesitic volcanic eruptions

58 Oceanic Oceanic Convergence Oceanic crust is destroyed Friction of plates melts sediments and crust Molten magma erupts in form of island arcs

59 Continental Continental Convergence No subduction Tall mountains uplifted Himalayans

60 Transform Boundary Features Oceanic Transform Fault ocean floor only Continental Transform Fault cuts across continent All transform faults occur between mid-ocean ridge segments Pearson Education, Inc.

61 Global Hot Spots Scattered around the earth are approximately 40 fixed areas of volcanic activity known as hot spots Hot spots remain stationary as the plates move and are useful for tracing plate movements As ocean plate moves over a hot spot, a chain of volcanos is formed.

62 2011 Pearson Education, Inc. Global Hotspot Locations

63 Hawaiian Islands Hot Spot