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M. Anderson, 2009.  3 major provinces  Continental margins ◦ Shallow-water areas close to shore  Deep-ocean basins ◦ Deep-water areas farther from.

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Presentation on theme: "M. Anderson, 2009.  3 major provinces  Continental margins ◦ Shallow-water areas close to shore  Deep-ocean basins ◦ Deep-water areas farther from."— Presentation transcript:

1 M. Anderson, 2009

2  3 major provinces  Continental margins ◦ Shallow-water areas close to shore  Deep-ocean basins ◦ Deep-water areas farther from land  Mid-ocean ridge ◦ Submarine mountain range

3  Zones separating the part of a continent above sea level from the deep‑sea floor.  The true geologic margin of a continent ‑ where continental crust changes to oceanic crust ‑ is somewhere beneath the continental slope.

4  Passive Continental Margin ‑ Trailing end of a continental plate.  Posses broad continental shelves, a continental slope and rise.  Flat abyssal plains are adjacent to the rise. They lack the seismic and volcanic activity.

5  Active ◦ Associated with convergent or transform plate boundaries ◦ Much tectonic activity  Convergent active margin  Oceanic-continental convergence  Where oceanic lithosphere is subducted.  Seismicity, a young Mt. range and andesitic volcanism.  The shelf is narrow and descends directly into a trench. Example: western South America

6  Continental shelf  Shelf break  Continental slope  Continental rise

7  Continental Shelf ‑ where the sea floor slopes very gently seaward.  It ends at a steep drop, the shelf ‑ slope break at @ 135m.  In the Pleistocene, sea level was much lower, @ 130m.  Much of the sediment on continental shelves accumulated in stream channels and floodplains.  Some glaciers extended onto the exposed shelves.  Affected by waves and tidal currents.

8  Change in gradient from shelf  Average gradient 4 o  Submarine canyonscut into slope by turbidity currents  Submarine canyons cut into slope by turbidity currents ◦ Mixture of seawater and sediments ◦ Move under influence of gravity ◦ Erode canyons ◦ Deposit sediments at base of slope

9  Turbidity Currents ; sediment‑ water mixture denser than normal seawater  Flow down‑slope to the deep‑sea floor.  Coarsest particles are deposited first ‑ forming a graded bed.  Deposits accumulate as a series of overlapping submarine fans, forming a large part of the continental rise.

10  Continental rise ‑ gently sloping area from the slope to a trench.  Unaffected by surface processes and transport is controlled by gravity. Where most of the sediments are eventually deposited. Much of the sediment is transported by turbidity currents.  Deep ocean Basin ‑ seaward of the continental margin.

11 Monterey Submarine Canyon  Deep, steep sided submarine canyons occur on continental shelves, but are best developed on the continental slopes.  Some Canyons can be traced across the shelf to associated streams on land. Some can not.  Strong currents move through these canyons and are probably responsible for their erosion.

12  Transition between continental crust and oceanic crust  Turbidite deposits ◦ Graded bedding ◦ Submarine fans  Distal end of submarine fans becomes flat abyssal plains

13  The product of turbidity currents in km water depth  Present/future targets of the oil industry once shallower resources are exhausted

14  Flat abyssal plains are adjacent to the rise. They lack the seismic and volcanic activity.  Abyssal Plains are the flattest, most featureless areas of Earth A result of sediment deposition.  Found adjacent to Continental rises.  Common in the Atlantic, rare in the Pacific.  Along active margins, sediments are trapped in an ocean trench so abyssal fans fail to develop.

15  Very flat depositional surfaces from base of continental rise  Suspension settling of very fine particles  Sediments cover ocean crust irregularities  Well-developed in Atlantic and Indian oceans

16  Abyssal hills ‑ average @ 250m high.  They are common on the sea floor and underlie thick sediments on the abyssal plains.

17 Abyssal plains Fig. 3.11

18  Oceanic Trenches  Where lithospheric plates are consumed by subduction.  Long, narrow features restricted to active continental margins.  Oceanic trenches are the sites of greatest oceanic depth.  Marianas Trench : 11,000 m deep.  The crust here is cooler and slightly denser than elsewhere.

19  Trenches show a huge negative gravity anomaly;  The crust is held down and is not in equilibrium.  Seismic activity occurs at or near the trenches.  They have Benioff zones in which earthquake foci become progressively deeper in a landward direction.  Most intermediate and deep  earthquakes occur in such zones.  They are associated with volcanoes. (W. So. America)

20  Seamounts are isolated volcanic mountains scattered across the ocean floor.  Most common in the Pacific Ocean, seamounts generally rise more than 1,000 meters above the sea floor, often forming islands. Emperor, Marshall and Hawaii seamounts

21  When the action of plate tectonics moves a seamount-formed island away from the mid-ocean ridge, the ocean crust sinks, pulling the island beneath the surface.  These submerged, often flat-topped, seamounts are called guyots.

22  Poke through sediment cover  Below sea level: ◦ Seamounts, tablemounts, or guyots at least 1 km (0.6 m) above sea floor ◦ Abyssal hills or seaknolls are less than 1 km  Above sea level: ◦ Volcanic islands

23  Landward side of ocean trench  Island arc ◦ Chain of islands, e.g., Japan, Hawaii, Aleutians  Continental arc ◦ Volcanic mountain range, e.g., Andes Mountains

24  As the South American continent moved west, in the Cretaceous Period (140mya) the Nazca plate subducted forming a trench.  By 130mya igneous activity began and a Volcanic arc was formed.  By 90 mya the trench had migrated west and a new volcanic arc formed along the west coast, while mountains to, now in the interior, ceased activity.

25  Longest mountain chain  On average, 2.5 km (1.5 miles) above surrounding sea floor  Wholly volcanic  Basaltic lava  Divergent plate boundary  Central rift valley, faults, and fissures  Seamounts  Pillow basalts  Hydrothermal vents ◦ Deposits of metal sulfides ◦ Unusual life forms  Fracture zones transform faults  Fracture zones and transform faults

26  Oceanic ridge ◦ Prominent rift valley ◦ Steep, rugged slopes ◦ Example: Mid-Atlantic Ridge  Oceanic rise ◦ Gentler, less rugged slopes ◦ Example: East Pacific Rise

27  When lava flows enter the ocean, or when lava outpourings actually originate within an ocean basin, the flows outer zones quickly congeal.  The lava within the flow is able to move forward by breaking through the hardened surface, when this occurs over and over.  The lava flow resembles large bed pillows stacked one upon another.

28 ALVIN: Woods Hole Submersible  1970'S @ 2,500m in the Galapagos Rift in E. Pacific Ocean Basin hydro‑thermal vents were first observed.

29  Hydrothermal vents ◦ Heated subsurface seawater migrates through cracks in ocean crust  Warm-water vents <30 o C or 86 o F  White smokers >30 o C 30 o C <350 o C or 662 o F  Black smokers > 350 o C

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