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Continental Margins and Ocean Basins. Continental Margins. Continental margins facing the edge of diverging plate boundaries are called passive margins. There is relatively little earthquake or volcanic activity associated with these margins.

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Continental Margins and Ocean Basins

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continental margins
Continental Margins
  • Continental margins facing the edge of diverging plate boundaries are called passive margins.
  • There is relatively little earthquake or volcanic activity associated with these margins.
  • Because these surround the Atlantic Ocean they are also called Atlantic Margins.
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Continental Margins
  • Continental margins near the edges of converging plate boundaries are called active margins.
  • These margins exhibit a lot of earthquake and volcanic activity.
  • Because of their prevalence in the Pacific they are also called Pacific type margins.
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Continental Margins
  • Continental margins have two main divisions.
  • The continental shelf is a shallow, nearly flat region of the margin.
  • The continental slope steeply slopes from the shelf seaward.
continental shelves
Continental Shelves
  • The continental shelf is the shallow submerged extension of the continent.
  • Continental shelves resemble the continental landmass much more than the deep-ocean floor.
  • They have hills, depressions and minerals like those on the continent. For example oil, as well as other important minerals.
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Continental Shelves
  • The broad continental shelf extends from the shore line in a gentle slope, about 1.7 meters pre kilometer (about 9 feet per mile).
  • Along the Atlantic Ocean the shelves may be as wide as 350 kilometers (220 miles) and have a depth of up to 140 meters(460 feet).
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Continental Shelves
  • The depth of the water over the shelves is 75 meters (250 feet).
  • This means that large areas of the the shelves are accessible to mining and drilling activities.
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Continental Shelves
  • The continental slope.
  • The continental slope is the transition between the gently sloping shelf and the deep ocean bottom.
  • Continental slopes are formed from the sediments that reach the edge of the shelf and are transported over the side.
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Continental Shelves
  • Continental slopes have a relatively steep incline.
  • A 25-degree slope is the greatest incline discovered so far.
  • In general continental slopes at active margins are steeper than those at passive margins.
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Continental Shelves
  • The shelf break marks the abrupt transition from the continental shelf to the continental slope.
  • The depth of the shelf break is surprisingly constant world wide, about 140 meters (460 feet).
submarine canyons
Submarine Canyons
  • Submarine canyons cut into the continental shelf and slope, often terminating on the deep sea-floor in fan shaped wedges of sediment.
  • Occasionally there are landslides of sediment down the trenches triggered by earthquakes
continental slope
Continental Slope
  • At the bottom of continental shelves along passive margins there is an accumulation of sediment called the continental rise.
  • Sediments from the shelf slowly descend to the ocean floor along the whole continental slope to produce a continental rise.
ocean basins
Ocean Basins
  • The Deep Ocean Basins
  • As you move away from the continental margins the seafloor is blanketed with sediments up to 5 kilometers (3 miles) thick over the basalt rock of the ocean floor.
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Ocean Basins
  • The deep ocean floor consist mainly of oceanic ridge systems and the adjacent sediment covered plates.
  • Deep ocean basins may be rimmed by trenches or masses of sediment.
  • The flat expanse of the ocean bottom is interrupted by islands, abyssal hills, extinct volcanoes and guyotes
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Ocean Basins
  • The ocean ridges 65,000 kilometers (40,000 miles). This is more than 1 & 1/2 times the Earth’s circumference.
  • These ridges rise about 2 kilometers (1.25 miles) above the seafloor in some places.
  • In some places they project above the surface of the ocean to form islands, like Iceland, the Azores and Easter Island.
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Ocean Basins
  • Hydrothermal Vents
  • Hydrrothermal vents were discovered in 1977 by Robert Ballard and J. F. Grassle of Woods Hole Oceanographic Institution.
  • They were diving in the submersible, Alvin, about 3 kilometers (1.9 miles) down near the Galapagos Islands.
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Ocean Basins
  • They saw rock chimneys up to 20 meters (66 feet) high.
  • They saw, what they now know was, dark mineral laden water blasting from these chimneys.
  • The temperature of that water was measured at 350-degrees C. (660-degrees F).
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Ocean Basins
  • It was only because to the great pressure at that depth that prevented water at that temperature from flashing into steam.
  • These smoking underwater chimneys quickly received the nick name “black smokers”.
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Ocean Basins
  • It is believed that water descends through fissures and cracks in the ocean ridge floor until it comes into contact with very hot rocks associated with active seafloor spreading.
  • The super heated water dissolves minerals and gases and escapes upward through the vents.

The Ocean Basins

A Hydrothermal vent area on the ocean floor.

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Ocean Basins
  • Other vents have been discovered on the Mid-Atlantic Ridge east of Florida, in the Sea of Cortez east and south of Baja California,and on the Juan de Fuca ridge off the coast of Washington and Oregon.
  • Hydrothermal vents are now thought to be common on ridges especially in areas of rapid plate spreading.
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Ocean Basins
  • Entire communities of organisms were found associated with hydrothermal vents in this deep, dark, cold part of the ocean.
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Ocean Basins
  • Abyssal Plains
  • The abyssal plains are flat featureless expanses of ocean floor found between the continental margins and the ocean ridges.
  • They are most common in the Atlantic Ocean, less common in the Indian Ocean and rare in the active Pacific Ocean.
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Ocean Basins
  • Most of the sediment that forms on the abyssal plains seems to be of terrestrial or shallow-water origin.
  • The deep sediment layers mask irregularities in the underlying crust.
  • These plains are punctuated by abyssal hills, and extinct volcanoes.
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Ocean Basins
  • Seamounts
  • Guyots
  • Trenches
  • Island Arcs
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Ocean Basins
  • The End