Continental Margins and Ocean Basins

1. Continental Margins and Ocean Basins

2. 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.

3. 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.

4. 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.

5. 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.

6. 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).

7. 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.

8. 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.

9. 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.

10. 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).

12. 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

14. Submarine canyon showing turbidity currents.

15. 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.

16. 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.

17. 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

18. 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.

19. 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.

20. 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).

21. 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”.

22. 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.

23. The Ocean Basins A Hydrothermal vent area on the ocean floor.

24. 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.

25. A “Smoker”

26. Ocean Basins • Entire communities of organisms were found associated with hydrothermal vents in this deep, dark, cold part of the ocean.

27. 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.

28. 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.

29. Ocean Basins • Seamounts • Guyots • Trenches • Island Arcs

30. Ocean Basins • The End