1. 20.2

2. Continental Margins The line that divides the continental crust from the oceanic crust is not always obvious. Shorelines are not the true boundaries between the ocean floor and continents. The real boundary lies offshore beneath the ocean and thick sediments.

4. Continental Shelf The gently (<1°) sloping platform at the edge of the continent. The shelf is generally thought to be an extension of the continent and not really a part of the ocean basin. The average water depth on the shelf is about 75 meters, varying from zero at the shoreline to about 150 meters near its edge.

5. A typical continental shelf is 60 kilometers wide, but it exceeds 100 kilometers off the Florida coast and is less than a few kilometers wide in places along the West Coast of South America. The rock underlying the thin veneer of sediments is granite similar to the basement rock elsewhere beneath the continents.

6. The width of the continental shelf varies. On the west coast of S. America it is only a few kilometers wide. The widest shelves extend out about 1,280 km from Siberia and Alaska into the Arctic Ocean.

7. Continental Slope The continental slope marks the transition between the shelf and deep ocean floor. It has an average slope of 3-6°. This may not sound like much, but over a distance of 100 kilometers water depth increases from 75 meters to 4000 meters.

8. Typically, continental slopes are crisscrossed by a series of deep submarine canyons the origin of which is controversial. Some represent drowned stream valleys, but others were clearly never above sea level and can not have been cut downward by stream erosion.

9. Continental Rise Represents the accumulation of sediment at the base of the continental slope. Result is a gentler slope and the buildup of "turbidite" deposits. Uplifted turbidite deposits are common along the coastline of southern California, particularly at Blacks and Torrey Pines beaches north of San Diego.

10. Southern California turbidites are thought to form during major earthquakes which cause sediments to slide off the edge of the shelf and accumulate on the ocean floor as "fining upward" sequences of sedimentary rocks.

12. Deep Ocean Basins Deep ocean basins have their own distinct features such as: submerged volcanic mountains, deep trenches, and plains.

14. Trenches The deepest feature on earth’s surface. The deepest place in the world is the Marianna Trench – over 11,000 m deep. Located between the western Pacific Ocean near Guam. Trenches are associated with earthquakes and located along the Ring of Fire.

16. At least 22 trenches have been identified although not all are classified as major. Of this number, 18 are in the Pacific Ocean, three in the Atlantic Ocean, and one in the Indian Ocean

17. In order to better illustrate the actual depth of the Mariana Trench, consider the following; if Mount Everest, which is the tallest point on earth at 8,850 meters (29,035 feet), were set in the Mariana Trench, there would still be 2,183 meters (7,166 feet) of water left above it.

18. Abyssal plain The ocean floor (covers about 30% of the earth's surface). The average water depth is around 5000 meters. Consists of a layer of unconsolidated sediment underlain by sedimentary rock and pillow basalt

19. Extremely vast flat areas that lie in the deep ocean basins where ocean depth is often great than 4 km. Flattest regions on earth.

20. Mid-Ocean Ridges The most prominent feature of the Ocean basin are the mid-ocean ridges. They are underwater mountain ranges. Form when plates pull away from each other. Have a narrow depression, or rift, where magma reaches the sea floor and forms new crustal rock.

21. Different parts of the ridge separate at different rates, the new crust breaks into a series of faults called fracture zones, that run perpendicular to the ridge.

22. Seamounts Submerged volcanic mountains at least 1,000 m high are called seamounts. Abyssal hills are less than 1 km in height. Both are generally associated with hot spots. Seamounts rise above the surface and form islands such as Hawaii and the Canary Islands.

23. As plate tectonics carries seamounts away from mid- ocean ridges, the crust sinks. The seamounts sink with it. Submerged seamounts are called guyots or tablemounts.