Features of the Ocean Floor Chapter 20.2

The Ocean Floor The ocean floor can be divided into two major areas. The continental margins are shallower portions of the ocean floor made up of continental crust and a thick wedge of sediment. The deep ocean basin is made up of oceanic crust and a thin sediment layer.

Continental Margins Continental Margins Shorelines are not the true boundaries between the ocean floor and the continents. The real boundary lies offshore, beneath the ocean and the thick sediments.

Continental Shelf The continental shelf is the part of the continent that is covered by ocean water. In general, the slope of the shelf drops about 0.12m for every 100m, and the water above the continental shelf is about 60m deep. While the continental shelf is under water, it is not part of the deep ocean basin, it is part of the continental margin.

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

Continental Shelf During the glacial period the more of the continental shelf was exposed to weathering and erosion since so much water was in the form of ice, and currently the continental shelf is a smoother version of the land surface above the shoreline.

Continental Slope A continental slope is a steep slope at the seaward edge of a continental shelf. The boundary between the continental crust and the oceanic crust is found at the base of the continental slope. Along the continental slope, the ocean depth increases to several thousand meters within a distance of a few kilometers.

Submarine Canyons Submarine canyons are areas where the continental shelf and slope are cut by deep v-shaped valleys. These valleys are generally associated with the mouths of major rivers.

Turbidity Currents However, some canyons have been caused by turbidity currents. Turbidity currents are very dense currents that carry large amounts of sediment down the continental slope, and form when landslides of various materials run down a slope. At the bottom of the continental slope these sediments are deposited and form a raised wedge called a continental rise.

Turbidity Current

Deep Ocean Basins Deep Ocean Basins The deep ocean basins have features such as broad, flat plains, submerged volcanic mountains, gigantic volcanic mountain ranges, and deep trenches. On the deep ocean basins, the mountains are higher and the plains are flatter than any found on the continents.

Trenches Trenches are deep valleys in the ocean floor. In fact, they are the deepest features on the earth’s surface. The deepest trench in the world is the Mariana Trench, located in the western Pacific Ocean near the island of Guam, and is over 11,000m deep. Most trenches are located near the Ring of Fire and are associated with earthquakes, volcanic mountain ranges, and volcanic island arcs.

Abyssal Plains Abyssal plains are extremely vast flat areas that lie in the deep ocean basins where the ocean depth is often greater than 4km. Abyssal plains cover about half of the deep ocean basin and are the flattest regions on earth.

Mid-Ocean Ridges Mid-ocean ridges are a continuous series of underwater mountain ranges that run along the floors of all oceans, and in a few places, such as Iceland, these mid-ocean ridges actually rise above sea level.

Mid-Ocean Ridge Mid-ocean ridges form with plates pull away from each other. Along the crest of the ridge are rifts, which are narrow depressions where magma reaches the sea floor and form the earth’s newest crustal rock.

Mid-Ocean Ridge The new rock is less dense and is elevated, however when it cools it becomes more dense and moves away from the rift. Because 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.

Seamounts Seamounts are submerged volcanic mountains that are at least 1,000m high. Submerged volcanic mountains that are less than 1,000m are called abyssal hills. Seamounts that rise above the surface of the ocean form oceanic islands such as Hawaii.

Guyots Both of these features are associated with hotspots. As these volcanic islands erode away and sink back into the ocean they are called guyots. Guyots are flat-topped, submerged seamounts also called tablemounts.