Ch. 20 The Ocean Basins Ch Features of the Ocean Floor

Two Major Divisions of the Ocean Floor 1. Continental Margins—shallower portions made up of continental crust and a thick wedge of sediment. 1. Continental Margins—shallower portions made up of continental crust and a thick wedge of sediment. 2. Deep Ocean Basin—made up of oceanic crust and a thin sediment layer. 2. Deep Ocean Basin—made up of oceanic crust and a thin sediment layer.

Continental Margins The line that divides continental crust from oceanic crust is almost always offshore. The line that divides continental crust from oceanic crust is almost always offshore. Continental Shelf—part of a continent’s edge covered by ocean water. Slopes gently away from the shore. Continental Shelf—part of a continent’s edge covered by ocean water. Slopes gently away from the shore. Average depth of about 60 meters. Average depth of about 60 meters. Width of the continental shelf varies, but averages about 70 km wide. Width of the continental shelf varies, but averages about 70 km wide.

Affected by changes in sea level. Affected by changes in sea level. During ice ages, sea levels fall, and more continental shelf is exposed to weathering and erosion. During ice ages, sea levels fall, and more continental shelf is exposed to weathering and erosion.

Continental Slope Steeper slope at edge of the continental shelf. Steeper slope at edge of the continental shelf. Continental slope’s base is the boundary between the continental and oceanic crust. Continental slope’s base is the boundary between the continental and oceanic crust. Ocean depth rapidly increases along the continental slope. Ocean depth rapidly increases along the continental slope.

May be cut by deep V-shaped valleys called submarine canyons, which could have been caused by river flows or turbidity currents (underwater sediment landslides). May be cut by deep V-shaped valleys called submarine canyons, which could have been caused by river flows or turbidity currents (underwater sediment landslides). Along the base of the continental slope is a raised wedge of sediments called the continental rise. Along the base of the continental slope is a raised wedge of sediments called the continental rise.

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Deep Ocean Basins Contain higher mountains and flatter plains than any found on the continents! Contain higher mountains and flatter plains than any found on the continents! Trenches—deepest feature of the earth’s surface. Mariana Trench is over 11,000 meters deep. Associated with subduction zones and all of their features, such as earthquakes, volcanic island arcs, and volcanic mountain ranges. Trenches—deepest feature of the earth’s surface. Mariana Trench is over 11,000 meters deep. Associated with subduction zones and all of their features, such as earthquakes, volcanic island arcs, and volcanic mountain ranges.

Abyssal Plains Vast flat areas in the deep ocean basins where depth is often greater than 4 km. Vast flat areas in the deep ocean basins where depth is often greater than 4 km. Flattest regions on earth, covering about half of the deep ocean basins. Flattest regions on earth, covering about half of the deep ocean basins. Covered with sediment—thin in the Pacific, but much thicker in the Atlantic Ocean, which is not bordered by trenches. Covered with sediment—thin in the Pacific, but much thicker in the Atlantic Ocean, which is not bordered by trenches.

Mid-Ocean Ridges Underwater mountain ranges that run along all ocean floors. Underwater mountain ranges that run along all ocean floors. Result from divergent plate boundaries. Result from divergent plate boundaries. A rift valley runs along the crest (top) of the ridges. A rift valley runs along the crest (top) of the ridges. New magma rises up through the rift. New magma rises up through the rift.

Warm, new crust is less dense than cold, older crust away from the ridge, so the colder, denser rock sinks, and elevation decreases as you move away from the ridge. Warm, new crust is less dense than cold, older crust away from the ridge, so the colder, denser rock sinks, and elevation decreases as you move away from the ridge. Fracture zones result from different parts of the ridge separating at different rates…they are faults that run perpendicular to the ridge. Fracture zones result from different parts of the ridge separating at different rates…they are faults that run perpendicular to the ridge.

Seamounts Submerged volcanic mountains at least 1000 m high. Submerged volcanic mountains at least 1000 m high. Abyssal hills—submerged volcanic mountains less than 1000 m. Abyssal hills—submerged volcanic mountains less than 1000 m. Both are usually associated with hotspots. Both are usually associated with hotspots. Seamounts that rise above the surface become volcanic islands. Seamounts that rise above the surface become volcanic islands. Guyots (GEE-oze)—sunken volcanic islands that have had their tops eroded flat by wave action. Guyots (GEE-oze)—sunken volcanic islands that have had their tops eroded flat by wave action.