Ch. 20 The Ocean BasinsCh. 20 The Ocean Basins

Ch. 20.2 Features of the Ch. 20.2 Features of the Ocean FloorOcean Floor

Two Major Divisions of the Two Major Divisions of the Ocean FloorOcean Floor

• 1. 1. Continental MarginsContinental Margins—shallower —shallower portions made up of continental crust portions made up of continental crust and a thick wedge of sediment.and a thick wedge of sediment.

• 2. 2. Deep Ocean BasinDeep Ocean Basin—made up of —made up of oceanic crust and a thin sediment oceanic crust and a thin sediment layer.layer.

Continental MarginsContinental Margins

• The line that divides continental crust from The line that divides continental crust from oceanic crust is almost always offshore.oceanic crust is almost always offshore.

• Continental ShelfContinental Shelf—part of a continent’s —part of a continent’s edge covered by ocean water. Slopes edge covered by ocean water. Slopes gently away from the shore.gently away from the shore.

• Average depth of about 60 meters.Average depth of about 60 meters.

• Width of the continental shelf varies, but Width of the continental shelf varies, but averages about 70 km wide.averages about 70 km wide.

• Affected by changes in sea level.Affected by changes in sea level.

• During ice ages, sea levels fall, and During ice ages, sea levels fall, and more continental shelf is exposed to more continental shelf is exposed to weathering and erosion.weathering and erosion.

Continental SlopeContinental Slope

• Steeper slope at edge of the Steeper slope at edge of the continental shelf.continental shelf.

• Continental slope’s base is the Continental slope’s base is the boundary between the continental boundary between the continental and oceanic crust.and oceanic crust.

• Ocean depth rapidly increases along Ocean depth rapidly increases along the continental slope.the continental slope.

• May be cut by deep V-shaped valleys May be cut by deep V-shaped valleys called called submarine canyonssubmarine canyons, , which which could have been caused by river could have been caused by river flows or flows or turbidity currentsturbidity currents (underwater sediment landslides).(underwater sediment landslides).

• Along the base of the continental Along the base of the continental slope is a raised wedge of sediments slope is a raised wedge of sediments called the called the continental risecontinental rise..

• stopstop

Deep Ocean BasinsDeep Ocean Basins

• Contain higher mountains and flatter Contain higher mountains and flatter plains than any found on the continents!plains than any found on the continents!

• TrenchesTrenches—deepest feature of the earth’s —deepest feature of the earth’s surface. Mariana Trench is over 11,000 surface. Mariana Trench is over 11,000 meters deep. Associated with subduction meters deep. Associated with subduction zones and all of their features, such as zones and all of their features, such as earthquakes, volcanic island arcs, and earthquakes, volcanic island arcs, and volcanic mountain ranges.volcanic mountain ranges.

Abyssal PlainsAbyssal Plains

• Vast flat areas in the deep ocean basins Vast flat areas in the deep ocean basins where depth is often greater than 4 km.where depth is often greater than 4 km.

• Flattest regions on earth, covering Flattest regions on earth, covering about half of the deep ocean basins.about half of the deep ocean basins.

• Covered with sediment—thin in the Covered with sediment—thin in the Pacific, but much thicker in the Atlantic Pacific, but much thicker in the Atlantic Ocean, which is not bordered by Ocean, which is not bordered by trenches.trenches.

Mid-Ocean RidgesMid-Ocean Ridges

• Underwater mountain ranges that Underwater mountain ranges that run along all ocean floors.run along all ocean floors.

• Result from divergent plate Result from divergent plate boundaries.boundaries.

• A A rift valleyrift valley runs along the crest (top) runs along the crest (top) of the ridges. of the ridges.

• New magma rises up through the rift.New magma rises up through the rift.

• Warm, new crust is less dense than Warm, new crust is less dense than cold, older crust away from the ridge, cold, older crust away from the ridge, so the colder, denser rock sinks, and so the colder, denser rock sinks, and elevation decreases as you move elevation decreases as you move away from the ridge.away from the ridge.

• Fracture zonesFracture zones result from different result from different parts of the ridge separating at parts of the ridge separating at different rates…they are faults that different rates…they are faults that run perpendicular to the ridge.run perpendicular to the ridge.

SeamountsSeamounts

• Submerged volcanic mountains at least Submerged volcanic mountains at least 1000 m high.1000 m high.

• Abyssal hillsAbyssal hills—submerged volcanic —submerged volcanic mountains less than 1000 m.mountains less than 1000 m.

• Both are usually associated with hotspots.Both are usually associated with hotspots.

• Seamounts that rise above the surface Seamounts that rise above the surface become volcanic islands.become volcanic islands.

• GuyotsGuyots (GEE-oze)—sunken volcanic islands (GEE-oze)—sunken volcanic islands that have had their tops eroded flat by that have had their tops eroded flat by wave action.wave action.