14.the Ocean Floor

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PrenticeHallEARTH SCIENCE

Tarbuck Lutgens


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Chapter

14The Ocean Floor


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The Blue Planet

14.1 The Vast World Ocean

Nearly 71 percent of Earths surface iscovered by the global ocean.

Oceanography is a science that draws onthe methods and knowledge of geology,chemistry, physics, and biology to study all

aspects of the world ocean.


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The World Ocean


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Geography of the Oceans


The world ocean can be divided into fourmain ocean basinsthe Pacific Ocean, the

Atlantic Ocean, the Indian Ocean, and theArctic Ocean.

The Arctic Ocean is about 7 percent of the size

of the Pacific.

The Pacific Ocean is the largest and has thegreatest depth.

The Atlantic Ocean is about half the size of thePacific and not quite as deep.

The Indian Ocean, largely a southern hemispherebody, is slightly smaller than the Atlantic.


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Mapping the Ocean Floor


The topography of the ocean floor is asdiverse as that of the continents.

Bathymetry is the measurement of oceandepths and the charting of the shape ortopography of the ocean floor.

Todays technologyparticularly sonar,satellites, and submersiblesallowsscientists to study the ocean floor in amore efficient and precise manner than

ever before.


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The Topography of the Ocean


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Sonar Sonar is an acronym for sound navigation and

ranging. It is also referred to as echo sounding. Sonar works by transmitting sound waves

toward the ocean bottom.


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Sonar Methods


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Satellites Satellites are able to measure small differences

by bouncing microwaves off the ocean surface. Using this new technology, scientists have

discovered that the ocean surface is notperfectly flat.

Differences in the height of the ocean surfaceare caused by ocean-floor features.


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Satellite Methods


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Submersibles Submersibles are small underwater crafts used

for deep-sea research. Today, many submersibles are unmanned and

operated remotely by computers. Theseremotely operated vehicles (ROVs) can remainunderwater for long periods.


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14.2 Ocean Floor Features

The ocean floor regions are the continentalmargins, the ocean basin floor, and the

mid-ocean ridge.


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Continental Margins


In the Atlantic Ocean, thick layers ofundisturbed sediment cover the continental

margin. This region has very little volcanicor earthquake activity.

A continental margin is the zone oftransition between a continent and the

adjacent ocean basin floor.


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Atlantic Continental Margin


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Continental MarginsIn the Pacific Ocean, oceanic crust plungesbeneath continental crust. This force results

in a narrow continental margin thatexperiences both volcanic activity andearthquakes.



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Continental Margins


Continental Shelf A continental shelf is the gently sloping

submerged surface extending from the shoreline. Continental shelves contain important mineral

deposits, large reservoirs of oil and natural gas,and huge sand and gravel deposits.


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Continental Margins


Continental Slope A continental slope is the steep gradient that

leads to the deep-ocean floor and marks the

seaward edge of the continental shelf. A submarine canyon is the seaward extension of

a valley that was cut on the continental shelfduring a time when sea level was lowera canyoncarved into the outer continental shelf, slope, andrise by turbidity currents.

A turbidity current is the downslope movement ofdense, sediment-laden water created when sandand mud on the continental shelf and slope are

dislodged and thrown into suspension.


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Submarine Canyons


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Continental Margins


Continental Rise

A continental rise is the gently sloping surface

at the base of the continental slope.


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Ocean Basin Floor


The ocean basin floor is the area of thedeep-ocean floor between the continental

margin and the oceanic ridge.

Trenches form at the sites of plate convergencewhere one moving plate descends beneath

another and plunges back into the mantle.

Deep-Ocean Trenches


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Ocean Basin Floor


Abyssal Plains An abyssal plain is a very level area of the

deep-ocean floor, usually lying at the foot of thecontinental rise.

The sediments that make up abyssal plains arecarried there by turbidity currents or aredeposited as suspended sediment settles out.

Seamounts and Guyots A seamount is an isolated volcanic peak that

rises at least 1000 meters above the deep-oceanfloor, and a guyot is an eroded, submerged

seamount.


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Abyssal Plain Cross Section


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Mid-Ocean Ridges


Hydrothermal Vents Hydrothermal vents form along mid-ocean

ridges. These are zones where mineral-richwater, heated by the hot, newly-formed oceaniccrust, escapes through cracks in the oceaniccrust into surrounding water.


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Types of Seafloor Sediments

14.3 Seafloor Sediments

Ocean-floor sediments can be classifiedaccording to their origin into three broad

categories: terrigenous sediment,biogenous sediment, and hydrogenoussediment.

Terrigenous Sediment Terrigenoussediments consist primarily of

mineral grains that were eroded from continentalrocks and transported to the ocean.


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Biogenous Sediment Biogenous sediments consist of shells and

skeletons of marine animals and algae.- Calcareous ooze is thick, common biogenous

sediment produced by dissolving calciumcarbonate shells.

- Siliceous ooze is biogenous sedimentcomposed of silica-based shells of single-celledanimals and algae.


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Hydrogenous Sediment Hydrogenous sediment consists of minerals

that crystallize directly from ocean water throughvarious chemical reactions.


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Biogenous Sediments

Radiolaria

Foraminifera


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Energy Resources

14.4 Resources from the Seafloor

Oil and natural gas are the main energyproducts currently being obtained from the

ocean floor.

Gas hydrates are compact chemical structuresmade of water and natural gas.

Gas Hydrates

Most oceanic gas hydrates are created whenbacteria break down organic matter in ocean-floor sediments.


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Gas Hydrates


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Other Resources


Other major resources from the ocean floorinclude sand and gravel, evaporative salts,

and manganese nodules.Sand and Gravel The offshore sand-and-gravel industry is second

in economic value only to the petroleum industry.


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Other Resources


Manganese Nodules

Evaporative Salts When seawater evaporates, the salt increases in

concentration until it can no longer remaindissolved. When the concentration becomeshigh enough, the salts precipitate out of solutionand form salt deposits.

Manganese nodules are hard lumps ofmanganese and other metals (like cobalt, copper,and iron) that precipitate around a small object.

The most economically important salt is halite

common table salt.


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Manganese Nodules

14.the Ocean Floor

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