14.1 the vast world ocean - mr. baker's earth science...

394 Chapter 14

14.1 The Vast World Ocean

Reading StrategyBuilding Vocabulary Draw a table similarto the one below that includes all thevocabulary terms listed for the section. As youread the section, define each term in yourown words.

Key ConceptsHow much of Earth’ssurface is covered bywater?

How can the world oceanbe divided?

How does the topographyof the ocean floorcompare to that on land?

What types of technologyare used to study theocean floor?

Vocabulary◆ oceanography◆ bathymetry◆ sonar◆ submersible

How deep is the ocean? How much of Earth is covered by theglobal ocean? What does the ocean floor look like? Humans havelong been interested in finding answers to these questions.However, it was not until relatively recently that these simplequestions could be answered. Suppose, for example, that all ofthe water were drained from the ocean. What would we see?Plains? Mountains? Canyons? Plateaus? You may be surprised tofind that the ocean conceals all of these features, and more.

The Blue PlanetLook at Figure 1. You can see why the “blue planet” or the “waterplanet” are appropriate nicknames for Earth. Nearly 71 per-cent of Earth’s surface is covered by the global ocean. Althoughthe ocean makes up a much greater percentage of Earth’s surfacethan the continents, it has only been since the late 1800s that theocean became an important focus of study. New technologieshave allowed scientists to collect large amounts of data about theoceans. As technology has advanced, the field of oceanographyhas grown. Oceanography is a science that draws on the methodsand knowledge of geology, chemistry, physics, and biology tostudy all aspects of the world ocean.

Vocabulary Term Definition

oceanography a.

bathymetry b.

sonar c.

submersible d. ?

?

?

?

SouthernHemisphere

NorthPole

NorthernHemisphere

SouthPole

Figure 1 The World Ocean These viewsof Earth show the planet is dominated bya single interconnected world ocean.

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FOCUS

Section Objectives14.1 Recognize that most of Earth’s

surface is covered by water.14.2 List Earth’s four main ocean

basins and identify theirlocations.

14.3 Describe the topography ofthe ocean floor and compareit to land.

14.4 Identify and describe threemajor technologies used tostudy the ocean floor.

Build VocabularyWord Parts Before students read thissection, ask them to write the meaningsof the prefix bathy- and the suffix -metry.Then have them write what they thinkthe word bathymetry means. After stu-dents read the section, have them discusswhether their prediction was correct.

Reading Strategya. science that studies all aspects of theworld’s oceanb. measurement of ocean depths andcharting the shape of the ocean floorc. echo sounding to measure ocean depthd. small underwater craft used fordeep-sea research

INSTRUCT

The Blue Planet

One OceanPurpose Students see how Earth’socean basins are connected.

Materials world globe

Procedure Have students point outthe regions on the globe where oceansconnect.

Expected Outcome Students willsee that the Atlantic, Pacific, and IndianOceans connect in the regionsurrounding Antarctica. The Atlanticand Pacific Oceans connectwith the Arctic Ocean.Visual, Logical

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Section 14.1

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Geography of the OceansThe area of Earth is about 510 million square kilometers. Of this total,approximately 360 million square kilometers, or 71 percent, is repre-sented by oceans and smaller seas such as the Mediterranean Sea andthe Caribbean Sea. Continents and islands comprise the remaining 29 percent, or 150 million square kilometers. The world ocean canbe divided into four main ocean basins—the Pacific Ocean, theAtlantic Ocean, the Indian Ocean, and the Arctic Ocean. These oceanbasins are shown in Figure 2.

The Pacific Ocean is the largest ocean. In fact, it is the largest singlegeographic feature on Earth. It covers more than half of the ocean sur-face area on Earth. It is also the world’s deepest ocean, with an averagedepth of 3940 meters.

The Atlantic Ocean is about half the size of the Pacific Ocean, andis not quite as deep. It is a relatively narrow ocean compared to thePacific. The Atlantic and Pacific Oceans are bounded to the east andwest by continents.

The Indian Ocean is slightly smaller than the Atlantic Ocean, butit has about the same average depth. Unlike the Pacific and Atlanticoceans, the Indian Ocean is located almost entirely in the southernhemisphere.

The Arctic Ocean is about 7 percent of the size of the Pacific Ocean.It is only a little more than one-quarter as deep as the rest of theoceans.

What are the four main ocean basins?

IndianOcean

PacificOcean

AtlanticOcean

60˚

30˚

0˚

30˚

60˚

90˚ 0˚30˚60˚120˚150˚ 30˚ 60˚ 90˚ 120˚ 150˚

Arctic Ocean

Distribution of Land and Water

Figure 2

Human-EnvironmentInteraction The fourmain ocean basins are thePacific Ocean, the AtlanticOcean, the Indian Ocean,and the Arctic Ocean. Predicting What is the longitude of theeasternmost point of thePacific Ocean? What is the longitude of thewesternmost point of theAtlantic Ocean?

AnswerPredicting easternmost pointof Pacific: approximately 70°W;westernmost point of Atlantic:approximately 100°W

Geographyof the OceansIntegrate MathGeometry and Projections Explainto students that the map pictured onthis page is a Mercator projection.Because Earth is a sphere, two-dimensional representations of Earthinvariably have some distortion. Onprojections such as this, the degreeof distortion increases with distancefrom the equator. Ask: Where on thisprojection is distortion greatest? (thenorthern and southern extremes, or thepoles) What landmasses do you thinkare the most distorted? (Greenlandand Antarctica) Explain in geometricterms why these landmasses are sodistorted. (Since Earth is a sphere, linesof longitude come closer together asdistance from the equator increases.These lines meet at the poles. To representa spherical body as a two-dimensionalgrid, the lines run parallel rather thanmeeting at the poles, and the featuresbetween them become stretched.)Visual, Logical

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Customize for Inclusion Students

Visually Impaired Provide students with arelief world map or globe. Allow students toexperience the interconnected nature of theworld’s oceans by helping them trace theoutlines of the oceans with their fingers.Students can also compare the sizes of Earth’s

four main ocean basins. This learning tool canbe used by both visually impaired students andtactile learners. (A relief map of the ocean floorcan be used in a similar way to allow studentsto experience deep ocean trenches, ridges,seamounts, and guyots.)

Answer to . . .

Pacific Ocean, AtlanticOcean, Indian Ocean,

Arctic Ocean


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PhilippineTrench

AleutianTrench

KurileTrench

Japan Trench

Mariana Trench

Java (Sunda)Trench

Kermadec Trench

MiddleAmericaTrench

Tonga Trench

East

Pac

ific

Ris

e

Juan deFucaRidge

Hawaiian Is.

Em

peror

Seam

ounts

RyukyuTrench

NorthAmerica

Arctic Ocean

Pacific Ocean

Australia

BeringAbyssal Plain

Eltanin Fracture Zone

BellingshausenAbyssal Plain

Mapping the Ocean FloorIf all the water were drained from the ocean basins, a variety of featureswould be seen. These features include chains of volcanoes, tall mountain ranges, trenches, and large submarine plateaus. Thetopography of the ocean floor is as diverse as that of continents. Thetopographic features of the ocean floor are shown in Figure 3.

An understanding of ocean-floor features came with the develop-ment of techniques to measure the depth of the oceans. Bathymetry

Figure 3 The topographyof the ocean floor is asvaried as the topography ofthe continents. The oceanfloor contains mountainranges, trenches, and flatregions called abyssalplains. Interpreting DiagramsList all of the features youcan identify in the figure.

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Mapping theOcean FloorUse VisualsFigure 3 Have students examine themap showing the topographic featuresof the ocean floor. Ask: In which oceanbasin are most of the oceanictrenches located? (Pacific) Whichocean basins contain oceanic ridges?(Atlantic, Indian) What is the majorundersea geological feature in theAtlantic Ocean? (Mid-Atlantic Ridge)What kind of geological featureis the Hawaiian islands a part of?(a linear chain of undersea volcanoes)

Build Science SkillsPosing Questions Have studentswrite one or more questions they haveabout the characteristics of the oceanfloor. Ask them to formulate eachquestion so that it could be used as thebasis for scientific research. (Samplequestions: Does the chemicalcomposition of seawater vary fromplace to place around the world?What methods can be used toaccurately measure the speed ofocean currents?) After students havewritten their questions, ask them todescribe how they might go aboutanswering their questions. (Sampleanswers: Collect and analyze seawatersamples from a variety of locations.Design an experiment or field study to testdifferent devices used to measure waterspeed.) If necessary, assist students inphrasing their questions so that they canserve as the basis for a scientific inquiry.Verbal, Logical

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Section 14.1 (continued)

On average, the depth of the oceans is morethan four times the elevation of thecontinents. The average elevation of thecontinents is about 840 m above sea level. The

average depth of the oceans is 3729 m. IfEarth’s solid mass were perfectly smooth andspherical, ocean water would cover it all to adepth of more than 2000 m.

Facts and Figures

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Puerto-RicoTrench

Peru-Chiletrench

South SandwichTrench

Mid-Atlantic Ridge

Mid-IndianR

idg

e

Southwest Indian Ridge

Southeast Indian Ridge

Greenland

Asia

Africa

SouthAmerica

Arctic

Mid-Ocean Ridge

GibbsFracture

Zone

Demerara

Abyssal Plain

St. PaulFracture

Zone

AtlanticOcean

Weddell Abyssal Plain

Red SeaRift

IndianOcean

Key: transform fault

(bathos � depth, metry � measurement) is the measurement of oceandepths and the charting of the shape or topography of the ocean floor.

The first understanding of the ocean floor’s varied topography didnot unfold until the historic three-and-a-half-year voyage of the HMSChallenger. From December 1872 to May 1876, the Challenger expedi-tion made the first—and perhaps still the most comprehensive—study of the global ocean ever attempted by one agency. The 127,500kilometer trip took the ship and its crew of scientists to every ocean

For: Links on oceans

Visit: www.SciLinks.org

Web Code: cjn-5141

Integrate Social StudiesChallenger Expedition Explain tostudents that the journey of the HMSChallenger is considered by many to bethe birth of the science of oceanography.Before the Challenger Expedition,enough information about Earth’s oceanshad been collected to make scientistsand sailors alike realize that an extensiveocean survey would be of great benefit.The success of the expedition inspiredthe launching of many subsequentocean research surveys. Point out tostudents that the expedition took placeduring a time when most ships usedwind and sail for propulsion, andsubmarine cables carried mosttransoceanic communications. Ask: Inwhat ways would data on oceancurrents, prevailing winds, andweather patterns collected byChallenger scientists have been usefulto others? (added to knowledge aboutthe oceans; provided help to sailors andship captains trying to chart the bestcourse for a journey) In what wayswould depth measurements andother data about the nature of theocean bottom have been helpful toothers? (assist companies in makingdecisions about where and how to laysubmarine cables)Verbal

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The Challenger Expedition took data from362 locations scattered throughout theAtlantic, Pacific, and Indian Oceans. Scientiststraveling with the expedition took charge ofdifferent research aspects. Matthew Maury(1806–1873), an American naval officer, wasin charge of charts and instruments. EdwardForbes (1815–1854) was a British biologistwho had already done extensive research inshallower waters around Britain and in the

Aegean Sea. Forbes led the expedition’scollection and analysis of biological specimens.Before the expedition, Forbes had predictedthat life would not be found below about2000 m in the deep sea. Challenger Expe-dition findings proved that life existed at leastas deep as 6000 m; it is now known that lifeexists even at the bottom of the deepest oceantrenches.

Facts and Figures

Answer to . . .

Figure 3 mid-ocean ridges, trenches,abyssal plains, seamounts

Download a worksheet on oceansfor students to complete, and findadditional teacher support fromNSTA SciLinks.


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except the Arctic. Throughout the voyage, they sampled various oceanproperties. They measured water depth by lowering a long, weightedline overboard. Today’s technology—particularly sonar, satel-lites, and submersibles—allows scientists to study the ocean floor ina more efficient and precise manner than ever before.

Sonar In the 1920s, a technological breakthrough occurred with theinvention of sonar, a type of electronic depth-sounding equipment.Sonar is an acronym for sound navigation and ranging. It is alsoreferred to as echo sounding. Sonar works by transmitting sound wavestoward the ocean bottom, as shown in Figure 4A. With simple sonar,a sensitive receiver intercepts the echo reflected from the bottom. Thena clock precisely measures the time interval to fractions of a second.Depth can be calculated from the speed of sound waves in water—about 1500 meters per second—and the time required for the energypulse to reach the ocean floor and return. The depths determined fromcontinuous monitoring of these echoes are plotted. In this way a pro-file of the ocean floor is obtained. A chart of the seafloor can beproduced by combining these profiles.

In the last few decades, researchers have designed even moresophisticated sonar to map the ocean floor. In contrast to simple sonar,multibeam sonar uses more than one sound source and listeningdevice. As you can see from Figure 4B, this technique obtains a profileof a narrow strip of ocean floor rather than obtaining the depth of asingle point every few seconds. These profiles are recorded every fewseconds as the research vessel advances. When a ship uses multibeamsonar to make a map of a section of ocean floor, the ship travelsthrough the area in a regularly spaced back-and-forth pattern. Not sur-prisingly, this method is known as “mowing the lawn.”

Figure 4 Sonar MethodsA By using sonar, oceanographerscan determine the depth of theocean floor in a particular area. B Modern multibeam sonarobtains a profile of a narrowswath of seafloor every fewseconds.

Outgoing signal

Reflected signal

Sea floor Sea floor

A B

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Build Math SkillsLine Graphs Have students graph sonarresults for a hypothetical transect of theocean bottom. Provide students withthe following time intervals for foursonar data points, each taken 10 kmapart. Sample data: 3.2 s, 5.5 s, 7.2 s,6.4 s. First, have students calculate thedepth for each data point (time/2 �1500 m/s). (3.2 s: 2400 m; 5.5 s: 4125m; 7.2 s: 5400 m; 6.4 s: 4800 m)Second, invite students to graph theirresults, placing distance (km) betweendata points on the x-axis and depth (m)on the y-axis. Have students connecttheir data points to create a line on thegraph. Ask students what the linerepresents. (a rough profile of part of theocean floor)Logical, Visual

Build Science SkillsInferring Remind students of thedifference between sound waves andmicrowaves. Sound waves are producedby vibrating matter. Microwaves are aform of electromagnetic energy. Ask:Which type of wave has more energy?(microwaves) Why can’t sound wavesbe used to gather ocean height datafrom satellites? (Sound waves must havea medium to travel through. Satellitesorbit high in Earth’s atmosphere, wherethere are too few molecules to transmitsound.)Logical, Verbal

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Satellites Measuring the shape of the ocean surface from space isanother technological breakthrough that has led to a better under-standing of the ocean floor. After compensating for waves, tides,currents, and atmospheric effects, scientists discovered that the oceansurface is not perfectly flat. This is because gravity attracts water towardregions where massive ocean floor features occur. Therefore, moun-tains and ridges produce elevated areas on the ocean surface. Featuressuch as canyons and trenches cause slight depressions.

The differences in ocean-surface height caused by ocean floor fea-tures are not visible to the human eye. However, satellites are able tomeasure these small differences by bouncing microwaves off the oceansurface. Figure 5 shows how the outgoing radar pulses are reflected backto a satellite. The height of the ocean surface can be calculated by know-ing the satellite’s exact position. Devices on satellites can measurevariations in sea-surface height as small as 3 to 6 centimeters. This typeof data has added greatly to the knowledge of ocean-floor topography.Cross-checked with traditional sonar depth measurements, the data areused to produce detailed ocean-floor maps, such as the one previouslyshown in Figure 3.

How do satellites help us learn about the shape ofthe seafloor?

Figure 5 Satellite MethodSatellites can be used to measuresea-surface height. The datacollected by satellites can be usedto predict the location of largefeatures on the seafloor. Thismethod of data collection is muchfaster than using sonar.

Satellite orbitSatellite

Outgoingradar pulses

Return pulsesfrom seasurface

Ocean bottom

Elevation in seasurface height

Build Reading LiteracyRefer to p. 216D in Chapter 8, whichprovides guidelines for this compareand contrast strategy.

Compare and Contrast Afterstudents have read the sections onbathymetric methods, have themcreate a table that compares simplesonar, multibeam sonar, and satellitebathymetry technologies in terms ofdata collection method used and thetype of data obtained. Ask students tosummarize advantages or disadvantagesof each technology with respect to theothers.

Simple Multibeamsonar sonar Satellite

Method Sound Sound Microwaveswaves waves

Type of Ocean Ocean Oceandata floor floor surface height,

depth depth correlated toocean depth

Advan- Simple More Most detailedtages to use detailed of all

Disadvan- Time- Time- Must betages consuming consuming cross-checked

with sonarmeasurements

Verbal, Visual

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Answer to . . .

Satellites bouncemicrowaves off the

ocean surface. Outgoing radar pulsesare reflected back to the satellite andcan be used to detect differences in seasurface height that can be correlatedto seafloor features.


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Section 14.1 Assessment

Reviewing Concepts1. How does the area of Earth’s surface

covered by the oceans compare with the areacovered by land?

2. Name the four ocean basins. Which of thefour ocean basins is the largest? Which islocated almost entirely in the southernhemisphere?

3. How does the topography of the oceanfloor compare to that on land? Name threetopographic features found on the oceanfloor.

4. What types of technology are used tostudy the ocean floor?

5. Describe how sonar is used to determineseafloor depth.

Critical Thinking6. Comparing and Contrasting Compare

and contrast the use of satellites andsubmersibles to collect data about thetopography of the seafloor.

7. Inferring Why is deep-sea exploration anddata collection difficult?

8. Assuming the average speed of soundwaves in water is 1500 meters persecond, determine the water depthin meters if a sonar signal requires4.5 seconds to hit the bottom andreturn to the recorder.

Submersibles A submersible is a small underwater craft used fordeep-sea research. Submersibles are used to collect data about areas ofthe ocean that were previously unreachable by humans. Submersiblesare equipped with a number of instruments ranging from thermometersto cameras to pressure gauges. The operators of submersibles can recordvideo and photos of previously unknown creatures that live in the abyss.They can collect water samples and sediment samples for analysis.

The first submersible was used in 1934 by William Beebe. Hedescended to a depth of 923 meters off of Bermuda in a steel spherethat was tethered to a ship. Since that time, submersibles have becomemore sophisticated. In 1960, Jacques Piccard descended in the unteth-ered submersible Trieste to 10,912 meters below the ocean surface intothe Mariana Trench. Alvin and Sea Cliff II are two other manned submersibles used for deep-sea research. Alvin can reach depths of4000 meters, and Sea Cliff II can reach 6000 meters.

Today, many submersibles are unmanned and operated remotelyby computers. These remotely operated vehicles (ROVs) can remainunder water for long periods. They collect data, record video, usesonar, and collect sample organisms with remotely operated arms.Another type of submersible, the autonomous underwater vehicle(AUV), is under development. Its goal is to collect long-term datawithout interruption.

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Build Science SkillsCommunicating Results Havestudents work in small groups toresearch information about submersiblesand the scientists who use them. Haveeach group investigate a different sub-mersible. Possibilities include WilliamBeebe’s sphere, Trieste, Alvin, Sea Cliff II,and Jason. Have the groups orallypresent their findings to the class.

ASSESSEvaluateUnderstandingTo assess students’ knowledge of sectioncontent, have them write a short para-graph comparing the ocean floor toEarth’s landmasses. Have them writeanother paragraph describing howsonar, satellites, and submersibles canbe used to gather data about the deepocean.

ReteachHave students create a timeline thatdescribes how bathymetric techniqueshave changed over the years since theChallenger expedition. Invite students toexplain their timelines and each of themethods shown on their timelines tothe class.

Solution8. 4.5 s/2 � 1500 m/s � 3375 m

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4. sonar, satellites, submersibles5. Sonar works by transmitting sound wavesto the ocean bottom. A receiver interceptsthe echo reflected from the ocean bottomand a clock measures the time it takes for thesound wave to travel to the ocean bottomand back.6. Both are used to find out more about theseafloor’s topography. Satellites use remotesensing to bounce microwaves off the sea


1. Nearly 71 percent of Earth’s surface iscovered by oceans, 29 percent is coveredby land.2. Pacific Ocean, Atlantic Ocean, IndianOcean, Arctic Ocean; Pacific Ocean; IndianOcean3. The topography of the ocean floor isas diverse as that of continents. Three topo-graphic features: mid-ocean ridges, trenches,abyssal plains.

surface to determine differences in height.Submersibles can be manned or unmanned,travel to deep areas, and record data withvideo and other instruments.7. The deep ocean is a harsh environment forhumans—cold, dark, and under high pressure.It is difficult to supply submersibles withpower for continuous use.


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14.2 Ocean Floor Features

Reading StrategyOutlining Before you read, make anoutline of this section. Use the greenheadings as the main topics and the blueheadings as subtopics. As you read, addsupporting details.

Key ConceptsWhat are the three mainregions of the oceanfloor?

How do continentalmargins in the AtlanticOcean differ from those inthe Pacific Ocean?

How are deep-oceantrenches formed?

How are abyssal plainsformed?

What is formed at mid-ocean ridges?

Vocabulary◆ continental margin◆ continental shelf◆ continental slope◆ submarine canyon◆ turbidity current◆ continental rise◆ ocean basin floor◆ abyssal plains◆ seamounts◆ mid-ocean ridge◆ seafloor spreading

Oceanographers studying the topography of the ocean floor havedivided it into three major regions. The ocean floor regions arethe continental margins, the ocean basin floor, and the mid-oceanridge. The map in Figure 6 outlines these regions for the North AtlanticOcean. The profile at the bottom of the illustration shows the variedtopography. Scientists have discovered that each of these regions has itsown unique characteristics and features.

Figure 6 Topography of theNorth Atlantic Ocean BasinBeneath the map is a profile ofthe area between points A and B.The profile has been exaggerated40 times to make the topographicfeatures more distinct.

I. Continental Margins

A. Continental Shelf

B. Continental Slope

C.

II.

A. ?

?

?

Continentalmargin

Oceanbasin floor

B

Mid-ocean ridgeContinentalmargin

A

Oceanbasin floor

A

B

Mid

-oce

anrid

ge

NorthAmerica

Africa

FOCUS

Section Objectives14.5 List the three main regions of

the ocean floor.14.6 Differentiate between the

continental margins of theAtlantic and Pacific Oceans.

14.7 Explain the formation of newocean floor at deep-oceantrenches, abyssal plains, andmid-ocean ridges.

Build VocabularyConcept Map Have students make aconcept map using the term ocean floorfeatures as the starting point. All thevocabulary terms in this section shouldbe used.

Reading StrategyC. Continental Rise

II. Ocean Basin FloorA. Deep-Ocean TrenchesB. Abyssal PlainsC. Seamounts and Guyots

III. Mid-Ocean RidgesA. Seafloor SpreadingB. Hydrothermal Vents

INSTRUCTUse VisualsFigure 6 Point out to students that theprofile shown below the map is a sideview of the ocean floor along the linebetween points A and B on the map.Ask: Why do the topographic featureshave to be exaggerated to make themmore distinct? (The scale of the map isso large that the elevation differencesbetween ocean floor, continental margin,and mid-ocean ridge would not be visible.If the map were 40 times larger, theexaggeration wouldn’t be necessary, butthe map would be too large to print.)Look back at the map of the oceanfloor shown in Figure 3. How woulda profile of the Pacific Ocean basindiffer from this profile of the AtlanticOcean? (The profile of the Pacific Oceanbasin would not show a central mid-oceanridge. Instead, depending on how thetransect line is drawn, it would showtrenches, chains of volcanic islands,or coral atolls.)Visual, Logical

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Section 14.2


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Continental MarginsThe zone of transition between a continent and the adjacent oceanbasin floor is known as the continental margin. In the AtlanticOcean, thick layers of undisturbed sediment cover the continentalmargin. This region has very little volcanic or earthquake activity.This is because the continental margins in the Atlantic Ocean are notassociated with plate boundaries, unlike the continental margins ofthe Pacific Ocean. In the Pacific Ocean, oceanic crust is plungingbeneath continental crust. This force results in a narrow continen-tal margin that experiences both volcanic activity and earthquakes.Figure 7 shows the features of a continental margin found along theAtlantic coast.

Continental Shelf What if you were to begin an underwaterjourney eastward across the Atlantic Ocean? The first area of ocean flooryou would encounter is the continental shelf. The continental shelf isthe gently sloping submerged surface extending from the shoreline. Theshelf is almost nonexistent along some coastlines. However, the shelfmay extend seaward as far as 1500 kilometers along other coastlines.On average, the continental shelf is about 80 kilometers wide and 130 meters deep at its seaward edge. The average steepness of the shelfis equal to a drop of only about 2 meters per kilometer. The slope is soslight that to the human eye it appears to be a horizontal surface.

Continental shelves have economic and political significance.Continental shelves contain important mineral deposits, large

reservoirs of oil and natural gas, and huge sand and gravel deposits.The waters of the continental shelf also contain important fishinggrounds, which are significant sources of food.Figure 7 Atlantic Continental

Margin The continental marginsin the Atlantic Ocean are widerthan in the Pacific Ocean and are covered in a thick layer ofsediment. Explaining Why are continentalmargins in the Pacific Oceannarrower and associatedwith earthquakes andvolcanic activity?

Continental margin

Continental shelf

Continental crust

Continental slopeAbyssal plain

Oceanic crust

Submarinecanyons

Continental rise

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Build Reading LiteracyRefer to p. 392D, which provides theguidelines for this previewing strategy.

Preview Have students skim headings,titles of visuals, and boldfaced text forSection 14.2 Ocean Floor Features.Invite them to think in broad termsabout what they will read. Ask: What dothe green section heads have incommon? (They are all parts of the oceanfloor.) What do the blue section headshave in common? (Most are terms forsmaller features of the ocean, usuallyassociated with one of the three parts ofthe ocean.)Visual, Logical

Continental MarginsIntegrate BiologySunlight and Ocean Life Explain thatthe ocean bottom along the continentalmargins supports a greater variety ofliving organisms than other regionsof the ocean floor. Tell students thatsunlight penetrates ocean water to anaverage depth of about 300 meters.Ask: What kinds of organisms formthe basis of almost every food chain?(organisms capable of photosynthesis)Why does the continental shelfsupport a greater amount and varietyof life than deeper parts of the oceanfloor? (Sunlight can penetrate to thebottom of at least some parts of thecontinental shelf; algae and other photo-synthetic organisms can live on thebottom and serve as the basis for oceanfood chains along the continental shelf.Deeper regions of the ocean floor do notreceive sunlight and so do not supportphotosynthetic organisms that wouldform the basis of food chains.)Logical, Verbal

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Customize for English Language Learners

Tell students that the words abyss and floor areused as synonyms with reference to the deepocean bottom. Explain that the word abyssmeans a “bottomless depth” and historicallyhas been used to describe the unknown.Before humans discovered technologies fordeep sea exploration, the bottom of the ocean

was considered by many to be the abyssreferred to in many myths—a bottomless pitthat humans could not know or understand.The word floor means “the ground surface.”Its use as a descriptor of the ocean bottom ismore recent, and does not imply a sense of theunknown.


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Continental Slope Marking the seaward edge of the continen-tal shelf is the continental slope. This slope is steeper than the shelf,and it marks the boundary between continental crust and oceaniccrust. The continental slope can be seen in Figure 7 on page 402.Although the steepness of the continental slope varies greatly fromplace to place, it averages about 5 degrees. In some places the slopemay exceed 25 degrees. The continental slope is a relatively narrow fea-ture, averaging only about 20 kilometers in width.

Deep, steep-sided valleys known as submarine canyons are cut intothe continental slope. These canyons may extend to the ocean basinfloor. Figure 8 shows how submarine canyons are formed. Mostinformation suggests that submarine canyons have been eroded,at least in part, by turbidity currents.Turbidity currents are occasionalmovements of dense, sediment-richwater down the continental slope.They are created when sandand mud on the continen-tal shelf and slope aredisturbed—perhaps byan earthquake—andbecome suspended inthe water. Because suchmuddy water is denserthan normal seawater, it flows down theslope. As it flows down, it erodes and accumulates moresediment. Erosion from these muddy torrents is believed to be themajor force in the formation of most submarine canyons. Narrow con-tinental margins, such as the one located along the California coast, aremarked with numerous submarine canyons.

Turbidity currents are known to be an important mechanism ofsediment transport in the ocean. Turbidity currents erode submarinecanyons and deposit sediments on the deep-ocean floor.

Continental Rise In regions where trenches do not exist, thesteep continental slope merges into a more gradual incline known asthe continental rise. Here the steepness of the slope drops to about 6 meters per kilometer. Whereas the width of the continental slopeaverages about 20 kilometers, the continental rise may be hundreds ofkilometers wide.

Compare and contrast the continental slope andcontinental rise.

Figure 8 SubmarineCanyons Most evidence

suggests that submarinecanyons probably formed as rivervalleys during periods of low sealevel during recent ice ages.Turbidity currents continue tochange the canyons.

Turbidity current

Turbiditycurrent

Submarinecanyons

For: Links on ocean floor features


Web Code: cjn-5142

Use VisualsFigure 8 Have students examine thefigure. Explain to students that turbiditycurrents are made up of water thatcontains suspended particles of rock,sand, and mud, which increase thedensity of the water. Ask: How isdensity important to the action ofturbidity currents? (Gravity causes thedenser, heavier water to fall to the bottomand flow down the slope.) What happensto the sediment once it reaches thebottom of the slope? (The sedimentforms fan-shaped deposits.)Visual, Logical

Build Science SkillsInferring Explain to students thatthere are probably many kinds of eventsthat could set a turbidity current flowingdown a continental slope. Scientists donot yet completely understand thecomplex workings of turbidity currents.Ask: What kinds of events, besidesearthquakes, might set off turbiditycurrents? (Accept all reasonable answers.Possible answers: A heavy accumulation ofsediment particles that builds up over timealong an existing steep edge could reachthe point where it becomes too heavy tobe supported by the underlying sedimentsand begins to flow down slope. Humanactivities, such as underwater explosionsor mining activities, could create sediment-laden water that then flows down slope.)Logical, Verbal

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When a turbidity current loses its momentum,it gradually slows down and deposits its loadof sediment in a fan-shaped bed. Beds ofsediment deposited by turbidity currents arecalled turbidites. Heavier sediments settle outfirst and lighter sediments settle out on top of

them, creating a layered feature known asa graded bed. Each turbidity current eventcreates a separate graded bed that decreasesin sediment size from bottom to top. Layersof graded beds build up over time, leavinga record of turbidity current activity.

Facts and FiguresAnswer to . . .

Figure 7 In the Pacific Ocean,oceanic crust is being pushedbeneath continental crust.

Continental slope marksthe steep boundary

between continental crust and oceaniccrust. The continental rise occurs at theend of the continental slope and has amore gradual decline.

Download a worksheet on ocean floorfeatures for students to complete,and find additional teacher supportfrom NSTA SciLinks.


404 Chapter 14

Ocean Basin FloorBetween the continental margin and mid-ocean ridge lies the oceanbasin floor. The size of this region—almost 30 percent of Earth’s sur-face—is comparable to the percentage of land above sea level. Thisregion includes deep-ocean trenches, very flat areas known as abyssalplains, and tall volcanic peaks called seamounts and guyots.

Deep-Ocean Trenches Deep-ocean trenches are long, narrowcreases in the ocean floor that form the deepest parts of the ocean. Mosttrenches are located along the margins of the Pacific Ocean, and manyexceed 10,000 meters in depth. A portion of one trench—the ChallengerDeep in the Mariana Trench—has been measured at a record 11,022 meters below sea level. It is the deepest known place on Earth.

Trenches form at sites of plate convergence where one movingplate descends beneath another and plunges back into the mantle.Earthquakes and volcanic activity are associated with these regions. Thelarge number of trenches and the volcanic activity along the margins ofthe Pacific Ocean give the region its nickname as the Ring of Fire.

Abyssal Plains Abyssal plains are deep, extremely flat features. Infact, these regions are possibly the most level places on Earth. Abyssalplains have thick accumulations of fine sediment that have buried anotherwise rugged ocean floor, as shown in Figure 9. The sedimentsthat make up abyssal plains are carried there by turbidity currentsor deposited as a result of suspended sediments settling. Abyssalplains are found in all oceans of the world. However, the AtlanticOcean has the most extensive abyssal plains because it has few trenchesto catch sediment carried down the continental slope.

Seamounts and Guyots The submerged volcanic peaks thatdot the ocean floor are called seamounts. They are volcanoes that havenot reached the ocean surface. These steep-sided cone-shaped peaksare found on the floors of all the oceans. However, the greatest numberhave been identified in the Pacific. Some seamounts form at volcanichot spots. An example is the Hawaiian-Emperor Seamount chain,shown in Figure 3 on page 396. This chain stretches from the HawaiianIslands to the Aleutian trench.

Once underwater volcanoes reach the surface, they form islands. Overtime, running water and wave action erode these volcanic islands to nearsea level. Over millions of years, the islands gradually sink and may dis-appear below the water surface. This process occurs as the moving plateslowly carries the islands away from the elevated oceanic ridge or hot spotwhere they originated. These once-active, now-submerged, flat-toppedstructures are called guyots.

What are abyssal plains?

Q Have humans ever exploredthe deepest ocean trenches?Could anything live there?

A Humans have indeed visitedthe deepest part of the oceans—where there is crushing highpressure, complete darkness, andnear-freezing water tempera-tures. In January 1960, U.S. NavyLt. Don Walsh and explorerJacques Piccard descended tothe bottom of the ChallengerDeep region of the MarianaTrench in the deep-diving sub-mersible Trieste. It took morethan five hours to reach the bot-tom at 10,912 meters—a recorddepth of human descent thathas not been broken since. Theydid see some organisms that areadapted to life in the deep: asmall flatfish, a shrimp, andsome jellyfish.

2800

3600 fathoms

0 10 miles 20 30 40 50 60

OceanAbyssal plain Volcanic peak

Oceanic crust

Layers ofsediment

Figure 9 Abyssal Plain CrossSection This seismic cross sectionand matching sketch of a portionof the Madeira abyssal plain inthe eastern Atlantic Ocean showshow the irregular oceanic crust isburied by sediments.

404 Chapter 14

Sediment BuildupPurpose Demonstrate to students howlayers of sediment build up over time onthe ocean floor, covering irregular rockto form a flat abyssal plain.

Materials small glass aquarium tank,rocks, pebbles, coarse gravel, coarse andfine aquarium sand, bone meal or otherfine material that settles out but doesnot dissolve quickly in water

Procedure Use rocks, pebbles, andgravel to create a model of an irregularocean floor in the bottom of theaquarium tank. Fill tank 2/3 full of water.As students watch, sprinkle the top ofthe water with coarse sand and let itsettle to the bottom. After it has settled,add a layer of fine sand and let it settle.Repeat alternating layers of sand andother materials.

Expected Outcome As the layersbuild up, the bottom of the tank willslowly be covered with a smooth layerthat models the appearance of anabyssal plain.

Visual, Kinesthetic

Ocean Basin Floor

Many students think that Earth’sfeatures, including the ocean floor, areunchanging or formed only in the past.To help students realize that tectonicactivity and plate motions continueto alter Earth’s surface, have theminvestigate rates of seafloor spreading.Explain to students that new crust iscreated at spreading centers. You maychoose to limit the scope of theirresearch to the Mid-Atlantic Ridge.(According to the United States GeologicalSurvey, the Mid-Atlantic Ridge is spreadingat a rate of about 2.5 cm per year.)Logical

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Customize for Inclusion Students

Visually Impaired Have students usemodeling clay to create a model of the AtlanticOcean floor, from the continental margin ofAfrica to the continental margin of NorthAmerica. Allow students to base their models

on Figure 6. After students have completedtheir models, help them to cut apart themodel along a transect line. Show them thecut side and explain that it is a profile viewof their ocean floor model.



Reviewing Concepts1. What are the three main regions of the

ocean floor?

2. How do continental margins in the AtlanticOcean differ from those in the Pacific Ocean?

3. What are trenches? How are deep-oceantrenches formed?

4. What are abyssal plains? How are abyssalplains formed?

5. What is formed at mid-ocean ridges?

Critical Thinking6. Comparing and Contrasting Compare

and contrast seamounts and guyots.

7. Applying Concepts Explain how turbiditycurrents are related to submarine canyons.

Mid-Ocean RidgesThe mid-ocean ridge is found near the center of most ocean basins. Itis an interconnected system of underwater mountains that have devel-oped on newly formed ocean crust. This system is the longesttopographic feature on Earth’s surface. It exceeds 70,000 kilometers inlength. The mid-ocean ridge winds through all major oceans similar tothe way a seam winds over the surface of a baseball.

The term ridge may be misleading because the mid-ocean ridge isnot narrow. It has widths from 1000 to 4000 kilometers and mayoccupy as much as one half of the total area of the ocean floor. Anotherlook at Figure 3 shows that the mid-ocean ridge is broken into seg-ments. These are offset by large transform faults where plates slide pasteach other horizontally, resulting in shallow earthquakes.

Seafloor Spreading A high amount of volcanic activity takesplace along the crest of the mid-ocean ridge. This activity is associatedwith seafloor spreading. Seafloor spreading occurs at divergent plateboundaries where two lithospheric plates are moving apart. Newocean floor is formed at mid-ocean ridges as magma rises betweenthe diverging plates and cools.

Hydrothermal Vents Hydrothermal vents form along mid-ocean ridges. These are zones where mineral-rich water, heated by thehot, newly-formed oceanic crust, escapes through cracks in oceaniccrust into surrounding water. As the super-heated, mineral-rich watercomes in contact with the surrounding cold water, minerals and metalssuch as sulfur, iron, copper, and zinc precipitate out and are deposited.

The Ocean Floor 405

Descriptive Paragraph Imagine you areabout to take an underwater journey in asubmersible across the Atlantic Ocean.Your journey begins at the coast, and youtravel out toward the mid-ocean ridge.Write a paragraph describing the oceanfloor features you will likely see on yourjourney.

Mid-Ocean RidgesBuild Science SkillsRelating Cause and Effect Remindstudents that seafloor spreading andhydrothermal vents occur where twocrustal plates are moving apart. Ask:Why don’t these features occur inregions where crustal plates aremoving together? (Both seafloorspreading and hydrothermal vents occurwhere magma rises up from below theEarth’s crust. Plates that are moving apartcreate a thin place in the crust betweenthem, through which magma can rise.Crustal plates that are moving togethermay form deep trenches where one plateslides beneath another, and sometimescreates a thin region in the crust throughwhich magma can rise.)Logical, Verbal

ASSESSEvaluateUnderstandingHave students draw and label a profileof the ocean floor that includes examplesof the following: continental margin,continental slope, continental rise, oceanfloor, mid-ocean ridge, abyssal plain,submarine trench, seamount, and guyot.

ReteachHave students work in pairs to producea set of flashcards for each of thesection’s vocabulary terms. Studentscan use the cards to quiz each other.

Student responses may vary but shouldinclude the continental shelf and conti-nental rise, the ocean basin floor withabyssal plains and possibly seamounts,trenches, or a mid-ocean ridge.

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sites where one plate descends beneathanother and plunges back into the mantle.4. Abyssal plains are deep, extremely flat fea-tures of the ocean floor. They are formed assediments from coastal regions are transport-ed far out to sea and settle to the oceanfloor, and as materials from the water col-umn above settle to the bottom.5. new ocean floor6. A seamount is an underwater volcano thathas not reached the surface of the water yet.A guyot is a volcanic island that has been


1. continental margin, ocean basin floor,mid-ocean ridge2. Continental margins in the Atlantic Oceanconsist of thick layers of undisturbed sedi-ment and there is little volcanic or earth-quake activity. In Pacific Ocean margins,oceanic crust is being pushed beneath conti-nental crust, leaving narrow margins withvolcanic and earthquake activity.3. Trenches are long, narrow creases in theseafloor. They are formed at convergence

Answer to . . .

deep, extremely flatregions of the ocean floor

eroded and sunk back under the water’ssurface.7. Turbidity currents consist of dense, mud-choked water that flows down the continen-tal slope. As the currents flow, they erodeand accumulate more sediment, creatingsubmarine canyons.


406 Chapter 14

Explaining Coral Atolls—Darwin’s Hypothesis

Coral atolls are ring-shaped structures that oftenextend several thousand meters below sea level.Corals are colonial animals about the size of anant. They are related to jellyfish and feed with sting-ing tentacles. Most corals protect themselves by

precipitating a hard external skeleton made of cal-cium carbonate. Coral reefs occur where coralsreproduce and grow over many centuries. Theirskeletons fuse into large structures called coral reefs.

The Problem with CoralsCorals require specific environmental conditions togrow. For example, reef-building corals grow bestin waters with an average annual temperature ofabout 24ºC. They cannot survive prolonged expo-sure to temperatures below 18ºC or above 30ºC.Reef-building corals also need clear sunlit water. Asa result, the limiting depth of most active reefgrowth is only about 45 meters.

Gathering DataHow can corals—which require warm, shallow, sunlitwater no deeper than a few dozen meters to live—create thick structures like coral atolls that extendinto deep water? The naturalist Charles Darwin wasone of the first to formulate a hypothesis on theorigin of atolls. From 1831 to 1836, he sailed aboardthe British ship HMS Beagle during its famous voyagearound the world. In various places Darwin noticeda series of stages in coral-reef development.Development begins with a fringing reef, like the oneshown in Figure 10A. The fringing reef forms alongthe sides of a volcanic island. As the volcanic islandslowly sinks, the fringing reef becomes a barrier reef,

as shown in Figure 10B. Figure 10C shows the finalstage of development of the atoll. The volcano sinkscompletely underwater but the coral reef remainsnear the surface.

Darwin’s HypothesisFigure 10 is a drawing that summarizes Darwin’shypothesis about atoll formation. As a volcanicisland slowly sinks, the corals continue to build thereef upward. This explained how living coral reefs,which are restricted to shallow water, can buildstructures that now exist in much deeper water. Thetheory of plate tectonics supports Darwin’s hypoth-esis.Plate tectonics explains how a volcanic islandcan become extinct and experience a change in ele-vation over long periods of time. As the hot oceanseafloor moves away from the mid-ocean ridge, itbecomes denser and sinks. This is why islands alsosink. Darwin’s hypothesis is also supported by evi-dence from drilling that shows volcanic rock isbeneath the oldest and deepest coral reef structures.Atolls owe their existence to the gradual sinking ofvolcanic islands containing coral reefs that buildupward through time.

Fringingcoral reef Volcanic

island

Oceanic crust

Barrierreef Atoll

Lagoon

Figure 10 Formation of a Coral Atoll A A fringing coral reef formsaround a volcanic island. B As the volcanic island sinks, the fringing reefslowly becomes a barrier reef. C Eventually, the volcano is completelyunderwater and a coral atoll remains.

A B C

406 Chapter 14

Explaining CoralAtolls—Darwin’sHypothesisHave students use an atlas to identifythe locations of coral atolls worldwide.Ask: At what latitudes are the majorityof Earth’s coral reefs found? (tropicallatitudes) What does Darwin’shypothesis of atoll formation implyabout the relationship between therate of growth of coral reefs and therate of subsidence of volcanoes? (Inregions where atolls form, corals are ableto build reefs at the same rate that thevolcano beneath them subsides.)According to the theory of platetectonics, what would cause a volcanoto sink below the ocean surface? (Overtime, hot ocean seafloor moves away frommid-ocean ridges, cooling and sinking as itbecomes more dense. Volcanic islands thusare gradually lowered below the watersurface.)Visual, Verbal

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14.3 Seafloor Sediments

Reading StrategySummarizing Make a table like the onebelow that includes all the headings for thesection. Write a brief summary of the text foreach heading.

Key ConceptsWhat are the three typesof ocean-floor sediments?

What does terrigenoussediment consist of?

What is the compositionof biogenous sediment?

How is hydrogenoussediment formed?

Vocabulary◆ terrigenous

sediment◆ biogenous

sediment◆ calcareous ooze◆ siliceous ooze◆ hydrogenous

sediment

Except for steep areas of the continental slope and the crest of themid-ocean ridge, most of the ocean floor is covered with sediment.Some of this sediment has been deposited by turbidity currents. Therest has slowly settled onto the seafloor from above. The thickness ofocean-floor sediments varies. Some trenches act as traps for sedimentoriginating on the continental margin. The accumulation mayapproach 10 kilometers in thickness. In general, however, accumula-tions of sediment are much less—about 500 to 1000 meters.

Generally, coarser sediments, such as sand, cover the continentalshelf and slope while finer sediments, such as clay, cover the deep-oceanfloor. Figure 11 shows the distribution of the different types of ocean-floor sediments. Varioustypes of sediment accumu-late on nearly all areas ofthe ocean floor in the sameway dust accumulates in allparts of your home. Eventhe deep-ocean floor, farfrom land, receives smallamounts of windblownmaterial and microscopicparts of organisms.

Actions at Boundaries

I. Types of Seafloor Sediments• Terrigenous sediments originated

on land.

• Biogenous sediments are biologicalin origin.

• ?

Terrigenous Coarse nearshoredeposits Fine clay (mud) Biogenous Calcareous ooze Siliceous ooze

Figure 11 Distribution ofOcean-Floor Sediments Coarse-grained terrigenous depositsdominate continental marginareas. Fine-grained clay, or mud,is more common in the deepestareas of the ocean basins. Infer Why is it more common tofind fine-grained sediments inthe deepest areas of the oceanbasins?

FOCUS

Section Objectives14.8 List the three types

of ocean floor sediments.14.9 Describe the formation of

terrigenous, biogenous, andhydrogenous sediments.

Build VocabularyWord Parts Have students break thewords terrigenous, biogenous, andhydrogenous into parts, using adictionary to find the meaning of eachpart. (terri- is from terra which means“earth” or “ground”; bio- means “life”;hydro- means “water”; -genous means“producing”, “yielding”, “origin.”)

Reading StrategyActions at BoundariesI. Types of Seafloor Sediments

• Terrigenous sediments originated on land.They come from minerals of continentalrocks. These sediments are composed of sandand gravel.

• Biogenous sediments are biological in origin.They come from shells and skeletons ofmarine animals and algae. These sedimentsare composed of calcareous ooze, siliceousooze, and phosphate-rich material.

• Hydrogenous sediments originated in oceanwater. They are crystallized through chemicalreactions. These sediments are composed ofmanganese nodules, calcium carbonates, andevaporites.

INSTRUCTBuild Science SkillsPredicting After students have readthe introduction, ask: Why do sedimentdeposits near the continental marginstend to be thicker than those on floorof the open ocean, far from land?(Water from rivers and runoff from coastalland transports land sediments tomargins. This sediment source does notexist in the open ocean.)Logical, Verbal

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Section 14.3

Answer to . . .

Figure 11 They are less dense andtransported further than coarsersediments that settle closer to shore.

Most seafloor sediments contain the remains ofmicroscopic organisms that once lived near theocean surface. When these organisms die, theirhard parts can settle onto the ocean floor,where they may become buried and preservedover time. The deep ocean floor has become a

repository for sediments representing millionsof years of Earth’s history. They are usefulrecorders of climate change because thenumbers and types of organisms living nearthe surface change with the climate.

Facts and Figures


408 Chapter 14

Types of Seafloor SedimentsOcean-floor sediments can be classified according to their origin

into three broad categories: terrigenous sediments, biogenoussediments, and hydrogenous sediments. Ocean-floor sediments areusually mixtures of the various sediment types.

Terrigenous Sediment Terrigenous sediment is sediment thatoriginates on land. Terrigenous sediments consist primarily ofmineral grains that were eroded from continental rocks and trans-ported to the ocean. Larger particles such as gravel and sand usuallysettle rapidly near shore. Finer particles such as clay can take years tosettle to the ocean floor and may be carried thousands of kilometers byocean currents. Clay accumulates very slowly on the deep-ocean floor.To form a 1-centimeter abyssal clay layer, for example, requires asmuch as 50,000 years. In contrast, on the continental margins near themouths of large rivers, terrigenous sediment accumulates rapidly andforms thick deposits. In the Gulf of Mexico, for instance, the sedimentis many kilometers thick.

Biogenous Sediment Biogenous sediment is sediment that isbiological in origin. Biogenous sediments consist of shells andskeletons of marine animals and algae. This debris is producedmostly by microscopic organisms living in surface waters. Once theseorganisms die, their hard shells sink, accumulating on the seafloor.

The most common biogenous sediment is calcareous ooze.Calcareous ooze is produced from the calcium carbonate shells oforganisms. Calcareous ooze has the consistency of thick mud. Whencalcium carbonate shells slowly sink into deeper parts of the ocean,they begin to dissolve. In ocean water deeper than about 4500 meters,these shells completely dissolve before they reach the bottom. As aresult, calcareous ooze does not accumulate in the deeper areas ofocean basins.

Other biogenous sediments include siliceous ooze and phosphate-rich material. Siliceous ooze is composed primarily of the shells ofdiatoms—single-celled algae—and radiolarians—single-celled animalsthat have shells made out of silica. The shells of these organisms areshown in Figure 12. Phosphate-rich biogenous sediments come fromthe bones, teeth, and scales of fish and other marine organisms.

Name two types of biogenous sediments.

Q Do we use diatoms in anyproducts?

A Diatoms are used in filters forrefining sugar, straining yeastfrom beer, and cleaning swim-ming pool water. They also aremild abrasives in householdcleaning and polishing productsand facial scrubs; and absorbentsfor chemical spills. You usediatoms in a variety of householdproducts such as toothpaste,facial scrubs, and cleaningsolutions.

408 Chapter 14

Use VisualsFigure 11 Have students examine themap showing the distribution of marinesediments in theworld’s oceans. Ask:What kinds ofterrigenous sedimentsare shown on themap? (coarse nearshore deposits and fineabyssal clay) How do the locations oftwo typesof terrigenous deposits differ? (Fine-grained clays are found farther fromlandmasses; coarser deposits are closerto landmasses.) According to the map,which type of biogenous sedimentappears to be more common?(biogenous calcareous ooze) Which typeof sediment is found along the westcoast of North America? The eastcoast of North America? (biogenoussiliceous ooze; terrigenous coarsenearshore deposits)Visual, Logical

Types of SeafloorSedimentsBuild Science SkillsClassifying Have students collectinformation about products that containdiatomaceous earth. Suggest tostudents to look for these products inlocal grocery, hardware, auto supply,pool supply, and garden supply stores.By reading the labels, students cancollect data about the uses for eachproduct. Challenge students to identifythe general applications diatomaceousearth is used for (filters, abrasives,absorbents) and classify each productthey investigated according to itsgeneral application. (Sample answers:filters: pool cleaning equipment; abrasives:toothpaste, garden insect control;absorbents: materials for cleaningchemical spills)Logical, Interpersonal

Build Reading LiteracyRefer to p. 362D in Chapter 13, whichprovides the guidelines for this Use PriorKnowledge strategy.

Use Prior Knowledge Before studentsread this section, have them make a listof the kinds of materials they think makeup the sediment deposits on the oceanfloor. After they have read the section,have students revise their lists.Logical, Intrapersonal

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Customize for English Language Learners

Help students understand chemical terms inthis section by explaining their derivations. Tellstudents that sulfides are substances thatcontain the element sulfur. Carbonates are

substances that contain the element carbon.The term evaporites is derived from the wordevaporation.



Reviewing Concepts1. What are the three types of ocean floor

sediments?

2. What does terrigenous sediment consist of?

3. What is the composition of biogenoussediment?

4. How is hydrogenous sediment formed?

Critical Thinking5. Comparing and Contrasting Compare and

contrast calcareous ooze and siliceous ooze.

6. Predicting Would you expect to find moreevaporites in an area of warm water thatreceives large amounts of sunlight such as theRed Sea or in an area of cold water that receivesless sunlight such as the Greenland Sea?

The Ocean Floor 409

Figure 12 BiogenousSediments The microscopic shells of radiolarians andforaminifers are examples ofbiogenous sediments. Thisphotomicrograph has beenenlarged hundreds of times.

Hydrogenous Sediment Hydrogenous sedimentconsists of minerals that crystallize directly from ocean waterthrough various chemical reactions. Hydrogenous sedimentsmake up only a small portion of the overall sediment in theocean. They do, however, have many different compositions andare distributed in many different environments. Some of themost common types of hydrogenous sediment are listed below.

• Manganese nodules are rounded, hard lumps of manganese,iron, and other metals. These metals precipitate around anobject such as a grain of sand. The nodules can be up to 20 centimeters in diameter and are often scattered acrosslarge areas of the deep ocean floor.

• Calcium carbonates form by precipitation directly fromocean water in warm climates. If this material is buried andhardens, a type of limestone forms. Most limestone, how-ever, is composed of biogenous sediment.

• Evaporites form where evaporation rates are high and there isrestricted open-ocean circulation. As water evaporates from suchareas, the remaining ocean water becomes saturated with dissolvedminerals that then begin to precipitate. Collectively termed “salts,”some evaporite minerals do taste salty, such as halite, or commontable salt. Other salts do not taste salty, such as the calcium sulfateminerals anhydrite (CaSO4) and gypsum.

Origin of Sediments An oceanographeris studying sediment samples from theBahama Banks. The sediments have a highamount of calcium carbonate. They arelabeled biogenous but are later found tocontain no shells from organisms that typi-cally make up calcareous ooze. What otherexplanation is there for the origin of thesesediments?

The Ocean Floor 409

Biogenous DepositsPurpose Students observe similaritiesand differences among the types oforganisms that form biogenoussediment deposits.

Materials prepared microscope slidesof diatoms, foraminifera, and radiolaria;or photographic slides of photomicro-graphs of these organisms and a slideprojector

Procedure Have students examineprepared slides under the microscopeor view projected photographic slides.Inform students that these organismsare single-celled members of thephylum protista. Diatoms are algae(plant-like protists) that make theirown food through photosynthesis.Foraminifera and radiolaria are animal-like protists that feed on othermicroscopic organisms. Have studentsobserve similarities and differencesamong the organisms shown.

Expected Outcome Students willobserve that these organisms all havehard structures that vary in shape fromspecies to species and can be preservedafter the organisms’ soft tissues decay.

Visual

ASSESSEvaluateUnderstandingPresent students with a blank tablethat lists the three major categories ofseafloor sediments down the leftcolumn. The center column should betitled Origin. The right column shouldbe labeled Composition. Have studentsfill in the Origin and Compositioncolumns.

ReteachHave each student write ten questionsthat cover the content of this section.Invite students to use their questions toquiz each other.

They are hydrogenous, formed fromcalcium carbonate precipitating directlyfrom seawater.

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Answer to . . .

calcareous ooze,siliceous ooze

5. Calcareous ooze and siliceous ooze bothhave the consistency of thick mud and areexamples of biogenous sediments. Calcareousooze is formed from the calcium carbonatetests, or hard parts, of organisms. Siliceousooze is formed from the siliceous tests oforganisms. Calcareous ooze is not foundbelow depths of 4500 m.6. area of warm water with lots of sunlightbecause these conditions are more favorablefor evaporation


1. terrigenous, biogenous, hydrogenous2. mineral grains weathered from continentalrocks3. shells and skeletons of marine animals andalgae4. minerals crystallize directly from the waterthrough chemical reactions


14.4 Resources from the Seafloor

Reading StrategyIdentifying Details Copy the concept mapbelow. As you read, complete it to identifydetails about resources from the ocean.

Key ConceptsWhich ocean resources areused for energyproduction?

How are gas hydratesformed?

What other resources arederived from the ocean?

Vocabulary◆ gas hydrates◆ manganese nodule

The ocean floor is rich in mineral and organic resources. Recoveringthem, however, involves technological challenges and high costs. Astechnology improves we are able to access some of these resources moreefficiently. However, other resources, such as manganese nodules,remain untouched.

Energy ResourcesMost of the value of nonliving resources in the ocean comes from theiruse as energy products. Oil and natural gas are the main energyproducts currently being obtained from the ocean floor. Otherresources have the potential to be used as a source of energy in the future.

Oil and Natural Gas The ancient remains of microscopicorganisms are the source of today’s deposits of oil and natural gas.These organisms were buried within marine sediments before theycould decompose. After millions of years of exposure to heat fromEarth’s interior and pressure from overlying rock, the remains weretransformed into oil and natural gas. The percentage of world oil pro-duced from offshore regions has increased from trace amounts in the1930s to more than 30 percent today. Most of this increase is due to thecontinual update of the technology used by offshore drilling platformssuch as the one shown in Figure 13.

Major offshore reserves exist in the Persian Gulf, in the Gulf ofMexico, off the coast of southern California, in the North Sea, and in theEast Indies. Additional reserves are probably located off the north coastof Alaska and in the Canadian Arctic, Asian seas, Africa, and Brazil. One

a. ? b. ? c. ?

Sand and Gravel

consist of are used for sometimes contain

410 Chapter 14

Figure 13 Offshore drilling rigstap the oil and natural gasreserves of the continental shelf.These platforms are near SantaBarbara, California. Inferring What changes to themarine environment may occur asa result of drilling for oil?

410 Chapter 14

FOCUS

Section Objectives14.10 Identify ocean resources used

for energy production.14.11 Explain how gas hydrates

are formed.14.12 List other types of ocean

resources.

Build VocabularyParaphrase Have students explain, intheir own words, the meanings of thenew vocabulary terms in this section,plus the terms evaporative salts andhalite. Have students write theirdefinitions and share them.

Reading Strategya. rock fragments and shells of marineorganismsb. landfill, recreational beaches,concretec. diamonds, tin, platinum, gold,titanium

INSTRUCT

Energy Resources

Students may have the misconceptionthat sand contains only the mineralsilicon. However, the term sand refersmore to particle size (smaller than gravelbut larger than silt) than to mineralcontent. Ask students to explain howthey think sand is formed. Guide themto understand that sand particles aremade up of minerals that comeprimarily from pulverized rocks, whichhave varying compositions. Sandparticles can also come from pulverizedshells of marine organisms.Logical

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Section 14.4


The Ocean Floor 411

environmental concern about offshore petroleum exploration is the pos-sibility of oil spills caused by accidental leaks during the drilling process.

Gas Hydrates Gas hydrates are compact chemical structuresmade of water and natural gas. The most common type of natural gasis methane, which produces methane hydrate. Gas hydrates occurbeneath permafrost areas on land and under the ocean floor at depthsbelow 525 meters.

Most oceanic gas hydrates are created when bacteria breakdown organic matter trapped in ocean-floor sediments. The bacteriaproduce methane gas along with small amounts of ethane and propane.These gases combine with water in deep-ocean sediments in such a waythat the gas is trapped inside a lattice-like cage of water molecules.

Vessels that have drilled into gas hydrates have brought up sam-ples of mud mixed with chunks of gas hydrates like the one shown inFigure 14A. These chunks evaporate quickly when they are exposed tothe relatively warm, low-pressure conditions at the ocean surface. Gashydrates resemble chunks of ice but ignite when lit by a flame, asshown in Figure 14B. The hydrates burn because methane and otherflammable gases are released as gas hydrates evaporate.

An estimated 20 quadrillion cubic meters of methane are locked upin sediments containing gas hydrates. This amount is double theamount of Earth’s known coal, oil, and natural gas reserves combined.One drawback to using gas hydrates as an energy source is that theyrapidly break down at surface temperatures and pressures. In thefuture, however, these ocean-floor reserves of energy may help pro-vide our energy needs.

What happens when gas hydrates are brought tothe surface?

BA

Figure 14 Gas HydratesA A sample from the ocean floorhas layers of white, ice-like gashydrate mixed with mud. B Gas hydrates evaporate whenexposed to surface conditions,releasing natural gas that can beburned.

For: Links on ocean resources


Web Code: cjn-5144

Use VisualsFigure 14 Have students examine thephotos of gas hydrates. Ask: Why doyou think the burning gas hydratedoes not burn the hands of theperson holding it? (Accept reasonablehypotheses. This demonstration could bedone only with a large chunk of gashydrate. As the gas hydrate slowlydissociates, it releases methane from allsurfaces. Methane is less dense than air,so it quickly rises and tends to concentrateabove the sample. A combustible mix ofmethane and air is reached just above theupper surface, so the flame is confined tothat region. At the lower surface, there isnot enough methane to combust. Theperson’s hands are contacting only thelower portion of the hydrate, whichremains cool enough to handle. It wouldbe more prudent to wear asbestos glovesor place the hydrate on a metal screen, asthere is always a risk of getting burned.)Logical, Verbal

Build Science SkillsInferring Remind students that whenwater freezes, the molecules arrangethemselves in a lattice structure thattakes up more space than liquid water.This lattice contains open spaces thatare large enough to hold methanemolecules. If methane molecules enterthose spaces during lattice formation, ahydrate may be produced. The presenceof methane molecules inside the cavitieshelps hold the hydrate crystal together.Point out to students that methane isproduced by bacteria involved in thedecomposition of organic matter. Ask:Why do gas hydrates form onlywhere there is plenty of decayingorganic matter? (Bacteria involved indecomposition produce methane, whichis required for gas hydrate formation.)Logical, Verbal

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Answer to . . .

Figure 13 Drilling platforms mayprovide structure for some marineorganisms to live on; drilling maydisturb some organisms or destroyhabitats; an oil spill could be harmfulto organisms in the immediate area

Gas hydrates evaporatequickly at surface

temperature and pressure.

Download a worksheet on oceanresources for students to complete,and find additional teacher supportfrom NSTA SciLinks.


Figure 15 These manganesenodules lie 5323 meters on thePacific Ocean floor south of theisland of Tahiti. Applying Concepts How domanganese nodules form?

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Other ResourcesOther major resources from the ocean floor include sand and

gravel, evaporative salts, and manganese nodules.

Sand and Gravel The offshore sand-and-gravel industry issecond in economic value only to the petroleum industry. Sand andgravel, which include rock fragments that are washed out to sea and

shells of marine organisms, are mined by offshore barges usingsuction devices. Sand and gravel are used for landfill, to fill inrecreational beaches, and to make concrete.

In some cases, materials of high economic value are associ-ated with offshore sand and gravel deposits. Gem-qualitydiamonds, for example, are recovered from gravels on the conti-nental shelf offshore of South Africa and Australia. Sedimentsrich in tin have been mined from some offshore areas ofSoutheast Asia. Platinum and gold have been found in depositsin gold-mining areas throughout the world. Some Florida beachsands are rich in titanium.

Manganese Nodules As described earlier, manganesenodules are hard lumps of manganese and other metals that pre-cipitate around a smaller object. Figure 15 shows manganesenodules on the deep-ocean floor. They contain high concentra-tions of manganese, iron, and smaller concentrations of copper,

Evaporative Salts

4. When all of the water has evaporated, placethe beaker and its remaining contents on thebalance and record the measurement.

Analyze and Conclude1. Comparing How did the mass of the beaker

and salt before the water was added compareto the mass of the beaker and salt after thewater evaporated?

2. Drawing Conclusions What happened tothe salt when the water evaporated?

3. Predicting How could the oceans be used asa source of salt?

Materials400 mL beaker, table salt, tablespoon, balance,glass stirrer

Procedure1. Place the empty beaker on the balance and

add between 3 and 5 tablespoons of the salt.Measure the combined mass of the balanceand the salt. Record the measurement andremove the beaker from the balance.

2. Add 100 mL of water to the beaker and stiruntil the salt is dissolved.

3. Place the beaker in a warm, sunny area andallow the water to evaporate.

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Other Resources

Students may not think that ordinarytap water contains minerals that wouldprecipitate out during evaporation.Guide students to understand that,because water is the universal solvent,there is virtually always somethingdissolved in it. Heat tap water in a panwith a black Teflon© coating. When thewater boils off, a white residue remainsof mineral salts. Explain that there area number of physical and chemicalprocesses that can be used to removenon-water molecules and ions from asample of water. Have students look upinformation about the effectiveness ofvarious water purification methods, suchas distillation, active carbon filtration,and ozone treatment systems.Logical, Verbal

Evaporative Salts

Objective Students observe that anysalt dissolved in water is left behindwhen the water evaporates.

Skills Focus Observing, Measuring,Comparing, Drawing Conclusions,Predicting

Prep time 10 minutes

Class Time 10–15 minutes to set up;5 minutes to make final measurements

Expected Outcome Students find thatall of the salt they put into the water isleft behind when the water evaporates.

Analyze and Conclude1. The masses should be equal.2. It is left behind (precipitates out).3. Certain areas with proper conditionscould be used to evaporate water andcollect solid salt.Kinesthetic, Logical

For Enrichment

Have students evaporate water takenfrom a variety of sources (tap water,rainwater, bottled water) to determineif an evaporite is left behind. Studentscould test any evaporite they obtain forthe presence of minerals.

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In Earth’s geologic history, there have beenincidents in which an entire body of salt waterevaporated away. The evidence for thisincludes extensive deposits of evaporiteminerals, including one in the MediterraneanSea. Evidence suggests that, about 6 millionyears ago, the Mediterranean Sea was cut off

from the Atlantic Ocean at the Strait ofGibraltar. The Mediterranean Sea almostcompletely evaporated within a few thousandyears, leaving a thick deposit of evaporites onthe hot, dry basin floor. A half million yearslater, water re-entered the Mediterranean Sea,filling it with seawater again.

Facts and Figures



Reviewing Concepts1. What are the main energy resources from

the ocean?

2. How are gas hydrates formed?

3. What drawbacks are associated withharvesting ocean resources for energy use?

4. What other resources are derived from theocean?

5. What are the uses of evaporative salts?

6. What are manganese nodules? Why is itdifficult to recover them from the ocean?

Critical Thinking7. Making Generalizations How does

technology influence the availability ofresources from the ocean?

8. Inferring Near-shore mining of sand andgravel can result in large amounts ofsediments being suspended in water. Howmight this affect marine organisms living inthe area?

nickel, and cobalt, all of which have a variety of economic uses.Cobalt, for example, is important because it is required to pro-duce strong alloys with other metals. These alloys are used inhigh-speed cutting tools, powerful permanent magnets, and jetengine parts. With current technology, mining the deep-oceanfloor for manganese nodules is possible but not economicallyprofitable.

Manganese nodules are widely distributed along the oceanfloor, but not all regions have the same potential for mining.Good locations for mining must have a large amount of nodulesthat contain an optimal mix of copper, nickel, and cobalt. Sites like thisare limited. In addition, it is difficult to establish mining rights far fromland. Also, there are environmental concerns about disturbing largeportions of the deep-ocean floor.

Evaporative Salts When seawater evaporates, the salts increasein concentration until they can no longer remain dissolved. When theconcentration becomes high enough, the salts precipitate out of solu-tion and form salt deposits. These deposits can then be harvested, asshown in Figure 16. The most economically important salt is halite—common table salt. Halite is widely used for seasoning, curing, andpreserving foods. It is also used in agriculture, in the clothing indus-try for dying fabric, and to de-ice roads.

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Figure 16 Common table salt, orhalite, is harvested from the saltleft behind when ocean waterevaporates. About 30 percent ofthe world’s salt is produced byevaporating seawater.

Sand and Gravel Why are most sandand gravel deposits found on the conti-nental shelf? What type of sediment issand and gravel?

Use CommunityResourcesAsk the owner or manager of a localquarry to explain to the class how graveland sand are mined, their uses, andtheir economic value. If possible, havethe presenter bring samples of differenttypes of sand and gravel for students toexamine. Ask the presenter to discussthe differences between sand and gravelfrom marine and terrestrial sources.Interpersonal, Visual

ASSESSEvaluateUnderstandingHave students explain how gas hydratesand evaporative salts are formed. Askstudents to write a short paragraph oneach.

ReteachOn the board, write a skeleton of anoutline for the content of this section,with Energy Resources as Romannumeral I and Other Resources asRoman numeral II. Have students copythe skeleton and complete the outlineby adding the ocean resources discussedin this section, along with details abouttheir uses and how they form.

Sand and gravel are coarse sedimentsthat settle out more quickly than fine-grained sediments. Sand and gravel areterrigenous sediments.

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3. possibility of oil spills, possible effects onmarine habitats, gas hydrates break downquickly when brought to the surface4. sand and gravel, evaporative salts,manganese nodules5. used to season, cure, and preserve foods;used in agriculture, to dye fabric, and to de-ice roads6. Manganese nodules are round, hardlumps of manganese, iron, and other metalsthat precipitate around an object, such as agrain of sand.


1. oil and natural gas2. Most oceanic gas hydrates are formedwhen bacteria break down organic mattertrapped in seafloor sediments. The bacteriaproduce methane gas along with smallamounts of ethane and propane. These gasescombine with water in deep-ocean sedimentsin such a way that the gas is trapped insidea lattice-like cage of water molecules.

Answer to . . .

Figure 15 Manganese nodules formwhen metals such as manganese andiron precipitate around an object, suchas a grain of sand.

7. As technology improves, people may beable to retrieve resources more efficiently.8. Suspended sediments can make watercloudy, interfering with the amount of lightthat penetrates water. This can affect organ-isms that need light for photosynthesis.Suspended sediments can also affect filter-feeding organisms, preventing them fromfeeding properly.


14.1 the vast world ocean - mr. baker's earth science...

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