Oceanography, Chapter 2 Ocean Basins.pdf

8/14/2019 Oceanography, Chapter 2 Ocean Basins.pdf

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he ocean basins are a primary feature of the earth.s surface(see Plate 1). But our vision of the ocean bottom is obscuredby theoceanwaters.Thus the oceanbottom remainsamong he least knownfeatures of our planet. For example, he largest mountain range onearth (seePlate 5), the midocean idge, was not discovered until thelate nineteenth entury, and ts origins were finally understood n the1960s. .Oceanbasin exploration continues today, with new findings comingevery few months. Our improved understanding of the ocean basinshas changed our view of the earth as much as the exploration of theNew World changed human perspectives in the fifteenth and sixteenthcenturies.In this chapter we examine the ocean basins, their individual

features and those common to all the major basins:Distribution of continentsand ocean basins;Major featuresof each oceanbasin;Principal eaturesof the ocean toor; andMajor featuresof submerged ontinental margins.

DISTRIBUTION F LANDAND WATER Land and water are uneveniy distributed on the earth. The continents,which cover 29.2% of the earth's surface, break up the ocean intothe three basins, which we call the Atlantic, Pacific, and Indian oceans.The ocean basins, which cover 70.8% of the earth's surface, can beviewed as northward-projecting gulfs of the world-circling ocean aroundAntarctica (Fig. 2-1). Because of the connections of the ocean basinsaround Antarctica, the Southern Hemisphere (see Plate I) is dominatedby ocean (80.9%). The Northern Hemisphere contains most of theland but is still dominated by ocean (60.7%).

32 CHAP'fER TWO OCEAN BASINS


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FIGURE2-1The ocean s centeredon Antarctica. For ourconveniencet is considered o be three separateoceanbasins,separated s shown.

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HYPSOGRAPHICURVE-- Relationshipsbetween oceanic depths and land elevations can be shownin a hypsographic curve (Fig. 2-2), which indicates that earth's surfacehas two distinct levels. On the average, the land projects about 840meters (2755 feet) above sea level and ocean basins have an averagedepth of 3800 meters (12,500 feet) below sea level. Despite the greatoceanic depths (11 kilometers, or 7 miles, in the Marianas Trench)and the enormous height of the mountains (8.8 kilometers, or 5 miles,at Mount Everest), these numbers are truly insignificant when comparedwith the dimensions of the earth (radius 6371 kilometers, or 3823miles). If the earth were a smooth sphere with the continents planedoff to fill the ocean basins, it would be covered with water 2430 meters(7970 eet) deep. In short, the ocean is a thin film of water on a nearlysmooth globe, interrupted here and there by continents.

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FIGURE -2A hypsographic i8iI'aIn shows he fraction ofthe earth's surface n elevationor depth zonesof ODe ilometer (left side).The right side showsthe cumulative raction of earth's surfaceshallower han a given depth. Note that theearth's surfacehas wo dominant evels-onerepresentinghe land and he other the oceanbottom.

Continents nd ace., BainsI "~n;' r-~.7% ~4 159.3%--;-~20 0 20 40 80 80 100Percent Pwcent

PACIFICOCEAN The Pacific is the deepest and by far the largest ocean basin (Fig.2-3). Bordered by the Americas, Asia, and Australia, the Pacific occupiesmore than one-third of the earth's surface (Fig. 2-4) and contains morethan one-half of the earth's free water (see Fig. 2-3). Mountain buildingdominates Pacific shorelines. Along the Pacific's entire eastern margin(the western coasts of the Americas from Alaska to Peru), rugged,young mountains parallel the coastlines.These young mountains block rivers flowing into the ocean alongmost of its margins, especially on the west coast of North and South

FIGURE .3Distribution (in percent)of water on the earth'ssurface.Note the minute raction of the totalamountof water that occurs as freshwater akes,rivers, or atmosphericwater vapor.

Atmosphere, lakesand rivers (0.01%1Ground water (0.5%1

34 0tAPTER TWO OCEAN BASIN


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America. Consequently, reshwater and sediment lowing into the Pacificcome primarily from Asia. Submerged continental margins are narrowbecause he steep slopes of the young mountain ranges continue belowsea level. The net result is that the Pacific is less affected by thecontinents than either the Atlantic or Indian oceans.Islands are abundant in the Pacific, especially in the southernand western portions. As previously mentioned, these islands servedas stepping stones or human migrations across the Pacific. The smallestislands are low-lying sand slands, many of them associated with coralreefs. (Reefs are discussed in Chapter 14.)Most of the larger islands are volcanoes, many still active. Theinactive volcanoes are deeply eroded by waves and rivers. The HawaiianIslands are good examples of volcanic islands (see Plate 6). They arethe tops of a chain of enormous volcanoes (up to 10,600 meters or35,000 feet above the sea floor) that extends 3000 kilometers (1800miles) from Hawaii, with two active volcanoes, to Kure Island, a smallsand island on top of a deeply submerged volcano that is more than20 million years old. Most of the large islands in the western Pacificare actually pieces of continents that were separated from Australiaand Asia millions of years ago. (More about this in the next chapter.)

~-~c OCEANThe Atlantic Ocean s a narrow, S-shaped asin hat connects he twopolar oceanareas o the north and south (seeFig. 2-1). The boundarybetween he Atlantic and Indian oceans s usually drawn from SouthAfrica's Capeof Good Hope along ongitude 20oE o Antarctica. (Re-member hat longitude lines run north-south and measure angulardistanceeast and west from the prime meridian, which runs throughGreenwich,England. Latitude and longitude ines are shown in Fig.2-4.Both are discussedurther in Appendix 3.) The boundarybetweenthe Atlantic and Pacific oceans s drawn between Cape Horn at thesouthern ip of South America and the northern end of the AntarcticPeninsula.When the A;rctic Ocean s included as part of the AtlanticOcean, t meets he Pacific at the Bering Strait between Alaska andSiberia. Defined n this way, the Atlantic Ocean stretches rom theshoresof Antarctica northward across he North Pole to about 65N,where t borders the North Pacific.The Atlantic is relatively shallow, averaging3310meters 10,800feet) in depth, as a result of the large areasof submerged ontinentalmargins,severalshallow marginalseas,and the presenceof the large,relatively shallowMid-Atlantic Ridge. There are relatively few islandsin the Atlantic. Greenland, he world's largest island, is a piece ofthe North American continent that was separatedmany millions ofyearsago. Most other islandsare tops of volcanoes, ncluding severaon the Mid-Atlantic Ridge.The Atlantic Ocean receives arge amounts of river dischargeboth water and sediment.The Amazon and Congo (Zaire) rivers ft.owinto the equatorialAtlantic. Together hey discharge bout one-quarterof the world's river ft.ow nto the ocean. Other large rivers ft.ow ntomarginalseas.Freshwater rom several arge Asian rivers ft.owing ntothe Arctic Oceaneventually enters he Atlantic.The Arctic Ocean s an arm of the Atlantic (see Fig. 2-1). It isunique n several espects:Submerged ontinental margins orm one-

36 0tAPTER 1WO OCEAN BASINS


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third of its Ooor. t is almost completely surroundedby land. And-especially mportant-much of it is coveredby sea ce. During muchof the year, ts surface s coveredby pack ce (sea ce in thick chunksthat freeze ogether,break apart, and freeze again) o a thickness of3 to 4 meters (10 to 12 feet). In the central Arctic this ice cover ispermanent,although t partially melts, thinning to about 2 meters (6feet) by late August.Being almost entirely surroundedby land, the Arctic Ocean sgreatly nfluencedby rivers that dischargeconsiderableamounts ofsedimentonto the continental shelves. Large amounts of sedimentdepositedon the bottom of the Arctic basins came from extensiveerosionof the surroundingands by glaciersduring the ice agesof thepast severalmillion years.

INDIAN OCEAN The Indian Ocean s primarily a Southern Hemisphere ocean (see Fig.2-4). Its Pacific boundary runs through Indonesia and extends fromAustralia southward to Antarctica along longitude lSOOE. t is thesmallest of the major ocean basins. Continental shelves around theIndian Ocean are relatively nalTOW. The large amount of sedimentdeposited in the northern Indian Ocean makes the basin intermediatein depth.Three of the world's largest riyers (Ganges, Brahmaputra, Indus)discharge large amounts of sediments and water into the northernIndian Ocean (see Fig. 2-4). The Red Sea and the Arabian Gulf influencethe waters of the entire Indian Ocean because hey are areas of intensiveevaporation and produce large quantities of warm, salty water thatflow out into the main ocean basin as subsurface currents.There are relatively few islands in the Indian Ocean. Madagascar,the largest, was once part of the African continent, as were severalshallowly submerged banks. There are a few volcanic islands andgroups of carbonate islands, including atons-groups of islands thatring a shallow lagoon, usually located on top of a submerged volcano(see Plate 7). Most atolls are in the Pacific and Indian oceans becauseof the abundant volcanic seamounts n these two ocean basins. (Atollsare discussed further in Chapter 14 ~hen we discuss coral reefs.)

---

MIDOCEANRIDGES ND RISESMidocean ridges and rises (Fig. 2-5) stand conspicuously above thedeep-ocean ftoor (see Plate 5). They constitute 23.1% of the earth'ssurface. The midocean ridge and rise system includes low-relief risesin the P,acific (shown schematically in Fig. 2-6) and high-relief ridgesin the Atlantic (shown schematically in Fig. 2-7).The rugged, high-relief Mid-Adandc Ridge stands 1 to 3 kilometers(0.6 to 1.9 miles) above he deep-ocean toor. It is 1.500o 2000kilometers(900 to 1200 miles) across and has a prominent steep-sided centralvalley (Fig. 2-8), 25 to 50 kilometers (15 to 30 miles) wide and 1 to2 kilometers (0.6 to 1.2 miles) deep. It is bordered by rugged mountainswhose shallowest peaks come to within 2 kilometers (1.2 miles) ofthe sea surface. The closest analogues on land to the Mid-AtlanticRidge are the mountains and the long, narrow, lake-filled East Mricanvalleys (see Fig. 2-7). The East Pacific Rise is much less rugged (seeFig. 2-5).

37DOCEAN RIDGESAND RISES


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FIGURE 2-6Schematic representation of the ocean-bottomtopography along the East Pacific Rise, a broad,low rise of the ocean bottom with manyvolcanoes. Note that there is no central riftvalley as in the Mid. Atlantic Ridge (seeFig. 2-7). 1500

FIGURE2.7Profiles of the rift valley of the Mid-AtlanticRidge and the Tanganyika ift in East Africa.Note the similarity in form aod size of the twofeatures.Rift Valley andLake ~anganyika ~

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Distancekilometers)Faults fractureswhere wo piecesof the crust have movedpasteach other) cut across he ridges and offset them, forming segmentsof the ridges, as seen n Fig. 2-8. Earthquakesoccur when crustalsegmentsmove past each other along aults (see Fig. 2-9). There arenumerousand frequent shallow-focuseatthquakes occurring within70 kilometers 42 miles) of the earth's surface]along he Mid-AtlanticRidge (Fig. 2-9) as well as volcanoesand volcanic islands, ncludingIceland.The East PacificRise ies near the easternmargin of the Pacificbasin. Unlike the Mid-Atlantic Ridge, it is a vast, low bulge on theocean loor, approximatelyequal n size o North and South Americacombined.The rise standsabout 2 to 4 kilometers (1.2 to 2.4 miles)above the adjacentocean bottom. Intersecting the North Americancontinent n the Gulf of California, ts continuation, he Juan de Fucaand Gorda ridges, reappearsoff the Oregon coast and extends ntothe Gulf of Alaska.In the Indian Ocean, he high-relief Mid-Indian Ridge (similar tothe Mid-Atlantic Ridge) ntersects Africa-Asia in the Red Sea area(see Fig. 2-5). On the south, the Mid-Indian Ridge system branchesto join the system of active ridges and rises that circle Antarctica.Near Madagascar,a low section of the ridge system permits deepwaters o movebetween he deeperparts of the Indian and he Atlanticoceans.

MIDOCEAN RiDGeS AND RISES 39


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FIGURE2-8Artist's reodjtioo shoWiDI the topopaphy or thebottom of the North Atlantic Ocean depictedwith all the water removed. Depths are Doted infeet.

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~20..FIGURE .9Earthquakesmark the locationsof midocean"dies and trenches.

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Narrow belts of frequent shallow-focusearthquakes un alongthe crests of oceanic rises (Fig. 2-9). These earthquakescoincideclosely with the axis of the rift valley. Indeed, earthquakes ave beenused o pinpoint active ridge systems n little-known ocean areas.Midoceanic idge systemsare also sites of high heat ftow. Areasof unusually high heat flow occur in narrow bands in rift valleys,causedby intrusions of molten roc~s. Hydrothermal vents dischargehot waters n areasof recent volcanic eruptions.

FRACTUREONES The ocean ftoor is cut by hundreds of fracture zones. These zones arelong, narrow (10- to l00-kilometer, or 6- to 6O-mile) belts of irregulartopography with many rugged ridges; some are nearly 4000 kilometers(2400 miles) long. Fracture zones displace segmentsof midocean ridges(see Fig. 2-8).Fracture zones continue into the continents. The MendocinoFracture Zone comes onto land at Cape Mendocino, California, whereit extends southeastward as the San Andreas Fault to co~ect withthe East Pacific Rise in the Gulf of California. Most of the earthquakesin California occur on or near the San Andreas Fault.One of the earth.s largest fracture zones s the R~~ Fracture~ (seeFig. 2-8) which offsets the Mid-Atlantic Ridge by 950 kilometers(600 miles) near the equator. It is part of a group of large faults; theentire zone consists of deep valleys about 100 kilometers (~ miles)wide, separated by ridges, one of which comes to the sea surface.The Romanche Fracture Zone contains one of the deepest partsof the Atlantic, which reaches a depth of 7~ meters (26,100 feet).The gap in the Mid-Atlantic Ridge is an important pathway for bottomwaters from the western Atlantic ftowing into the nearly isolated deepbasins of tk~ eastern Atlantic Ocean. (We learn about movements ofdeep-water masses,n Chapter 8.)

The deep-oceanBoor (deeper than 4000 meters, or 13,00> eet) occupiesabout 29.5% of the earth's surface. Low abyssal bills [less than 100>meters (3300 feet) abOve the surrounding ocean bottom] cover about80% of the Pacific deep.ocean bottom, about 50% of the Atlantic, andare abundant in the Indian Ocean. These hills-probably the mostcommon feature on the earth's surface-are typically about 200 meters(600 feet) high and about 6 kilometers (4 miles) across. Most appearto be extinct small volcanoes.Immense areas of exceedingly flat ocean bottom, called abyssalplains, occur near continents. These areas are the flattest portions ofthe earth's surface. They have slopes of less than I in 100>,comparableto the flattest parts of the continents. Abyssal plains commonly occurat the seawardmargins of the deep-sea ans that make up the continentalrises. Most abyssal plains are covered with thick sediment depositswhich have buried any rough topography. Abyssal plains are especiallycommon in marginal seas, such as the Gulf of Mexico, the Caribbean,and the many small ocean basins around the Pacific Ocean margin.

TRENCHESTrencbeSo-theeepestparts of the ocean toor-occur near the oceanbasin margins. They are relatively narrow (see Fig. 2-10) and steep-sided eatures, ypically several ens of kilometers wide and about 3~ 41


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TABLE 2-1Characteristics f Trenches. AVERAGEWIDlH(kilometers)DEPTH(kilaneters ) lENGTH(kilometer1)R8"Oof

. After R. W. Fairbridce. 1966. Tren


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FIGURE -11IDdonesia, complex sland-an:system.Activevolcanoesoccur on the larger slands. Someofthe smaller slandsnear the trench are built ofdeformedsedimentdeposits.Depths greater han6 kilometers 3.6 miles) are shown n black. 1200 E

VOLCANOES Volcanoes and volcanic islands are common features on the oceanftoor, usually projecting 1 kilometer (0.6 mile) or more above thesurrounding ocean ftoor. There are tens of thousands of volcanoes onthe Pacific ftoor. Most volcanic activity on land occurs near the edgesof the ocean basin, especially around the Pacific.Many active volcanoes occur in the middle of ocean basins, farfrom midocean ridges or island arcs. Such volcanoes form chains ofvolcanic peaks, such as the Hawaiian Islands (see Plate 6). Theseoceanic volcanoes are called shield volcanoes. As mentioned earlier,these are among the largest mountains on earth. (Origins of suchvolcanoes are discussed in the next chapter.)Submerged volcanoes persist in the ocean for millions of years.A volcano builds a cone while it is active, and then eventually becomesdormant, often millions of years later. Over millions of years, volcanoessubside and are eroded. Eventually they become submerged banks orthe foundation for coral reefs and atolls (plate 7).Volcanic island arcs are associated with active mountain buildingand trenches. The Aleutian Islands southwest of Alaska, for instance,are a single chain with a trench on the seaward side, as shown in Fig.

VQ.CANQES 43


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2-10. The Indonesian-Philippine area has two trenches (see Fig.2-11) witll many active volcanoes. Some of the largest recorded andmost destructive eruptions occuned in Indonesia. Tambora on Sumbawaerupted in 1815, killing 92,000 people and caused an unusually coldsummer n 1816.Snow fell in New England on July 4. In 1883,Krakatoa.between Java and Sumatra, killed 36.000 people when it erupted.Volcanic eruptions are well known on land. Exploration of theocean bottom shows that volcanic eruptions are especially commonon mid-ocean ridges. On land and on the ocean bottom. volcanic lava(molten rock) erupts at temperatures between 900 and 12000C. t flowsfor a while and then solidifies, first at the surface and finally throughoutthe ftow. Further cooling turns the lava into volcanic rock.There are two modes of oceanic volcanic eruptions. Some lavasform tranquil flows in which the surface cools first. forming an insulatedcover for the still-molten material in the interior of the ftow. Whenthis type of. lava flows into the ocean. it forms piUow lavas (Fig.2-12). rounded massesof volcanic rock. Oceanic depths seem to favorsuch tranquil flows. for they are quite common features of the deepocean. The other type of marine volcanic eruption [usually in relativelyshallow waters (see Fig. 2-13)] is explosive, producing large amountsof volcanic ash. small pieces of volcanic rock. and glass formed byquenching of molten rock.Not all volcanic activity forms volcanoes. Large areas of oceanbottom are smooth plains of volcanic rock. Apparently lava flowedout in large volumes. covering previous topography over extensiveareasof the ocean bottom. These arch~~ pIaim (or aprons) surroundvolcanic groups or islands and extend tens or hundreds of kilometersfrom the volcanoes themselves. As we shall see in the next chapter.volcanic eruptions on the ocean floor are common at midocean ridges.

FIGURE 2-12Toothpaste nd pillow lavas are formed bysubmarine olcanic erupCjonsn the axial vaDeyof the Mid-Atlantic Ridge.These eaturesareprobably ess han IO.(xx) earsold. Thesubmersible'semote-controlled rm s pickingup a rock.


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(8)(b)

FIGURE -13Eruption of Kovachi, a submarine olcano n theSolomon slands, South Pacific, n October 1968.(a) Eruptions and explosionsejectedwater andsteam60 to 90 meters 200 o 300 eet) into theair and discoloredsurfacewaters or 130kilometers SOmiles). (b) An island ormed byan eruption n 1961has since beenerodedawayto form a submerged ank.

MARGINALOCEAN BASINS Marginal oceanbasinsare arge oceanbottom depressionsying nearcontinents. They are separated ro~ the open ocean by submarineridges, islands, or parts of continents. Marginal basins are usuallymore than 2 kilometers (1.2 miles) deep and so their bottom watersare partially isolated rom oceanwaters at comparabledepths outsidethe basin.MARGINALOCEAN BASINS 45

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FIGURE .14The Red ~ (upper center)with the Gulf ofAqaba 1eft)and the Gulf of s.uezGower iaht).The dark band on the riaht is tbe Nile Valley;the river is the liaht meanderingine in the darkband. Note the straight sidesof the Red Sea.

These small seas are transitional between continents and deep-ocean basins. They commonly have an oceanic crust overlain by thicksediment deposits. Being near coasts, they receive the discharge ofmany large rivers. The sediment brought by these rivers accumulatesin the basins, resulting in deposits many kilometers thick. These depositscontain about one-sixth of all known ocean sediments. Nearly allmarginal ocean basins are located in areas where crustal deformationor mountain building is active.Three kinds of associations are typical of marginal oc~an basins.The most common is that of a basin associated with island arcs andsubmarine volcanic ridges that partially isolate surface waters andmay completely isolate the subsurface waters. The many small basinsaround Indonesia and along the Pacific margin of Asia are examples.A second type of marginal sea lies between continents-for in-stance, the Mediterranean Sea between Europe and Mrica, the BlackSea (now almost completely landlocked), and the Gulf of Mexico andCaribbean Sea between North and South America (see Fig. 2-4).The third type is the long, narrow marginal sea surrounded bycontinents. The Red Sea (shown in Fig. 2-14) and the Gulf of Californiaare examples of such basins.Because of their proximity to large landmasses, marginal seasare subjected to greater extremes in climate than occur in the open


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ocean. In high latitudes marginal seas are cold enough during winterto freeze over. In midlatitudes, where the world's deserts are located,marginal seas like the Mediterranean Sea and Arabian Gulf are alsoexposed to much evaporation and little rainfall, so their waters aremuch warmer and saltier than average seawater. Because of theirmarked response to regional and seasonal weather conditions, waterscirculating through marginal seas have a recognizable influence onadjacent open waters.Around Europe and North America shallow seasoccupy depressedareas at the edge of continental blocks, such as Hudson Bay in Canada..The North Sea of north em Europe is another such flooded continentalarea, leaving only the British Isles above sea level at present.

CONTINENTSANDCONTINENTALMARGINS Four north-south-trendingand masses-Eurasia-Africa, he Americas,Antarctica, and Australia-interrupt the oceanbasins.As we shall seein later chapters, he locationsof these and masses ave an mportanteffect on oceancurrents.Connectionsamongcontinental and masses re rather narrow,usually ow-lying and easily looded, even by relatively small changesin sea evel. As we shall see n Chapter 11, relatively recent changesin sea evel have greatly modified he coastal eatureswe see oday.Large changesn sea evel haveopenedup connections etweenoceanbasins,such as the flooding of Central America, which connected heAtlantic and Pacific oceans,and the isolation of the MediterraneanSea rom the Atlantic (more about this in later chapters).Continentalmargins onsistof continental helf, continental lope,and continental ise (Fig. 2-15).The continental helf s the submergedtop of the edgeof the continental andmass.From the shoreline, heshelf slopesgently toward the continentalshelf break (averagedepthabout 130meters,or 430 eet), where t joins the steepercontinentalslope.Off Antarctica he shelf break occurs at about 500meters (I~feet) because he weight of the Antarctic ice sheethas depressed hecontinentalmargin nearly 400 meters (1300 eet).

On the average, he continental shelf is about 70 kilo*ters (40miles) across.Around much of the Pacific n areasof active mountainbuilding, continental shelvesare only a few tens of kilometers wide.Wide continental shelvesgenerally occur where continental marginshave not experiencedmountain building for many millions of years.The widest shelvesoccur around the Arctic Sea.FIGURE 2-15Block diaaramsbowina he North Americancontinentalmarlin in the Atlantic Ocean.Notethe vertical exaaeration.

/' ShelfbreakOutershelfInnershelf

i..2~-siQ 0 '200Continentalshelf \ 800Continentalslope / Distance1000 (kilometenl-- Upper

contin.,talrise\'" Lowercontinentalri. Abyssalplan

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FIGURE -16Schematic epresentation f the continentalmargin, showing he continentalshelf, thecoastalplain, the continentalslope,and thecontinental ise. Note that a submarine anyoncuts across he shelf break and extends o thebaseof the continentalslope,where t ftows outonto the continental ise.

The coastal plain is an emergent continental shelf (Fig. 2-16)Where the coastal plain is absent and mountains form the coastline,as in southern California, the continental margin is rugged, broken bysubmarine ridges and basins. Continental shelves near high-latitudecoasts tend to be ru'gged, another effect of the glaciers that havecovered these areas several times in the last few million years.Deep troughs, cut by glaciers, extend across high-latitude shelvesand connect the deep, flooded indentations called fjords with the oceanNearby shelf areas were also substantially modified by the recenadvance and retreat of the glaciers. As we shall see in Chapter 11marine processes have not yet had enough time to completely altersuch coastlines.Continental slopes are the submerged outer edges of continentalblocks. On an ocean-free earth, they would be the most conspicuoussurface feature-cliffs several kilometers high. The most spectaculacontinental slopes occur where a range of coastal mountains bordersa deep trench. On the western coast of South America, the peaks othe Andes Mountains reach elevations around 7 kilometers (4 miles)and the nearby Peru-Chile Trench is 8 kilometers (5 miles) deep-atotal relief of 15 kilometers (9 miles) within a few hundred kilometers.Such slopes are common around the Pacific.

Submarine canyons (see Fig. 2-16) are common on continentashelvesand slopes.Typically they follow curved paths and have tributarychannels leading into them, much like river valleys on land. Somsubmarine canyons are as large as the Grand Canyon of the ColoradoRiver. A few are obviously associatedwith the mouths of major rivers-for instance, Hudson Canyon or the Congo (Zaire) Canyon. In thescases, the upper parts of the canyons were apparently cut when selevel was much lower than its present level. Many submarine canyonhave no obvious connection with large rivers.Continental rises at the base of continental slopes are formed byaccumulations of sediments eroded from the continents and depositeat the base of the slope. The thick deposits cover the transitionsbetween continents and ocean basins.

SUMMARYThe ocean is centered around Antarctica, with three gulfsextending northward to form the Atlantic, Pacific, and Indianoceans. The solid earth surface has two primary levels-land and ocean bottom.

The PacificOcean s the largest,containingmore hahalf the earth's free water. It is borderedby areasof activmountainbuilding. The Atlantic Ocean s long and narrowconnecting he polar ocean egions. t is relatively shallow48 OtAPTER1WO OCEAN B~


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slope. Continental slopes are cut by submarine canyons,especially near the mouths of ~or rivers. The continentalrise at the base of the continental slope grades nto the nearlyftat abyssal plains.Volcanoes are among the most cOmmon features onthe ocean bottom; they are especially abundant on midoceanridges and are scattered across the ocean basins. Some ong-extinct volcanoes have flat tops, apparently cut by waveswhen the volcanoes were exposed to wave action and weath-ering at the sea swface.M8rIina1 ocean basins are large, deep depressionsnearthe continents. Most are formed by mountain building andcontain thick sediment deposits. Many are structurally tran-sitional between ocean and continent. Most marginal seasare associated with island arcs which cut them off from theopen ocean. A second type, such as the Mediterranean Sea,lies between continents. The third type is the long, narrowocean basin within a continent; the Red Sea is an example.Since the Waters in these seas are paitiany isolated fromthe open ocean hey sometimes onn distinctive water masses.Shallow seas such as the North Sea or Hudson Bay occuron the continents.Four nortb-south-treoding continental masses ntenuptthe oceans. Connections between continental masses areoften narrow and easily flooded by rises in sea level. Con-tinental margins consist of continental shelves, continentalslope, and continental rise. The continental shelf and as.sociated coastal plain are the tops of the edges of continentalblocks; they are separated by the shoreline. Continentalslopes are the submerged edges of the continental blocks.Continental rises are thick sediment deposits at the base ofthe slopes, where they join the deep-ocean floor.

many marxinal seas, and receives about two-thirds ofrld's river discharge. The Arctic Ocean is the nearlym of .the Atlantic. It is ice covered most ofbasins, and lies primarily in the Southern Hemisphere.Midocean.ridges occur in all ocean basins. They formmountain chain on earth. The Mid-Atlantic Ridge. It is rugged and hasdeep central rift valley. The Mid-Indian Ridge resemblesic Ridge. The East Pacific Rise has no centralvalley and is a broad, gentle rise on the ocean ftoor in

with the ruaed mountains of the Mid-Atlantic andridges. Midocean ridges nearly surround Ant-Midocean ridges are offset by bands of rough to-The deep-ocean ftoor is mostly covered by sedimentsmooth older topography and eventuallyabyssal plains, the ftattest parts of the earth's surface.near the continents at theof ocean basins. The greatest depths in the oceanccur in the trenches at the edges of the ocean basins;in the Pacific and are associated with areas ofSmaller ocean basins occur around the margins of they have been formed by mountain-Several are partially isolated from thebasins and are distinctly different from theContinents separate the ocean basins, especially inNorthern Hemisphere. Continental margins (continentaland slope) are the submergededgesof the continents.shelf break (average depth 130 meters, or 430 feet)transiti9.n from continental shelf to continental

QUESTIONSDraw a cross section showing he continental margin,the deep-oceantoor, and the adjacentcontinent. Labeland indicate ypical depths or each.a cross section of a rapidly spreadingmidoceanridge. Indicatewherevolcanicactivity and hydrothermalventsare most ikely to occur. Label significant eatures.On an outline map of the world, sketch he locationsof

earthquake belts, active volcanoes, midocean ridges, andtrenches.4. Briefty descn"be he geographic relations of the three ma,jorocean basins. Indicate how and where they connect andwhere and by what they are separated.5. Discuss the origins and potential economic significanceof marginal ocean basins.

ROBOT D. 1983. Exploring Our Living Planet.National Geographic Society, Washington, D.C. 366pp. WeD-illustrated presentation of ocean features andorigins.

Eo8lewood Cliffs, N.J. 813 pp. Comprehensive treat-ment of ocean basin geology.WEIND, JONATHAN. 986. Planet Earth. Bantam Books, To-ronto. 370 pp. Well written, nicely illustrated surveyof ocean, earth, and atmospheric and astronomicsciences.ENNETT, AaIESP. 1982. Marine Geology. Prentice-Hall,

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Oceanography, Chapter 2 Ocean Basins.pdf