Oceans &Coastlines

IntroductionDepth of the Ocean FloorSalinity & Temperature of the OceansOceanic CirculationCoastlinesWave ActionShorelines & the Sediment BudgetShoreline ProtectionSummary

The oceans are the planet's last great living wilderness, man's onlyremaining frontier on earth, and perhaps his last chance to prove himself arational species.

John L.Culliney

[The coastal zone is] rich in a variety of natural, commercial, recreational,industrial, and aesthetic resources of immediate and potential value to thepresent and future well-being of the nation.

Coastal Zone Management Act

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Introduction• Over two-thirds of Earth’s surface is ocean.• Global climate patterns are influenced by oceanic

circulation which in turn is controlled by the physicalcharacteristics of the oceans.

• The evolution of coastal landforms depends on theinteraction of wave action with processes in the continentalinterior and human actions.

Over two-thirds (~71%) of the planet is covered in seawater.The world’s oceans are unevenly distributed and lie mostly inthe Southern Hemisphere (88% seawater). The three majoroceans (Pacific, Atlantic, Indian Oceans) are connectedtogether along their southern margins by the Southern Oceanthat encircles Antarctica. Smaller, enclosed or partiallyenclosed water bodies are termed seas or gulfs (Fig. 1). All ofthese features are created by plate tectonic processes andmany continue to increase or decrease in size as plates divergeor converge (see the Plate Tectonics chapter for more on platetectonics and the seafloor).

It is in the various small seas that we can observe the mostobvious negative consequences of human activity on the oceanrealm in the form of overfishing, pollution, eutrophication, andhabitat destruction. These same consequences are visited on themargins of the major oceans.

Figure 1. Thedistribution of theworld's oceansand selected seas.Nearly 90% of theSouthernHemisphere iscovered byoceans.

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Today we appreciate the oceans for their direct and indirectimpacts on human activity. This chapter is divided into twohalves. The first part considers the physical characteristics ofthe oceans and begins with the depth of the ocean floor. Thefloor of the ocean is a few hundred meters below sea levelalong the margins of the continents but averages over 3 kmdepth between the major oceans and reaches a maximum depthof 11 km in the western Pacific. The ocean floor has thepotential to be a great source of mineral wealth in the futureand mining companies are already staking claims to potentiallylucrative sections of subsea real estate.

Ocean currents are influenced not only by the extent and depthof the oceans but also the salinity and temperature of oceanwaters. Ocean waters are thought to have originated frommeteorites and icy comets colliding with the early Earth. Bothtemperature and salinity change with depth and latitude andeach is influenced by atmospheric weather patterns. Eachfactor is examined in the section titled salinity andtemperature of the oceans.

The final section of the first half of the chapter examines howdepth, salinity, and temperature combine to generatecharacteristic patterns of oceanic circulation. Surface currentsare controlled by dominant wind patterns that are in turn linkedto the rotation of the planet. Deepwater circulation patterns arecontrolled by the density of ocean waters and the distributionof landmasses. Both sets of currents redistribute the Earth'sheat budget and play a crucial role in controlling climate.

In the second half of the chapter we focus on how humanactivity impacts the ocean margins and enclosed seas.Coastlines represent the fragile strip of land that borders theocean. Developed coastal areas are threatened with potentialloss of life and billions of dollars in property damage as a resultof storm impacts and long-term erosion. For exampleHurricane Fran (Fig. 2) devastated parts of several easternFigure 2. Damage

from HurricaneFran along thecoast of NorthCarolina. Notechange in positionof house indicatedby arrow. Imagescourtesy of USGSRecent Highlights-Hazards.

Average oceanarea and depth:Pacific Ocean

165,250,000 km2

4.28 kmIndian Ocean

73,440,000 km2

3.89 kmAtlantic Ocean82,440,000 km2

3.33 km

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states and was especially destructive in North Carolina wherewinds of over 100 mph generated more than $5 billion indamages.

The evolution of landforms along a coastline depends upon theinteraction of wave action with the shore with processes in thecontinental interior and human actions. The coastline is adynamic environment that advances or retreats depending uponthe balance between the supply of sediment and the materialremoved by erosion. The section on shorelines and thesediment budget examines this balance.

The National Park Service completed the tricky task ofrelocating the historical Cape Hatteras lighthouse to a sitefurther inland in July 1999, to protect the light from erosionthat threatened to topple the structure. Twenty-six of the thirtystates bordering an ocean or Great Lake are presentlyexperiencing net loss of their shorelines. Well-intentionedefforts at shoreline protection often resulted in theconstruction of coastal structures such as jetties, groins, orbreakwaters that altered the natural movement of sedimentalong the coastline and simply exacerbated existing erosionproblems. We finish by examining the interaction of shorelineerosion and shoreline protection efforts along the southernshore of Lake Erie.

Depth of the Ocean Floor• The average depth of the ocean floor is nearly 4 km and the

maximum depth is a little over 11 km along the MarianaTrench.

• Four principal depth zones can be identified in the oceans;continental shelf (and rise), abyssal plain, oceanic ridge,and oceanic trench.

• The oceanic ridge is a submarine mountain range thatoccupies much of the floor of the Atlantic Ocean.

The depth of the ocean floor varies from sea level to amaximum of over 11 km along the Mariana Trench in thewestern Pacific Ocean. In contrast, the highest landform on thecontinents is Mt. Everest, which is approximately 9 km (5.6

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miles) in elevation. In addition, the average elevation of theland surface is less than a kilometer but the average depth ofthe oceans is approximately 3.8 km (2.3 miles). We coulddump the continents in the ocean basins and still have plenty ofroom to spare.

Recent analysis of originally classified satellite data at theNational Oceanographic and Atmospheric Administration's(NOAA) National Geophysical Data Center (NGDC) hasallowed scientists to use slight variations in the elevation of theocean surface to determine the topography of the seafloor (Fig.3).

Topography of the Ocean FloorBeginning at the edge of the continents we can recognize fourprincipal depth zones in the oceans (Fig. 4). The first depthlevel is the continental shelf, the shallow ocean floor (0-150meters) immediately adjacent to continental land masses. Theshelf slopes gently toward the ocean from the coast withmaximum depths of a few hundred meters. The shelf may be arelatively wide zone (hundreds of kilometers) adjacent topassive margins (e.g., U.S. Atlantic Coast) or a narrow strip

Figure 3.Characteristics ofthe seafloor for theeastern Pacific andnorthwesternAtlantic Oceanbasins. Map ofseafloor topographyfrom satellitealtimetry fromNOAA's NationalGeophysical DataCenter.

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(tens of kilometers) landward of subduction zones (e.g., westcoast of South America).

The width of the shelf increases when sea level rises anddecreases during times of sea level decline. Rapid seafloorspreading associated during the Early Cretaceous period (138-100 million years ago) caused an expansion of the oceanicridge system, displacing water and raising sea level. Incontrast, sea level declined during the most recent Ice Age (2million -10,000 years ago) when substantial volume of oceanwater was locked up in ice sheets. The Atlantic shore of theNorth American continent was located near the present edge ofthe continental shelf during the Ice Age.

Beyond the shelf break the ocean floor steps down across thecontinental slope and continental rise oceanward of the shelf.The slope and rise mark the transition from the relativelyshallow shelf to the second depth level, the deep ocean floorknown as the abyssal plain (Figs. 4, 5). As its name suggests,the continental slope represents an increase in the gradient ofthe ocean floor from the adjoining shelf. The slope is oftendissected by submarine canyons formed during times of lowersea level. The canyons transported sediment from the shelf tothe continental rise. The rise is that section of the slope that hasbeen a site of sediment accumulation, resulting in an decreasein slope gradient toward the abyssal plain. There is anunbroken transition from slope to rise, to abyssal plain alongpassive margins where the continental and oceanic crust makeup part of the same plate. These margins are not characterizedby the volcanism and earthquake activity that distinguishesactive margins located along plate boundaries.

Much of the abyssal plain lies over four kilometers below theocean surface and represents the flattest portions of the Earth'ssurface. The plains are covered by layers of sedimentprecipitated in the oceans and are dotted with submarinevolcanoes (seamounts). The ocean floor rises to a third levelapproaching the oceanic ridge system, a submarine mountain

Figure 4.Diagrammatic viewof the principalfeatures of theocean floor includean elevatedoceanic ridge;deep, narrowtrenches; and agradual rise of theocean floor to thecontinents alongpassive margins.Note narrow shelfalong activemargin, broadshelf along passivemargin. Verticaland horizontaldimensions ofsome elementshave beenexaggerated.

chain that can be traced around the world. The ocean floor isrelatively shallow (less than 3 km) along the ridge system. Theocean ridge system dominates the floor of the Atlantic Ocean,occupying over half its width.

The final depth level is apparent in the narrow oceanictrenches found along active margins marked by the boundarybetween two plates (see Plate Tectonics chapter). The trenchesmark the locations of subduction zones where oceaniclithosphere descends into the mantle. Trenches, the deepestareas on the ocean floor, record depths of 7 to 11 kilometers (4-7 miles).

Figure 5. Principaltopographicfeatures of thefloor of thesouthern AtlanticOcean. Theoceanic ridgeoccupies morethan half the widthof the ocean floor.Image modified fromoriginal at NOAA'sNationalGeophysical DataCenter.

Think about it . . .1. Examine the maps and diagrams of ocean floor

topography at the end of the chapter and answer therelated questions.

2. Label as many features as you can in the image of theocean floor adjacent to Monterey Bay, California, foundat the end of the chapter.

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Salinity and Temperature of the Oceans• Salinity in surface waters is controlled by currents and

temperature and averages approximately 35 parts perthousand.

• Salinity values are relatively uniform in well-mixed surfacewaters of the open oceans but are more extreme inrestricted waters of coastal seas.

• Salinity and temperature change with depth with the mostrapid change occurring in a depth zone labeled thehalocline.

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• Ocean temperatures of 27oC are typical of tropical surfacewaters and temperatures of 2oC are typical for deep oceanwaters.

• Cold water is more dense than warm water, but ice (frozensolid water) is less dense than liquid water.

Salinity and LatitudeSeawater contains dissolved salts. The concentration of salt inseawater is salinity. Salinity varies around the world's oceansdepending on temperature and the mixing action of oceancurrents. Salinity is measured in parts per thousand (ppt; 10ppt = 1%) of salt in water. The salinity of the warm, well-mixed surface waters over much of the world's major oceanbasins ranges from 33 to 37 parts per thousand (Fig. 6).

Higher- and lower-salinity values are observed in smaller,restricted ocean basins and seas (Fig. 7). For example, salinityvalues of 20 to 30 ppt are recorded for the high-latitude ArcticOcean and values of over 40 ppt occur in the narrow tropicalRed Sea basin between north Africa and the Arabianpeninsula. Salinity is higher at low latitudes because hightemperatures at these locations promote evaporation whichremoves water but leaves the salt it contains behind. Salinityvalues are lower at high latitudes because of the lack ofevaporation, high precipitation, and the influx of freshwaterfrom melting ice sheets. The isolated Baltic Sea betweenSweden and Finland has salinity values that approachfreshwater along its northern shore.

Figure 6. Map ofsalinity at theocean surface.Numbersrepresent salinityvalues in parts perthousand. Salinityin the open oceanis greatest intropical regionsand decreases inthe isolated ArcticOcean. Mapgenerated atUniversity of Tokyowebsite.

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Salinity and DepthSalinity values are variable in the shallow (e.g. 0-200 m) oceanbut are much more uniform in deeper waters below 2,000meters (6,600 feet; Fig. 8). Salinity may decrease with depth inthe tropics but increases with depth at high latitudes (+60oN/S).The salinity in the Arctic Ocean (north of 70oN latitude)increases with depth from 30 to 35 ppt. Salinity in thisrelatively isolated ocean basin remains uniform below a depthof approximately 300 meters (1,000 feet).

The change of salinity occurs over a depth zone known as thehalocline. The depth range for the halocline is fromapproximately 200 to 1,000 meters (660-3,300 feet) but willshow some variation with location. Salinity is uniform with avalue of 34 to 35 ppt below the halocline.

Temperature and LatitudeSolar radiation strikes Earth more directly at the equator andtropics than in polar regions (Fig. 9). Radiation strikes Earth ata lower angle near the poles and the Sun’s rays must thereforepenetrate a greater thickness of atmosphere. Some of the solarradiation is scattered in the atmosphere and more heat energy islost near the poles as a result of scattering. Earth's surface at

Figure 7. High andlow salinity inrestricted seas inlow (Red Sea) andhigh (Baltic Sea)latitudes. Mapsgenerated atUniversity of Tokyowebsite.

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the equator receives 2.5 times more insolation, incoming solarradiation, than the atmosphere above the poles. The highestaverage annual ocean temperatures (~27oC) are present alongthe equator and temperatures decrease symmetrically to thenorth and south approaching 0oC at high latitudes (Fig. 10).

Water has two relatively unusual thermal properties that

make the oceans a great storage reservoir for heat energy andcontribute to global oceanic circulation patterns. First, the heatcapacity of a material is measured as the amount of heatrequired (in calories) to raise the temperature of 1 gram of thesubstance by 1oC. Materials with high heat capacity, such aswater, can absorb substantial quantities of heat without anysignificant change in temperature. The ability of the oceans tostore heat plays a crucial role in controlling global climatepatterns.

Figure 8. North-south profilethrough the PacificOcean along the155.5 meridianillustrating therange of salinitywith depth andlatitude. Numbersrepresent salinityvalues in parts perthousand. Crosssection generated atUniversity of Tokyowebsite.

Figure 9. Solarradiation isdistributed over awider area andmust penetrate agreater thicknessof atmosphere atthe poles, reducingthe amount ofsolar energyreaching Earth'ssurface.Consequently,oceantemperatures aregreater near theequator.

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Second, cold water can be both less dense and more dense thanwarm water. Water density increases as water temperaturedecreases down to approximately 4oC. Below that temperaturewater density decreases, especially when water changes statefrom a liquid to solid (ice) form. Consequently, dense coldwater can sink below less dense warm water but ice will floaton the ocean's surface.

Temperature and DepthThe major oceans can be divided into layers of relatively warmwaters at shallow depths and cold waters at greater depths (Fig.11). Surface waters are warmed by solar radiation and currentscause thermal mixing that results in relatively uniformtemperature distributions by latitude. Sunlight doesn't penetratemore than a few hundred meters below the ocean surface andthe impact of current activity diminishes with depth.

Temperatures exceed 20oC over much of the tropical ocean'ssurface but decline to a chilly 2oC below 2,000 meters (6,600feet) depth. The depth zone in which temperature decreasesrapidly is known as a thermocline. The base of thethermocline is at a depth of approximately 1,000 meters (3,300feet).

Figure 10. Map ofworld's oceansillustrating theaverage annualrange oftemperature withlatitude. Numbersrepresenttemperature indegreesCelsius. Mapgenerated atUniversity of Tokyowebsite.

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Oceanic Circulation• Surface ocean currents are driven by winds and involve

only 10% of ocean waters.• Oceanic circulation patterns generate current systems

known as gyres.• Fast-flowing western boundary currents redistribute heat

from the relatively warm tropics to cooler high latitudes• The Coriolis effect is the name of the apparent deflection of

ocean currents or winds to the right of their course in theNorthern Hemisphere and to the left of their course in theSouthern Hemisphere.

Think about it . . .Use the data in the thermocline exercise at the end of thechapter to plot two ocean temperature profiles and answerthe questions that follow.

Figure 11. North-south profilethrough the PacificOcean along the155.5 meridianillustrating therange oftemperature withdepth andlatitude. Crosssection generated atUniversity of Tokyowebsite.

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• The global conveyer belt moves heat energy from thetropics to the poles in surface waters and transports coldwaters to warmer location by deep ocean circulation.

Ocean CurrentsOcean surface currents are mainly controlled by climate(temperature, winds) but are also influenced by the distributionof continents and Earth's rotation. Surface currents involveapproximately 10% of the world's ocean waters. Sea level ishigher at the equator because of thermal expansion of warmwaters and diminishes toward the poles. The contrast in theelevation of the ocean surface is about 15 cm (6 inches). In theabsence of winds, water would simply flow away from theequator ("downhill") under the influence of gravity. Windsblowing over the ocean exert a frictional drag on surface watersand are the principal force in controlling oceanic circulation.Ocean currents follow wind directions except where windblows onland. The continents represent barriers to currents,deflecting them to the north or south of their course (Fig. 12).

Global atmospheric circulation patterns generate circular oceancurrent systems known as gyres that are centered on 30 degreeslatitude in each of the major ocean basins (Fig. 12). Circulationof the gyres is clockwise in the Northern Hemisphere andcounterclockwise in the Southern Hemisphere. Surface watermight take several months to a few years to complete thecircuit of a gyre.

Figure 12.Distribution ofocean currents.Note circularpatterns (gyres)with clockwisepattern north ofequator andcounterclockwisepattern south ofequator.

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Westerly winds cause water to pile up along the western sidesof major oceans. These concentrations of surface watergenerate fast-flowing western boundary currents thatredistribute warm tropical waters toward the poles (Fig. 12).These currents (e.g., Gulf Stream, Kuroshio, Brazil) can bethought of as marine rivers, relatively narrow (less than 100 kmacross) water masses that flow at speeds of 100 to 200 km/dayfor thousands of kilometers. The Gulf Stream can transportover 50 million cubic meters of water per second, hundreds oftimes more water than the Amazon, the world's largest river.

In contrast, the eastern boundary currents (e.g., Canary,California, Peru) that complete the eastern leg of each gyre arewider, carry less water, and move more slowly. The Canarycurrent, nearly 1,000 km (625 miles) wide, carries just a thirdof the volume of water in the Gulf Stream and travels at tens ofkilometers per day.

Coriolis EffectCurrents are deflected to the right of their course in theNorthern Hemisphere and to the left of their course in theSouthern Hemisphere: this pattern is termed the Coriolis effect(Fig. 13).

To an observer on earth, the path of a north- or south-directedwind or ocean current will appear to be deflected. Note that thewind or current doesn’t actually change direction, but theplanet beneath it has changed position. An object (rocket, airmass, ocean current, etc.) that travels directly north or south inthe Northern Hemisphere appears to be deflected to theright of its course when viewed from a location on the solid

Figure 13. Objectson Earth's equatortravel further (andfaster) than objectsat higher latitudes.It is this contrast invelocity that resultsin the Corioliseffect. Objectsmoving north fromthe equator have agreater componentof eastward motionthan objects athigher latitudesand thus appear todeflect to the rightof their course.

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Earth's surface. Objects are deflected to the left of their coursein the Southern Hemisphere. The net result of thesedeflections is the circular path of ocean currents.

Global Ocean Conveyer BeltSurface ocean currents carry warm water away from theequator and toward the poles. Deeper currents are driven bycontrasts in water density and are dependent upon temperatureand salinity contrasts below 1,000 meters. The pattern of deepcurrents is termed thermohaline circulation.

Currents in the North Atlantic cool as they approach thenorthern latitudes. Cold, salty (dense) water sinks in the NorthAtlantic Ocean south of Greenland and moves southward as theNorth Atlantic Deep Water (NADW) current at depths of 2 to 4km (1-2.5 miles; Fig. 14). When the NADW reaches Antarcticait is diverted to the Indian and Pacific Oceans by the Antarcticcircumpolar current. The deep water current eventually comesto the surface (upwelling) in the northern Indian and PacificOceans before returning to the Atlantic Ocean by a series ofsurface currents (Fig. 14). A complete loop may take 1,000years.

The sinking of this cold, dense water in the North Atlantic is akey step in the global conveyer belt. This system movesenergy from the tropics to the poles and back again and servesto moderate Earth's climate.

Figure 14. Globaloceanic circulation.Cold water sinks innorthern AtlanticOcean and travelssouthward in deepwater beforeupwelling in theIndian and PacificOceans. Surfacecurrents returnwarm water toAtlantic Ocean.

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Coastlines• Over a quarter of the U.S. population lives in counties

along the Atlantic or Gulf Coasts.• Ten hurricanes have inflicted over a billion dollars of

damages on sites along the East Coast since 1980 andHurricane Andrew was the most expensive natural disasterin U.S. history.

Think about it . . .1. A shipment of rubber elephants falls overboard in the

northern Pacific Ocean at location A on the map below.What path do the elephants subsequently follow?

a) A to G to B to F to E to A c) A to G to C to E to Ab) A to E to C to G to A d) A to E to F to B to G to A

2. How would the deflection of ocean currents be altered inthe Northern Hemisphere if Earth’s rotation changeddirection from west to east to east to west? Complete thestatement below using one of the choices that follow.Ocean current directions would ____________ becausecurrents would be deflected to the _______________.

a) stay the same; right of their courseb) stay the same; left of their coursec) switch direction; right of their coursed) switch direction; left of their course

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• Coastal landforms are dependent on the interaction of waveaction with fluvial processes in the continental interior andhuman activity.

• Long-term processes such as climate cycles (hundreds ofyears) and tectonic history (thousands to millions of years)can raise or lower sea level or coastlines, respectively.

Author Henry Beston considered the sound of the ocean on ashore to be one of the three great elemental sounds of nature(the other two where the rain, and the wind in woods). Bestonshould have known, he built a small cabin and lived alone onCape Cod, Massachusetts, for a year, recording hisobservations of the changing coastline in a highly regardedbook, The Outermost House (1928).

Our culture places a premium on living along the fringe of thecontinent facing the ocean. Over 70 million people live incounties along the Atlantic and Gulf Coasts and coastalproperty values in Florida rose by over 50% in the last 10years. In an effort to ensure shoreline access to the public,Cape Hatteras, North Carolina, was designated (August 17,1937) the nation’s first national seashore. Other publicseashores followed, including Cape Cod (August 7, 1961). Theestablishment of Cape Cod represented an early effort to createparklands that were within easy access of millions ofAmericans.

Developed coastal areas are threatened with potential loss oflife and billions of dollars in property damage as a result ofstorm impacts and long-term erosion. The weather patterns thatbring balmy breezes on warm summer days can also generatedevastating hurricanes (Fig. 15). There were 30 U.S. weatherdisasters that resulted in over a billion dollars in damagesbetween 1980 to 1997. A third of these events were hurricanesor tropical storms that battered the Atlantic or Gulf Coast

Figure 15.Hurricane Franapproaches theU.S. coastline.Image from GoddardSpace FlightCenter's PublicPhotographic ImageRetrieval System.

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states. Hurricane Andrew (August 24, 1992) decimatedsouthern Florida and was the most expensive natural disaster inU.S. history (58 deaths, $30 billion damages). Many smallinsurance companies went bust following Andrew. State Farm,the nation’s largest insurance company, shelled out $3.7 billionin claims. Many in the insurance industry are nervous of afinancial meltdown following a cataclysmic hurricane thatgenerates damages similar to those of an unnamed storm thatkilled over 8,000 people in Galveston, Texas, September 8,1900.

The short-term evolution of landforms along a coastlinerepresents the interaction of wave action with fluvialprocesses in the continental interior and human activity. Thecoastline is a dynamic environment that advances or retreatsdepending upon the balance between the supply of sedimentand the material removed by wave erosion. This balance maybe upset by geologic processes that act at a variety of timescales. Seasonal variations in stream flow and storm activityaffect the volume of sediment supplied to the coast and the rateof erosion. Climate cycles that result in increasing ordecreasing sea levels will have long-term effects measured indecades or centuries. Finally, tectonic cycles measured inhundreds or thousands of years may continually revitalizerugged coastlines by periodic uplifts. The role plate tectonicsplays in influencing the physical character of the coastline isexemplified by the contrast between the sandy beaches of theAtlantic shore (passive margin) and the rocky headlands of theactive margin represented by the Pacific Coast (Fig. 16).

Figure 16. Rockycoastline withheadlands andbeachescharacteristic ofU.S. West Coast(top) and sandycoastline with low-lying beachestypical of the EastCoast (bottom).Images courtesy ofNOAA photocollection.

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Some areas in the Gulf of Mexico coastline are activelysubsiding. Sediment deposited in a delta at the mouth of theMississippi River is submerged below sea level duringcompaction. Subsidence rates are approximately 1 meter percentury. In the past this subsidence was compensated byadditional sediment supplied during flood events. However, theconstruction of levees along the river’s channel prevents theredistribution of sediment during flooding.

Sea level has fluctuated considerably during the geologicalpast. It was ~100 m (330 feet) lower during the last ice agewhen some of the water present in today’s oceans was lockedup in ice sheets (Fig. 17). Today sea level is believed to beincreasing at a rate of 10 to 15 cm (4-6 inches) per century inresponse to the global warming which causes melting of icesheets and thermal expansion of ocean waters. Increasing sealevels will have a much more significant impact on the low-lying East Coast than the rugged West Coast. Some islands inChesapeake Bay, between parts of Virginia and Maryland, thatwere inhabited by colonists four centuries ago are nowsubmerged below the waters of the estuary.

Figure 17. Highand low locationsof the Atlanticcoastline over thelast 5 million years.Image from USGSCoasts in Crisis.

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Wave Action• Wave size, speed, and direction are controlled by winds.• The wave shape moves but the water within it does not

travel.• Wavelength is the distance between adjacent waves.• Waves increase height and break along the shore when the

depth of water diminishes to less than the wave base.• Waves are refracted toward headlands.

Wave MotionShoreline erosion is tied to the interaction between waves andthe coastline. Wave action erodes, transports, and redistributessediment along the shoreline. Wave size, speed, and directionare controlled by winds. Water does not travel with waves butsimply moves vertically, tracing a circular path as a wavepasses. It is the shape of the wave (the waveform) that movesacross the ocean surface, not the water itself (Fig. 18).Consider the "wave" performed by a crowd at a sporting event.The wave passes around the stadium as each individual in turnstands up and sits down. The people in the stadium play therole of the water particles in ocean waves.

The distance between adjacent wave crests is termed thewavelength. Wave motion only affects the surface waters.Wave motion decreases downward, with increasing distancefrom the winds, to the wave base. The depth of the wave baseis approximately half the wavelength (Fig. 19).

Figure 18.Sequential wavemotion in openwater. Thewaveform moveswhile waterparticles follow acircular path andremain in place.

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Waves and CoastlinesWater piles up as waves approach shore because of the effectof friction between the wave and the seafloor above the wavebase (Fig. 20). Water in contact with the seafloor is slowed byfriction but water on the surface is unaffected and movesforward more rapidly, encroaching on preceding waves andpiling up to form taller, steeper waves.

The wave eventually collapses (breaks) forming surf thatwashes up the shore before flowing back down to sea.Turbulent flow in the surf zone, between the line of breakingwaves and the shore, can cause erosion by washing sandparticles from beaches and breaking rocks from headlands.Some of this material may also be transported along the shoreby currents in the surf zone.

Waves approaching a coastline are reoriented to follow theslope of the seafloor, this process is termed wave refraction.The seafloor will shallow more rapidly toward headlands thanadjoining bays and waves approaching a rugged coastline willbe refracted toward the resistant headlands (Fig. 21). One resultof this pattern is that wave erosion is concentrated onheadlands while adjoining bays become areas of deposition.Sediment eroded from the headlands are deposited in therelatively quiet waters of the bays to form beaches. Thecoastline is straightened as erosion wears away the headlandsand the bays are filled with sediment (Fig. 21).

Figure 20. Wavessteepen andwavelengthdecreases aswaves approachshore. Steepenedwaves eventuallycollapse (break)forming surf thatsurges up theslope of the shore.

Figure 19. Waterparticle motiondecreasesdownward fromwaves on theocean surfaceending at thewave base.

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Renewed tectonic activity may result in uplift of the coastlineand another cycle of erosion. The rugged coastline of Oregonand Washington is constantly revitalized as part of an activeplate boundary separating North America and the small Juan deFuca Plate. The coastline is dominated by rocky headlandsseparated by small (pocket) beaches.

Wave erosion associated with large storms can remove largesections of beach in a single storm (Fig. 22). Recent hurricaneson the East Coast and El Nino-induced storms along the WestCoast were responsible for substantial coastal erosion.

Figure 21. Waveerosion isconcentrated onheadlands (top)and bays becomeareas of deposition(left). The coastlineis straightened(bottom right) aserosion continuesas the headlandsare eroded backand the bays arefilled withsediment.

Figure 22. Beacherosion near SanDiego, California,following winterstorms (1997)associated with theEl Nino weatherphenomenon. Topimage taken inOctober 1997;bottom image fromApril 1998. Imagescourtesy of USGSCenter for CoastalGeology.

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Shorelines and the Sediment Budget• Twenty-six of the thirty states bordering an ocean or Great

Lake are presently experiencing net loss of their shorelines.• Waves that strike a beach at an angle will generate a

longshore current that transports sediment parallel to thebeach in the surf zone.

• The sediment budget is the balance between material addedto the shore by deposition and the material removed byerosion.

• Sediment entering the shoreline system is frequentlyreduced by human activity (dams, development) and is alsosubject to changing natural conditions (drought).

Shoreline ErosionThe short-term evolution of landforms along a coastlinerepresents the interaction of wave action with fluvialprocesses in the continental interior, and human activity. Thecoastline is a dynamic environment that advances or retreatsdepending upon the balance between the supply of sedimentand the material removed by wave erosion. Twenty-six of thethirty states bordering an ocean or Great Lake are presentlyexperiencing net loss of their shorelines (Fig. 23). The most

Figure 23. Coastalerosion rates alongthe Atlantic, Gulf,and Great Lakesshorelines. Imagemodified fromoriginal in Coasts inCrisis, an on-lineUSGS publication.

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rapid erosion rates are along the Gulf Coast (Louisiana) andAtlantic shore (South Carolina, Maryland, New Jersey).

Erosion rates in Louisiana are as high 20 meters per year.Much of the coastline of Louisiana is represented by sedimentdeposited in a delta at the mouth of the Mississippi River. Thedelta is submerged below sea level as the sediment becomescompacted. Subsidence rates are approximately 1 meter percentury. In the past this subsidence was compensated byadditional sediment supplied during flood events. However, theconstruction of flood control levees along the river’s channelacross the delta has robbed the delta of its primary source ofsediment.

Erosion is accelerated by the actions of storm surges, violentwaves associated with large storm events. Erosion is mosteffective on unconsolidated sediments of beaches or dunes.Rocky shorelines are less susceptible to erosion. Hurricanesgenerate waves that can destroy eastern beaches in a matter ofhours. West Coast erosion (Fig. 24) is particularly severe inassociation with winter storms, especially those occurringduring an El Nino year when sea level is higher than normal.Typical erosion rates for part of the California coastline nearSan Francisco are 0.2 meters per year. Yet, winter stormsduring 1997-1998 caused some cliff lines to recede over 10meters (33 feet), more than 50 times faster than normal.

Figure 24.Retreatingshorelines atPacifica (top) andMonterey Bay(bottom left),California. Twelvehomes werecondemned whenthe cliff at Pacificaretreated 10 metersbecause of winterstorms (1997-1998).The graph illustratesthe magnitude ofshoreline retreat (15meters) at theMonterey Bayexample. Lowerimage and originalgraph courtesy ofUSGS Center forCoastal Geology;upper image fromNASA AirborneTopographic Mappersite describing WestCoast projects.

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Sediment TransportWave action causes the erosion, transportation and re-deposition of sand along the Atlantic shore. In thisenvironment, the relative orientation of the waves to the beachcontrols the distribution of erosion and deposition. Waves thatstrike a beach at an angle (Fig. 25) will generate a longshorecurrent that transports sediment parallel to the beach in thesurf zone (Fig. 26). In addition, sediment on the beach is alsotransported laterally parallel to the shoreline. Sediment iscarried up the beach parallel to the direction of wave motion.Water washes back down slope carrying the sediment parallelto the slope of the beach. If the waves strike the beachobliquely the sediment is transported along the beach in azigzag pattern. Both mechanisms ensure that sediment istransported along the length of the beach (Figs. 26, 27).

Figure 25. Wavesstrike aWashington beachobliquely,generating alongshore currentthat transferssediment from leftto right along theshore. Imagecourtesy of USGSCenter for CoastalGeology.

Figure 26.Longshorecurrents aregenerated in thesurf zone wherewaves strikeobliquely againstthe shoreline.Sand particles aremoved along thebeach in a zigzagpattern andsediment istransported alongshore.

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Sediment transported along the beach can give rise to somecharacteristic landforms when it is eventually deposited in calmwaters of adjoining bays (Fig. 28). Sediment may block theentrance to the mouth of a bay to form a baymouth bar or mayonly partially block a channel to form a landform termed aspit.

Sediment BudgetThe beach (or shoreline) is not the final resting place for thesediment. It is an intermediate stop on a longer journey.Sediment is transported to the coast by streams, redistributedalong the coast by longshore currents, and eventually depositedoffshore.

Shoreline processes are influenced by the sediment budget:the balance between material added to the shore by depositionand the material removed by erosion. Sediment added to theshore comes from headland erosion or is delivered to the coastby stream flow (Fig. 29). Much like a financial balance sheet,the sediment budget remains in a state of equilibrium as long asthe sediment coming in is equal to the material that is lost.However, the material entering the shoreline system is

Figure 27. Addition ofsand to a beach as aresult of longshorecurrents generatedduring winter storms,Tomales Bay, north ofPoint Reyes,California. Left: Priorto storms. Right: Afterstorms, sand wastransported along theshoreline from the leftside of the image.Images courtesy ofUSGS Center forCoastal Geology.

Figure 28. MatagordaBay, Texas. The bayhas been almostcutoff from the Gulf ofMexico by narrowbaymouth barsformed from sedimenttransported along thecoastline bylongshore currents.Image courtesy ofNASA's Earth fromSpace.

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frequently reduced by human activity and is also subject tochanging natural conditions (Fig. 29).

The construction of dams on major rivers will reduce thevolume of sediment reaching the coast, resulting in sedimentstarvation. Sediment that would once have been depositedalong the shoreline is trapped in upstream reservoirs. Damswithin the Mississippi River drainage basin have reducedsediment supply to the Mississippi delta by approximately half.Drought conditions may also reduce streamflow and thusdiminish sediment transported to the shore by streams. Coastaldevelopment may result in the construction of structuresdesigned to reduce erosion (e.g., breakwaters, seawalls) or tocontrol the local depositional patterns to prevent infilling ofnavigation channels (Fig. 29).

The loss of sediment from any of these sources may causelongshore currents to cannibalize existing beaches and cause ashort circuit in the natural cycle of shoreline erosion, transportand deposition. The net result is the loss of beaches andincreased erosion of the shoreline, especially during largestorms.

Figure 29.Sediment supplyalong a coastlinecan be disruptedby human activitysuch as dredgingor building jettiesto prevent in-fillingof bays or streamchannels; or byshorelinedevelopment thatresults in theconstruction ofseawalls orbreakwaters toprevent erosion(and sedimentproduction) thatthreatens homesand otherbuildings.

Think about it . . .Scientists surveyed the Californian coastline to evaluateerosion and/or accretion associated with winter storms of1997-1998. Two images of the coastline around Ventura,California, are presented at the end of the chapter thatshow the coastline before and after the storms. Answer therelated questions.

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Shoreline Protection• Structures built to protect coastlines may prevent erosion of

part of the shoreline but can result in accelerated erosionelsewhere.

• Seawalls, groins, and breakwaters differ in their locationsand orientations relative to the shoreline but all act toprevent erosion and/or encourage deposition.

• Artificial beach nourishment occurs when sand is dredgedand pumped onto the beach from offshore.

• Lake Erie is the shallowest of the Great Lakes and issurrounded by the large population centers along U.S. andCanadian shores.

• Most of Lake Erie's shoreline is eroding and erosion ratesare greatest where glacial deposits form the shoreline andare least where bedrock forms the coast.

• Presque Isle in the east basin of Lake Erie is erodingbecause of shoreline protection measures in the central andwest basins.

Techniques that attempt to prevent beach erosion revolvearound methods to limit the removal of sediment along specificareas of the coast or involve adding material to areasundergoing erosion. Unfortunately, nearly all these methodshave shortcomings. Most of these methods aim to preventerosion but some recent regulations have recognized thaterosion will inevitably occur and have instead focused oncontrolling construction adjacent to eroding coasts. Floridaintroduced strict regulations that required buildings constructednear the shoreline to meet rigorous standards to preventdestruction from storm surges or high winds. No buildingsconstructed to these standards failed when Hurricane Opelstruck southern Florida in 1995. In contrast, 56% of all otherhabitable buildings in the storm's path were heavily damaged.

SeawallsSea walls are built to protect shoreline property owners fromreceding shorelines (Fig. 30). As such, they represent a barrierbetween waves and the shoreline. Waves are reflected backfrom the walls onto the adjoining beach and may promotebeach erosion. Unfortunately, erosion is often exaggeratedwhere the seawall ends, causing the shoreline to recede morerapidly on either side of the structure.

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GroinsGroins are wall-like structures built along beaches to act asbarriers to longshore currents (Fig. 31). A longshore currentwill lose velocity as it meets the groins, causing the current todeposit part of its sediment load on the upcurrent side of thegroin, thus building up the adjacent beach. However, as thecurrent passes the groin it picks up additional sediment on thedowncurrent side of the structure causing local erosion.

BreakwatersBreakwaters are barriers built offshore to protect part of theshoreline (Fig. 32). They act as obstacles to waves, preventingerosion and allowing the beach to grow behind the structure.However, the beach behind the breakwater often grows at theexpense of the adjacent unprotected shoreline.

Artificial Beach NourishmentArtificial beach nourishment occurs when sand is dredged andpumped onto the beach from offshore (Fig. 33). The beach will

Figure 30. Seawall(left) at base oferoding cliff, northof Monterey,California. Notehow erosion isexaggeratedwhere the seawallends. Imagecourtesy of USGSCenter for CoastalGeology.

Figure 31. A groinadjacent to CapeHatteraslighthouse, NorthCarolina, prior torelocation oflighthouse (1999).Image courtesy ofUSGS Center forCoastal Geology.

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grow if material is added to the beach faster than naturalprocesses remove it. This is a temporary fix because the sand iseroded again and must be replaced. Material added to manyEast Coast beaches remained for less than two years before thebeach returned to its prenourishment state. One successfuleffort was for Miami Beach, Florida, which spent $64 millionin the 1970s to stabilize and expand its beaches to meet theneeds of the booming tourism industry.

Example: Lake ErieLake Erie is the shallowest of the Great Lakes and was formedwhen glaciers scoured out a depression in the bedrock duringthe last Ice Age. The lake is the 11th largest in the world andrepresents the "North coast" of Ohio, Pennsylvania, andwestern New York. It covers over 26,000 km2 and has amaximum depth of 64 meters (Fig. 34). The lake is divisibleinto three separate basins that increase in depth from west to

Figure 32.Breakwaters,south shore ofLake Erie,Maumee Bay StatePark, Ohio. Imagecourtesy of the U.S.Army Corps ofEngineers.

Figure 33. Beachnourishmentproject on OceanCity Beach,Maryland. Notewider beach nearbottom of image.Pipeline pumpssand collectedoffshore by adredge ontobeach. Imagescourtesy of the U.S.Army Corps ofEngineers DigitalImages Library.

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east. Regional flow in the lake carries water from west to eastbut local currents may reverse that direction.

Ninety-five percent of the lake's shoreline in Ohio is eroding(Fig. 35). Average erosion rates are 10 to 80 cm per year (0.4-2.7 feet per year) but rates of up to 33 meters (100 feet) peryear have been recorded. Rates are largely controlled by thegeology of the coastline. More resistant rocks such assandstone erode slowly whereas glacial sediments and weakerrocks erode more rapidly. Economic losses from damages tostructures are estimated to be millions of dollars per year.

Erosion of the shorelines of Lake Erie is evident in the imagebelow that shows how the coastline receded southward,eroding the land along the northern edge of this subdivision.The white lines are roads. Notice how the east-west trendingroad near the center of the image is truncated by the cliff.

Further information on the costs and effects of coastal erosionin Ohio are provided at the Ohio Geological Survey website.Development around the Great Lakes has covered much of theland area, reducing sediment sources. Population around Lake

Figure 34. Map ofLake Erie. The mapwas modified from abathymmetry map atthe Great LakesForecasting Systemwebsite.

Figure 35. Belowright: Erosion ofthe southern shoreof Lake Erie atPainesville-on-the-Lake, Ohio. Notehow roads end attop of recedingcliff-line. Belowleft: The view fromthe lake (north)toward theshoreline (south).Image courtesy ofDr. Charles Carter.

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Erie alone has climbed from 3 million to over 14 millionpeople today.

Erosion control measures in the western half of the lake havereduced sediment supply and resulted in increased erosion ratesalong the shoreline in the east basin. Presque Isle is an unusualsand deposit that built outward from the Lake Erie shorelinenear Erie, Pennsylvania (Fig. 36). The construction of coastalstructures in Ohio to the west blocked the eastward flow ofsediment needed to replenish the deposit. The narrow neck thatconnects the island to the mainland is eroding as fast as 2.5meters per year. The U.S. Army Corps of Engineers has theresponsibility of coming up with a plan to protect Presque Isle.

Figure 36. Top:View of PresqueIsle, Pennsylvania,from the southwestlooking along theshore of Lake Erieto thenortheast. Erie,Pennsylvania, is tothe right of theimage. Currentdirections alongthe shoreline arefrom the bottom ofthe image towardthe top. Below:Breakwaters andgroins, PresqueIsle. Depositionoccurs behindbreakwaters thatform barriers toonshore currents.

Think about it . . .Create a concept map that illustrates the characteristics ofsediment erosion, deposition, and transport along theshoreline and the factors that affect these processes.

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Summary1. How much of Earth's surface is covered by oceans?Approximately 71% of the planet is covered by oceans. Thereare three major oceans (Indian, Pacific, Atlantic) that areconnected along their southern margins by the Southern Oceancircling Antarctica.

2. How does the depth of the oceans vary?The average depth of the ocean floor is nearly 4 km and amaximum depth of a little over 11 km has been recorded alongthe Mariana Trench in the western Pacific Ocean. Fourprincipal depth zones can be identified ranging from theshallow shelf along the continental margins (~100s meters), tothe near horizontal floor of the abyssal plain (4-5 km), rising tothe crest of the oceanic ridge (~3 km), and descending to thenarrow depths of the oceanic trenches (7-11 km).

3. What factors control variations in the salinity of theoceans?

Seawater contains dissolved salts. The concentration of salt inseawater is salinity. Salinity is measured in parts per thousand(ppt; 1 ppt = 0.1%, 10 ppt = 1%) of salt in water. Salinityvaries depending on temperature and the mixing action ofocean currents. Salinity is higher at low latitudes because hightemperatures at these locations promote evaporation whichremoves water but leaves the salt it contains behind. However,the mixing action of ocean currents ensures a consistentsalinity range of 33 to 37 parts per thousand for much of theopen ocean.

4. Where are salinity values highest and lowest?Salinity values are most extreme in restricted ocean basinswhere the effects of evaporation or stream inflow areexaggerated. Salinity values of over 40 ppt occur in the narrowtropical Red Sea basin between north Africa and the Arabianpeninsula. Salinity is lower at high latitudes because of the lackof evaporation, high precipitation, and the influx of freshwaterfrom melting ice sheets. Salinity values of less than 10 ppt arerecorded from the Baltic Sea in northern Europe.

5. How does salinity change with depth?Salinity increases with depth in the restricted northern oceanwaters (Arctic Ocean) but decreases slightly with depth in thetropical open ocean. Salinity is much more consistent at depth(2 km) below the halocline.

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6. What factors control the temperature of the oceans?Solar radiation is distributed over a wider area and mustpenetrate a greater thickness of atmosphere at the poles,reducing the amount of solar energy reaching Earth's surface.Consequently, ocean temperatures are greater near the equator.The highest ocean temperatures (~27oC) are present along theequator and temperatures decrease symmetrically to the northand south approaching 0oC at high latitudes.

7. How does temperature vary with depth?Temperature decreases significantly with depth. The effectsinsolation and the surface mixing of currents diminish withdepth. Temperature declines steadily to a depth ofapproximately 1,000 meters. Deeper waters have a uniformtemperature of 1 to 2oC.

8. What controls the direction of ocean currents?Winds generated by atmospheric circulation patterns representthe principal control on ocean currents but the distribution ofcontinents and the Coriolis effect also affect currents.Circulation patterns known as gyres control currents in theopen oceans. Currents form a clockwise pattern in gyres of theNorthern Hemisphere and a counterclockwise pattern south ofthe equator.

9. Which currents are important in global climate?Although all currents contribute to global climate patterns, thewestern boundary currents such as the Gulf Stream and Brazilcurrents have an especially significant role as they transportwarm tropical waters to higher latitudes.

10. What is the Coriolis effect?The Coriolis effect represents the deflection of currents to theright of their course in the Northern Hemisphere and to the leftof their course in the Southern Hemisphere. The Coriolis Effectresults from the contrast in the Earth's rotation velocity withlatitude.

11. What is thermohaline circulation?Thermohaline circulation occurs in deeper ocean waters and isdriven by density contrasts related to differences in watertemperature and salinity. Thermohaline circulation drives theglobal conveyer belt that causes surface waters to sink in thenorthern Atlantic Ocean and sends cold, deep currents through

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the world's oceans before upwelling in the northern Pacific andIndian Oceans.

12. What factors influence the development of coastallandforms?

The coastline is a dynamic environment that advances orretreats depending upon the balance between the supply ofsediment and the material removed by wave erosion. Seasonalvariations in stream flow and storm activity affect the volumeof sediment supplied to the coast and the rate of erosion.Climate cycles that result in increasing or decreasing sea levelswill have long-term effects measured in decades or centuries.Finally, tectonic cycles measured in hundreds or thousands ofyears may continually revitalize rugged coastlines by periodicuplifts.

13. How does water move in waves?The waveform is a shape that moves across the open ocean butthe water particles don't move with the wave but instead traceout a circular path while remaining essentially in place.

14. What happens when waves approach the coast?Material is eroded and redeposited by turbulent flow thatoccurs in the surf zone as waves break along the shoreline.Wave refraction results in wave action being concentrated onheadlands. Deposition occurs in the calmer waters of shelteredbays.

15. Where does coastal erosion occur?Coastal erosion occurs where erosion by wave action is notbalanced by local deposition of the eroded material and thesupply of sediment from streams. Erosion exceeds deposition,resulting in a loss of shoreline, along most of the U.S.coastline.

16. What is a longshore current?A longshore current is generated when waves strike the coast atan angle. The current forms in the surf zone and transportssediment laterally along the shoreline. Longshore currents canresult in the formation of characteristic depositional landformsparallel to the coastline.

17. What is the sediment budget?The sediment budget is the balance between material added tothe shore by deposition and the material removed by erosion.The construction of dams will reduce the volume of sediment

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reaching the coast. Drought conditions may also reducestreamflow and thus diminish sediment transported to the shoreby streams. Coastal development may result in the constructionof structures designed to reduce erosion (e.g., breakwaters,seawalls) or to control the local depositional patterns to preventinfilling of navigation channels (jetties).

18. Why are shoreline protection structures regarded as amixed blessing?

Seawalls, groins, and breakwaters act to prevent erosion and/orencourage deposition. However, structures built to protectcoastlines may prevent erosion of part of the shoreline but canresult in accelerated erosion elsewhere. Artificial beachnourishment occurs when sand is dredged and pumped onto thebeach from offshore but has a limited life span unless theprocesses that caused the original erosion are stopped.

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Image Analysis: Continental Margins1. The image below illustrates part of South America. The

ocean floor is represented by the blue colors. The deeperdepths are shown as darker blues. Label the followingfeatures on the image.

active margincontinental shelfabyssal plainAtlantic Ocean

passive margincontinental slopeoceanic trenchPacific Ocean

2. Label three active continental margins on the map belowwith the letter A and label three passive continentalmargins with a P.

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3. Which of the profile views below most accurately reflectsthe topography of the ocean floor along an east-west linefrom Australia to South America?

4. Sketch and label a profile of the ocean floor topographybetween North America and north Africa in the spacebelow.

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Image Analysis: Monterey Bay

The image below illustrates the topography of the ocean flooradjacent to Monterey Bay, California. The landforms ofCalifornia are shown on the top right side of the image in thegreen and gray colors. The blue colors represent the oceanfloor.

Label as many features as you can in the image.

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ThermoclinesThe thermocline marks a zone of relatively rapid temperaturechange between the warm surface currents and deeper coldwaters. This exercise will attempt to identify the depth range ofthis boundary by identifying the location of the thermocline.

1. Plot the data points from the table below on the graph onthe next page and sketch a best fit line for the data.

Depth (m) Data 1 Data 20 12 30100 12 24300 11 21500 9 171,000 8 131,500 7 102,000 6 93,000 5 8

2. These data come from subpolar and tropical oceans. Labelthe lines as subpolar and tropical.

3. Identify the approximate range of depths for thethermocline on each curve and label those parts of thecurves accordingly.

4. Circle the data points on the graph where you expect to findthe highest and lowest salinity values. Explain why youchose those points.

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Doing Science: Measuring Rates of CoastalErosion/AccretionScientists with the U.S. Geological Survey devised a plan toevaluate erosion (removal of material along the shoreline) oraccretion (addition of material) over 1,200 km (750 miles) ofshoreline following the winter storms of 1997 to 1998. Theyflew aerial surveys of the coastline before and after the stormseason and compared the images they acquired.

Two images of the coastline around Ventura, California, arepresented on page 43. One represents the coastline prior to thewinter storms and the other illustrates the state of the coastlineafter the storms. The accompanying graph shows the relativepositions of the beach before (blue squares) and after (redtriangles) the storms.

1. Examine the graph. What happened to the beach as a resultof the winter storms?

2. How much did the width of the beach increase/decreaseover the winter?

3. Assuming that these changes occurred over a six-monthperiod, the rapid rate of coastal erosion/accretionrepresented by the winter storms was approximately________ m/yr? (Note: this was an exceptional cycle oferosion/accretion and we shouldn’t consider such rates tobe “normal”).

a) 20 b) 50 c) 120 d) 200

4. Which photograph represents the beach in October 1997?

a) A b) B

A

x

x

x marksthesamelocationin eachimage

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B