Chapter 20 - Ocean and Coastal Systems

8/8/2019 Chapter 20 - Ocean and Coastal Systems

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Ocean and Coastal Systems 463

The Physical Environment: An Introduction to Physical Geography

CHAPTER 20: Ocean and Coastal Systems

Chimney Rock, Point Reyes, CASource: Michael Ritter

The oceans have a significant impact on the Earth system. Ocean water serves as a reservoirof heat, the source of energy to fuel hurricanes, and constantly sculpts the edges of thecontinents. Coasts are an interface between the lithosphere and hydrosphere. The erosional

and depositional work of ocean water creates spectacular landscapes and habitats for floraand fauna. In this chapter you will investigate the physical properties of oceans and theirimpact on the physical geography of Earth.


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Ocean and Coastal Systems Outline

y The Oceanso Oceans & Seaso The Ocean flooro The Nature of Ocean Water

y Water in Motiono Waves and water movemento Tideso tsunami

y Coastal Processes and landformso Erosional processes and landformso Depositional Processes and landforms

Beaches Spits and Bars Barrier Islands

y Types of Coastso

Submergedo Emergento Depositionalo Reefs

y Review


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Seas are salt-water bodies smaller than oceans and partially enclosed by land. Major seasinclude the Mediterranean, Baltic, Bering, Black, Caribbean, Coral, North, Red, and Yellow.Unlike inland lakes that can be salt or fresh water, there is a constant exchange of water

between ocean and sea. Several inland salt lakes like the Aral, Caspian, and Dead areerroneously named "seas".

The Ocean Floor

The deep-ocean floor has an average depth of 4000 meters (12,000 feet) with a significantvariation in the depth. The ocean basins are cut by deep trenches and large mountains risefrom the floor. The midocean ridge is the longest continuous chain of mountains on Earthwinding its way through all four oceans. Its width average 1600 kilometers (1000 mi) withridges rising an average of 1500 - 3000 meters ( 5,000 - 10,000 feet) above the floor. Alongits spine, new sea floor is created and spreads apart by lava pouring out of rifts along theocean ridge. Along the rugged volcanic topography of the ridge one finds dramatic underseavolcanoes called "black smokers".

Long narrow arc-shaped trenches cut across the deepest parts of the ocean floor. Trenchesare found near active plate margins where earthquakes and volcanoes are common.Converging plates in subduction zones recycle rock along the ocean trenches. Ocean-flooringdrilling along trenches and the midocean ridge indicates that rock is recycled over a period of2 to 3 million years. The deepest trenches are found along the Pacific "Ring of Fire".

Trenches are common along curved island arcs like the Aleutian Islands.

The vast submarine plains of the "ocean deep" are the abyssal plains. The abyssal plainscover 40% of the ocean floor with depths of 3000 - 6000 meters (10,000 - 20,000 ft). Winddeposition and volcanic eruptions have covered most in layers of brown and red clays. Someare covered with the remains of microscopic marine organisms known asooze. Thecelebrated White Cliffs ofDover are uplifted ooze deposits.

Figure OC.2 Features of the ocean floor.(Courtesy Naval Meteorology Program and Oceanography Command "The Underwater

World")

Continental Margins

The continental margins are comprised of three topographic features. The ocean floor slopesgradually upward toward the continents forming the continental rise. Approaching the edge


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of the continent the slope of the ocean floor steepens into thecontinental slope. At the top ofthe slope lies the continental shelf, a gently sloping platform of varying width.Alongpassive margins like the east coast of the United States, the continental shelf is wide.The continental shelf is much narrower alongactive margins bordered by mountains like thatfound along the west coast of the United States.

Figure OC.3 Topography of continental marginsCourtesy USGS Earthquakes Hazards Program - Southern California

(Source)

The Nature of Ocean Water

Sea water makes up the largest store of water in the hydrologic cycle. It is comprised ofnearly 60 chemical substances with common salt being the most abundant, 78% of the

dissolved solids. Ocean salinity varies from 32 - 37 parts per thousand. Salinity is lower nearland and in the polar regions (30 ppt).

Figure OC.4 Controls over ocean

salinity.(Courtesy Naval MeteorologyProgram and Oceanography

Command "Seawater, Sound & Ice")

A number of factors determine ocean salinity. Salinity is lower where freshwater rivers enterthe ocean. In the North Pacific precipitation exceeds evaporation thus diluting ocean water.

Water is more saline in subtropical oceans where evaporation exceeds precipitation. TheAtlantic ocean is the most salty while the Arctic ocean is the least.

The oceans exhibit three vertical temperature zones, 1) a surface layer of water water, 2) atransition zone of decreasing temperatures with depth, and 3) the cold waters of the deepocean. The zone of transition, known as the thermocline, is most noticeable where surfacewater is warmest. Polar water may have no thermocline as the surface temperature are verycold..


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Water in Motion

Waves and water movement

Waves are undulations in the surface of a water body. Most waves are created when kineticenergy is transferred to water by the frictional stress of wind blowing over it. The resultingtransfer causes a rise in water level producing a wave crest, followed by the sinking of thesurface creating a wave trough. The wave length is the distance between successive crests.The time required for successive crests to pass a point is the wave period.The waveheight is the distance between the crest of the wave and the still water level. Wave height isdetermined by (1) wind velocity, (2) duration of the wind, and (3) the fetch. The fetch is thedistance of uninterrupted flow over an open water surface. An increase in any these factorswill increase wave height and length. [See these effects using the "Savage Seas" WaveMachine. ]

Figure OC.5 WaveCharacteristics(Courtesy Naval MeteorologyProgram and Oceanography Command"Restless Sea")

The rise and fall ofoscillatory waves in an open water reflects the circular motion of waterparticles. There is relatively little forward motion by a water particle as a wave passes. It issimply the wave form and its energy that is transmitted across the ocean surface. Water

particles move in circular orbits that diminish with depth. The radius of the circular path isgreatest at the surface and decreases with toward the bottom of the wave. Larger wavesexhibit larger orbital radii and extend to a greater depth than smaller waves. At some point indeep water, the wave has no effect on the motion of the water. Thus a zone ofno wavemotion exists from the base of the wave to the ocean floor. "Observe an animation of wavemotion" (Courtesy NSF/TERC/McDougall Littell)

Swells are smooth, rounded waves that travel outward from a storm center or continue asbroad undulations of the ocean surface after the wind dies down. The wave slope isexpressed as the ratio of the wave height to wave length, ranging from 1:25 to 1:50. A wavewill become unstable at slopes greater than 1:7 and will fall over itself, orbreak.


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Figure OC.6 Plunging breakers.(Courtesy Naval Meteorology Program and OceanographyCommand "Restless Sea")

As a wave approaches the coast, a depth is reached offshore where the wave touches theocean floor. The tug of the ocean floor changes the circular wave motion into an ellipticalone; the water moves back and forth over the bottom as each wave passes. The frictionimparted from the floor slows the wave base. At a depth of 1.3 times the wave length, the

drag causes the top of the wave to rush forward, become unstable and break. Water in thebreaking wave is transported toward shore as a wave of translation.

Tides

Nearly all marine coastlines experience the rhythmic rise and fall of sea level calledtides. Thedaily oscillation in ocean level is a product of the gravitational attraction of the Moon andSun on Earth's oceans and varies in degree worldwide. Tidal action is an important force

behind coastal erosion and deposition as the shoreline migrates landward and seaward.

Causes of Tides

The gravitational attraction of the Sun is about half that of the of the Moon on the Earth.Gravitational attraction is a function of both the mass of the objects and the distance betweenthem. Even though the Moon is much smaller in mass than the Sun it is closer and thus has agreater influence on the Earth than does the Sun. The gravitational pull of the Moon and theSun stretches both solid and fluid surfaces of the Earth. This creates a tidal bulge in theatmosphere, the oceans and to a very slight extent the Earth's crust.

Figure OC. 7 The two forces that result in tidesCourtesy NOAA (Source)


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The Phys

calEnvironment:AnIntroductionto Physical Geography

Gravit i t t l rce responsi le for a ti al bul e.Inerti , t e tendency of movingobjects to continue moving in a straightline or stay motionless, also affects the tidal bulge.As the gravitational force draws water closerto the Moon the inertial force tries to keep itin

place. The tidal bulge forms as the gravitation force exceeds the inertial force on the nearside. The gravitational force ofthe far (opposite) side is less because itis farther away fromthe Moon. On this side, the inertial force exceeds the gravitational force. Here the waterattempts to keep going in a straightline, moving away from the Earth, creating another,smaller bulge. Thus tidal bulge, is greatest on the side ofthe Earth facing the Moon or Sun("near side") simply because it s closerthan the "far side" ofthe Earth.

Ti l rrents

Watching the tide "come in" one gets the impression that ocean wateris moving in and outalong the shoreline. The landward and seaward movement are a resultthe Earth rotating intoand out of a semi fixed tidal bulge as it changes its position relative to the Sun and Moon.Any point on Earth rotates through two bulges every 24 hours and 50 minutes producingtwo hi h tide and two l wtide called each day. The difference in height between

consecutive high and low tides is the ti l range. uring a high tide water moves landwardas a fl current. uring low tide water recedes seaward as anebb current. The two hightides and the two low tides do not have to be of equal height because the angle between theMoon and Earth changes each day. The tidal range is the difference in height between highand low tide

Figure OC.8 Flood and Ebb CurrentsCourtesy NOAA (Source)


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Spring and Neap Tides

The Sun and Moon are said to be inconjunction with they are aligned with the Earth. Thehighest tides, and greatest gravitational attraction, occur when the Sun and Moon are on thesame side of the Earth creating a spring tide. This occurs during the new moon phase. Asecond lower spring tide occurs when the Sun and Moon are on opposite sides of the Earthduring the full Moon phase. Aneap tide results when the Sun and moon are at right angles tothe Earth. There are two neap tides, at the first-quarter and second quarter phases of theMoon.

Figure OC.9 Spring and neap tides.(Courtesy Naval Meteorology Program andOceanography Command "Restless Sea")

Located on the northeast end of the Gulf of Maine between New Brunswick and Nova Scotia,

the Bay of Fundy is known for its high tidal range, as much as 48 feet (14 meters). Over a100 billions tons of water passes in and out of the bay every 12 1/2 hours each day. Watchthis amazing process occur below.

Earthquake-Generated Waves: Tsunami

A tsunami is a devastating water wave generated by an undersea earthquake. They arecommonly called "tidal waves", but are by no means created by the tides. The suddenslippage of the ocean floor near the source of an earthquake can send a train of seismic wavesacross the ocean. When the overriding plate along a subduction zone suddenly breaks free itmoves upward raising the sea floor and the water above. The waves move outward in ever-expanding circles, nearly imperceptible in deep ocean water. As they approach land, water

recedes from the shore. This dramatic action often entices the curious to investigate. But veryshortly, a water rises rapidly and rushes landward.


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The PhysicalEnvironment:AnIntroductionto Physical Geography

Figure OC.10 Formation of a tsunamiCourtesy USGS

Source: Surviving a Tsunami essons from Chile, Hawaii, and Japan,USGS Circular 1187 http://pubs.usgs.gov/circ/c1187/

Last accessed August 16, 2006

On ecember 26th, 2004 subduction between the Indian and Eurasian (more specifically theBurma) plates offthe coast offthe coast of Indonesia resulted in a magnitude 9 earthquake

and large tsunamithat devastated South Asia. [Watch "Violent Earth"fromNatio

al

Geo aphi .] The earthquake was a result of stresses being released when the Indian plateslid beneath the Burma plate. The resulting vertical movement ofthe ocean floor displacedhundreds of cubic meters of water and large waves propagated outward from the focus ofthequake. The massive tsunami devastated coastal regions of South Asia as it crashed ashore.

Coastal Landforms and Processes

A coastorthe coastal zoneis a dynamic region where land is sculpted and shaped by waveaction and currents. Barring the effects oftectonic uplift and sea level change, erosion is thedominate geomorphic process acting on coasts. Coastal sediments are subjectto multipleepisodes of erosion, transportation and deposition, though a net seaward transporttakes place

on a global scale. The deep ocean floor becomes the resting place forterrestrial sedimenteroded from the land.

The combined effect of waves, currents and tides resultin a variety of gradational processesacting in the coastal zone. Mostimportantis abrasion, caused by the scraping orimpact ofsediment carried by waterthrown against shore materials. Breakers are particularly effectiveatlifting larger rocks and hurling them againstthe shore.


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Hydraulic action caused by the direct impact of waves on the coast can be an effectivegeomorphic agent. Enormous pressures can build as water and air are compressed into rockfractures. Solution is locally important especially where soluble rock is exposed along theshore.

Figure OC.11 Longshore current in the surfzone.(Courtesy Naval Meteorology Program andOceanography Command "Restless Sea")

The water level in the surf zone increases as waves approach shore at an angle. The risingwater moves parallel to the shore as alongshore current. Beach drifting transports sandgrains along the beach as waves strike the shore at an oblique angle. Sediment is carriedlandward when water rushes across the beach as swash. Sediment is carried back toward theocean as backwash. The continual up rush and backwash carries sand in a zig-zag likemovement along the shore.

Depositional Coastal Landforms

Beaches

A beach is a deposit of loose sediment adjacent to a body of water. Though sand is commonto most beaches, a remarkable diversity of sediment size, from boulders to fine silt is foundon beaches around the world. Larger particles and steeper slopes are found where waveaction is high. Fine particles and gentle slopes are characteristic of beaches exposed to lowwave action.

OS.12 Black lava beach, Tahiti

Courtesy NOAA


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Most midlatitude beaches undergo a cycle of erosion and deposition following the seasonalchanges in wave action. During the winter, midlatitude storms are more vigorous producingmore wave action and erosion. Hence, beaches tend to narrow during the winter. Wave actionsubsides during the summer as storms weaken somewhat favoring deposition over erosionand producing broader beaches.

Spits and bars

A sand spitis one of the most common coastal landforms. A sand spit is a linear

accumulation of sediment that is attached to land at one end. Sand carried parallel to shore bylongshore drift may eventually extend across a bay or between headlands especially wherewater is relatively calm. Spits are typically elongate, narrow features built to several dozenfeet by wind and waves.

Figure OC.13 A barrier spit along the SouthCarolina coast. (Courtesy USGS)

Spits often form when wave energy decreases as a result of wave refraction in a bay. When a

coastline turns abruptly, wave energy is dissipated by divergence of wave trajectories,causing sediment to accumulate as the water loses its ability to transport.

Spits can extend across the mouth of a bay, but wave action is usually strong enough to washsand out to sea or be deposited in the embayment. They may curve into the bay or stretchacross connecting to the other side as a baymouthbar. When the bay is closed off by a bar it

becomes a lagoon.

Wave energy also dissipates in the lee of large sea stacks or islets. Wave refraction sweepssediment behind the obstruction from two directions, depositing it as a slender finger calleda tombolo.


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Figure OC.15 Sea cliffs withrocky headlands, sea caves,stacks and a pocket beach SanMateo County, California.Source:USGS ProfessionalPaper 1693

Sea caves form along lines of weakness in cohesive but well-jointed bedrock. Sea caves areprominent headlands where wave refraction attacks the shore.

Figure OC.16 Sea arch andstacks along Californiacoast. Source: Michael Ritter

A sea arch forms when sea caves merge from opposite sides of a headland. If the archcollapses, a pillar of rock remains behind as a sea stack.

Seaward of the retreating cliffs, wave erosion forms a broad erosional platform called a wave-cutbench orwave-cut platform.After the constant grinding and battering, eroded material is

transported to adjacent bays to become beaches or seaward coming to rest as a wave-builtterrace. If tectonic forces raise the bench above the water level a marine terrace forms.Some shorelines have several marine terraces creating during various episodes of uplift.


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Figure OC.17 Development of a wave cut notch. (Source:Wikipedia)

Coasts

The characteristics of a coast depend on its geologic structure, initial topographicconfiguration, and shoreline process that shape it. There are several ways and varying scalesone can classify these complex systems. Submerged coasts form when river mouths areflooded due to rising sea level or subsidence of land. A drop in sea level or rise in the landsurface creates an emergentcoast.Depositionalcoasts have abundant depositional featureslike deltas, bars, spits and reefs where new land has been built. Coral coasts are formed by

biological rather than physical processes.

Submerged Coasts

Ria coasts are formed by the submergence of river valleys emptying into the ocean."Ria" is aSpanish term for coasts with prominent headlands and embayments typical of these coasts.Wave action turns the smooth valley sides into receding cliffs with sandspits and tombolos common. Examples of ria coasts are found in New England and the

Atlantic coast of Europe, especially France.

Fjord (fiord ) coasts are form when glacial troughs are flooded due to a rise in sea level.Fiord coasts are deeply indented, with steep-walled valleys. Sandy beaches are rare assediment eroded from valley walls collects on the floor. Fjords are common in Scandinavia,British Columbia, Alaska, and Patagonia.


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Figure OC.17 Kenai Fjord National Park,

AlaskaCourtesy USGS

Emergent Coasts

Emergent coasts are a result of forces acting to raise the land surface or drop sea level. Theincredible weight of massive ice sheets during the the Pleistocene depressed the continentalsurfaces beneath them. The continents began to rebound as the ice melted and released theoverlying pressure. The rising surface lifted the shoreline above sea level forming glacialuplift coasts along continental margins.

Figure OC. 18 Wave-cut terraces on

San Clemente Island, California.Courtesy USGS (Source)

Raised shorelines and erosional features like wave cut terraces are also found along tectoniccoastswhere endogenic forces have uplifted the surface. Such coasts are common along themountain and island arcs of the Pacific Ocean.

Depositional Coasts

Barrier island coasts are those paralleled by deposits of sand separated from land by alagoon. There is some controversy over how barrier island coasts form. They begin asoffshore bars of submerged sediment that migrate landward unless stabilized. Barrier islandsare often cut by tidal inlets, openings that allow water to move landward and seaward withthe tides. Barrier islands border the Atlantic coastal plain of North America.


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Figure OC.19 Birdfoot orBalize delta of the MississippiRiverCourtesy USGS Source: Geomophology From Space

Delta coastsare those formed by the deposition of sediment at the mouth of a river that enters

the ocean. Deposition is caused by the rapid decrease in water velocity as it enters the ocean.Sand and silt are the first to deposit while the mixing of fresh and salt water cause clayparticles to bind together forming larger particles that settle to the bottom.

Reefs

Coral is a simple marine animal having a small cylindrical sac-like body called a polyp andan exoskeleton of calcium carbonate. As old colonies die new form on top, ultimatelyforming limestone. There are an estimated 9 million species making it one of the most diversemarine ecosystems. Corals are on the decline in many regions due to human activity.

Figure OC.20 Pillar coralCourtesy NOAA

Corals thrive in warm tropical water at depths of 10 - 55 m (30 - 18 ft) from about 30o N to30

oS. Warm, east coasts encourage their development while few are found in cooler,

western coastal environments. Water temperatures range from 18o to 29o C (64o - 85o F) 27%to 40% salinity. Bleaching and death result when water temperatures rise to high. There are


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cool water corals found deep, dark ocean water at temperatures as low as 4o

C (39o

F). Coralsalso require sediment-free water thus few are found near the mouths of rivers discharging inthe ocean.

There are three kinds of coral reefs. Fringing reefs are platforms of coral attached to land.They tend to be wider where wave action is prominent and the water well aerated. Barrierreefs form offshore with lagoon in between. Many form along slowing subsiding islands,growing at a rate that keeps them near sea level. Others form on continental shelves. Thelargest and one of the most well known is the Great Barrier Reef of Australia at over 2025 kmlong ( 1260 m) and 16 - 145 km (10-90 mi) wide . Atolls are circular reefs enclosing a lagoonformed from the subsidence and disappearance of a volcanic island cone.

Figure OC.21Triangular atoll in the

western PacificCourtesy NOAA

Figure OC.22Great Barrier Reef of Australia

CourtesyNASA/GSFC/LaRC/JPL,MISR

Team. Source Visible Earth

Figure OC.23 Fringing reefaround Howland Island, Pacific

OceanCourtesy NASA Astronaut

photograph ISS010-E-9287


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Review

Use the links below to review and assess yourlearning. Start with the "Important Terms andConcepts" to ensure you know the terminologyrelated to the topic of the chapter and conceptsdiscussed. Move on to the "Review Questions"to answer critical thinking questions aboutconcepts and processes discussed in thechapter. Finally, test your overall understanding

by taking the "Self-assessment quiz".

y Important Terms and Conceptsy Review Questionsy Self-assessment quiz

Additional Resources

Use these resources to further explore the worldof geography

Multimedia

"On the Beach"News Hourwith Jim Lehrer(PBS) August 22, 2003 report on California'scoastal erosion problem.

The Sea FloorThe Earth Revealed (Annenberg/CPB)This segment examines the research submersibles and indirect methods used to study the

bottom of the ocean, providing a look at volcanic activity, formations such as the continentalshelf and mid-ocean ridges, and life forms living at extreme depths. One-time, freeregistration may be required to view film. (29:00) Go to the Earth Revealed site and scroll to"The Sea Floor". One-time, free registration may be required to view film.

Conservation Medicine: What the Oceans are Telling Us (WGBH ForumNetwork) From the site: "This talk analyzes how human impacts on the world's oceans havedevastated populations, species and ecosystems at a rapid scale, and how methodologies toassess marine ecosystem health are grossly lacking."

Readings

"Amazing Atolls of the Maldives" (NASA Earth Observatory) This report investigates therole of climate and waves in the development of coral reefs that form atolls.

"Shifting Shoals and Shattered Rocks - How Man has Transformed the Floor of West-

Central San Francisco Bay" (USGS) 7.9MB


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Web Sites

USGS Coastal and Marine Geology Program

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Chapter 20 - Ocean and Coastal Systems

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