Chapter 16 The Oceans, Coastal Processes and Landforms

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Chapter 16The Oceans, Coastal Processes and
Landforms

• Geosystems 5e
• An Introduction to Physical Geography

Robert W. Christopherson Charlie Thomsen
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Homework

• Assignment 3 (book review) is due on March 16.

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Overview

• Coastal regions are unique and dynamic
  environments. Most of Earth's coastlines are
  relatively new and are the setting for continuous
  change. The land, ocean, and atmosphere interact
  to produce waves, tides, erosional features, and
  depositional features along the continental
  margins. The interaction of vast oceanic and
  atmospheric masses is dramatic along a shoreline.
  At times, the ocean attacks the coast with its
  erosive power at other times, the sea breeze,
  salty mist, and repetitive motion of the water
  are gentle and calming.

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Key Learning Concepts

• ---After this lecture you should be able to
• Describe the chemical composition of seawater and
  the physical structure of the ocean.
• Identify the components of the coastal
  environment and list the physical inputs to the
  coastal system, including tides and mean sea
  level.
• Describe wave motion at sea and near shore and
  explain coastal straightening as a product of
  wave refraction.
• Identify characteristic coastal erosional and
  depositional landforms.
• Describe barrier islands and their hazards as
  they relate to human settlement.
• Assess living coastal environments corals,
  wetlands, salt marshes, and mangroves.
• Construct an environmentally sensitive model for
  settlement and land use along the coast.

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1. Describe the salinity of seawater its
composition, amount, and distribution.

• Water acts as a solvent, dissolving at least 57
  of the elements found in nature. In fact, most
  natural elements and the compounds they form are
  found in the seas as dissolved solids, or
  solutes. Thus, seawater is a solution, and the
  concentration of dissolved solids is called
  salinity. Seven elements comprise more than 99
  of the dissolved solids in seawater chlorine
  (Cl), sodium (Na), magnesium (Mg), sulfur (S),
  calcium (Ca), potassium (K), and bromine (Br).
  Seawater also contains dissolved gases (such as
  carbon dioxide, nitrogen, and oxygen), solid and
  dissolved organic matter, and a multitude of
  trace elements. Salinity worldwide normally
  varies between 34 and 37 variations are
  attributable to atmospheric conditions above the
  water and to the quantity of freshwater inflows.
  The term brine is applied to water that exceeds
  the average of 35 salinity, whereas brackish
  applies to water that is less than 35. (See
  figure 16.2).

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Figure 16.2 Variation in Ocean Salinity and
Latitude. Salinity (green line) is principally a
function of climatic conditions. Specifically,
important is the moisture relation expressed by
the difference between evaporation and
precipitation (E-P).
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2. Analyze the latitudinal distribution of
salinity shown in Figure 16-2. Why is salinity
less along the equator and greater in the
subtropics?

• Generally, oceans are lower in salinity near
  landmasses, because of river discharges and
  runoff. Extreme examples include the Baltic Sea
  (north of Poland and Germany) and the Gulf of
  Bothnia (between Sweden and Finland), which
  average 10 or less salinity because of heavy
  freshwater runoff and low evaporation rates. On
  the other hand, the Sargasso Sea, within the
  North Atlantic subtropics, averages 38, and the
  Persian Gulf is at 40 as a result of high
  evaporation rates in an almost-enclosed basin.
  Deep pockets near the floor of the Red Sea
  register a very salty 225. In equatorial water,
  precipitation is high throughout the year,
  diluting salinity values to slightly lower than
  average (34.5). In subtropical oceanswhere
  evaporation rates are high due to the influence
  of hot, dry subtropical high-pressure
  cellssalinity is more concentrated, increasing
  to 36.5.

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3. What are the three general zones relative to
physical structure within the ocean?
Characterize each by temperature, salinity,
dissolved oxygen, and dissolved carbon dioxide.

• The ocean's surface layer is warmed by the Sun
  and is wind-driven. Variations in water
  temperature and solutes are blended rapidly in a
  mixing zone that represents only 2 of the
  oceanic mass. Below this is the thermocline
  transition zone, a region of strong temperature
  gradient that lacks the motion of the surface.
  Friction dampens the effect of surface currents,
  with colder water temperatures at the lower
  margin tending to inhibit any convective
  movements. Starting at a depth of 1-1.5 km
  (0.62-0.93 mi) and going down to the bottom,
  temperature and salinity values are quite
  uniform. Temperatures in this deep cold zone are
  near 0C (32F) but, due to its salinity,
  seawater freezes at about 2C (28.4F). The
  coldest water is at the bottom except near the
  poles, where cold water may be near or at the
  surface. (See Figure 16-3).

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Figure 16.3 The Oceans Physical Structure
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4. What are the key terms used to describe the
coastal environment?

• The coastal environment is called the littoral
  zone. (Littoral comes from the Latin word for
  shore.) The littoral zone spans both land and
  water. Landward, it extends to the highest water
  line that occurs on shore during a storm.
  Seaward, it extends to the point at which storm
  waves can no longer move sediments on the
  seafloor (usually at depths of approximately 60 m
  or 200 ft). The specific contact line between
  the sea and the land is the shoreline, and
  adjacent land is considered the coast. (See
  Figure 16-4).

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Figure 16.4 The Littoral Zone. The littoral
zone includes the coast, beach, and nearshore
environments.
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5. Define mean sea level. How is this value
determined? Is it constant or variable around
the world?

• Mean sea level is a calculated value based on
  average tidal levels recorded hourly at a given
  site over a period of years. Mean sea level
  varies spatially from place to place because of
  ocean currents and waves, tidal variations, air
  temperature and pressure differences, and ocean
  temperature variations.

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6. What interacting forces generate the pattern
of tides?

• Earth's orientation to the Sun and the Moon
  (astronomical relationships) produce the pattern
  of tides, the complex daily oscillations in sea
  level that are experienced to varying degrees
  around the world. Tides also are influenced by
  the size, depth, and topography of ocean basins,
  by latitude, and by shoreline configuration.
  Tides are produced by the gravitational pull
  exerted on Earth by both the Sun and the Moon.
  Although the Suns influence is only about half
  that of the Moon (46) because of the Sun's
  greater distance from Earth, it is still a
  significant force. Figure 16-6 illustrates the
  relationship among the Moon, the Sun, and Earth
  and the generation of variable tidal bulges on
  opposite sides of the planet.

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• Figure 16.6 The Cause of Tides. Gravitational
  relations of Sun, Moon, and Earth combine to
  produce the tides. Gravity and inertia are
  essential elements in understanding tides.
  Gravity is the force of attraction between two
  bodies. Inertia is the tendency of objects to
  stay still if motionless or to keep moving in the
  same direction if in motion. In the case of
  figure 16.6a for example, the corresponding tidal
  bulge on the Earths opposite side is primarily
  the result of farside waters remaining in
  position (being left behind) because its inertia
  exceeds the gravitational pull of the Moon and
  Sun.

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Do tides really move in and out along the
shoreline?

• Tides appear to move in and out along the
  shoreline, but they do not actually do so.
  Instead, the Earths surface rotates into and out
  of the relatively fixed tidal bulges as Earth
  changes its position in relation to the Moon and
  Sun. Hence, every day, most coastal locations
  experience two high (rising) tides, known as
  flood tides, and two low (falling) tides, known
  as ebb tides. The difference between consecutive
  high and low tides is considered the tidal range.

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7. What characteristic tides are expected during
a new Moon or a full Moon? During the
first-quarter and third-quarter phases of the
Moon?

• Figure 16-6a shows the Moon and the Sun in
  conjunction (lined up with Earthnew Moon or full
  Moon), a position in which their gravitational
  forces add together. The combined gravitational
  effect is strongest in the conjunction alignment
  and results in the greatest tidal range between
  high and low tides, known as spring tides. Figure
  16-6b shows the other alignment that gives rise
  to spring tides, when the Moon and Sun are at
  opposition. When the Moon and the Sun are
  neither in conjunction nor in opposition, but are
  more-or-less in the positions shown in c and d
  (first- and third-quarter phases), their
  gravitational influences are offset and
  counteract each other somewhat, producing a
  lesser tidal range known as neap tide.

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8. Is tidal power being used anywhere to generate
electricity? Explain briefly how such a plant
would utilize the tides to produce electricity.
Are there any sites in North America? Where are
they?

• The fact that sea level changes daily with the
  tides suggests an opportunity that these could be
  harnessed to produce electricity. The bay or
  estuary under consideration must have a narrow
  entrance suitable for the construction of a dam
  with gates and locks, and it must experience a
  tidal range of flood and ebb tides large enough
  to turn turbines, at least a 5 m range. Tidal
  power generation is possible at about 30
  locations in the world, although at present only
  two of them are actually producing electricity
  an experimental 1 megawatt station in Russia at
  Kislaya-Guba Bay, on the White Sea, since 1969
  and a facility in the Rance River estuary on the
  Brittany coast of France since 1967.
• According to studies completed by the Canadian
  government, the present cost of tidal power at
  ideal sites is economically competitive with that
  of fossil fuels, although certain environmental
  concerns must be addressed. Among several
  favorable sites on the Bay of Fundy, one plant is
  in operation. The Annapolis Tidal Generating
  Station was built in 1984 to test electrical
  production using the tides. Nova Scotia Power
  Incorporated operates the 20 megawatts plant.

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9. What is a wave? How are waves generated, and
how do they travel across the ocean? Does the
water travel with the wave? Discuss the process
of wave formation and transmission.

• Undulations of ocean water called waves travel in
  wave trains, or groups of waves. Storms around
  the world generate large groups of wave trains.
  A stormy area at sea is the generating region for
  these large waves, which radiate outward from
  their formation center. As a result, the ocean
  is crisscrossed with intricate patterns of waves
  traveling in all directions. The waves seen along
  a coast may be the product of a storm center
  thousands of kilometers away. Water within such
  a wave is not really migrating but is
  transferring energy through the water in simple
  cyclic undulations, which form waves of
  transition. In a breaker, the orbital motion of
  transition gives way to waves of translation, in
  which both energy and water move forward toward
  shore as water cascades down from the wave crest
  (See Figure 16-8).

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Figure 16.8 The orbiting tracks of water
particles change from circular motions and swells
in deep water (waves of transition) to more
elliptical orbits near the bottom in shallow
water (waves of translation).
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10. Describe the refraction process that occurs
when waves reach an irregular coastline. Why is
the coastline straightened?

• Generally, wave action is a process that results
  in coastal straightening. As waves approach an
  irregular coast, they bend and focus around
  headlands, or protruding landforms generally
  composed of more resistant rocks (See Figure
  16-9). Thus, headlands represent a specific
  point of wave attack along a coastline. Waves
  tend to disperse their energy in coves and bays
  on either side of the headlands. This wave
  refraction (wave bending) along a coastline
  redistributes wave energy so that different
  sections of the coastline are subjected to
  variations in erosion potential.

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Figure 19.9 Coastal Straightening. The process
of coastal straightening is brought about by wave
refraction (deflection from a straight path).
Wave energy is concentrated as it converges on
headlands.
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11. Define the components of beach drift and the
longshore current and longshore drift.

• Particles on the beach are moved along as beach
  drift, or littoral drift, shifting back and forth
  between water and land in the effective wind and
  wave direction. These dislodged materials are
  available for transport and eventual deposition
  in coves and inlets and can represent a
  significant volume. The longshore current
  transports beach drift. A longshore current
  generates only in the surf zone and works in
  combination with wave action to transport large
  amounts of sand, gravel, sediment and debris
  along the shore as longshore drift. See Figure
  16.10.

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Figure 16.10 Longshore Current and Beach Drift.
Longshore currents are produced as waves approach
the surf zone and shallower water. Longshore and
beach drift results as substantial volumes of
material are moved along the shore.
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13. What is meant by an erosional coast? What are
its features?

• The active margins of the Pacific along the North
  and South American continents are characteristic
  coastlines affected by erosional landform
  processes. Erosional coastlines tend to be
  rugged, of high relief, and tectonically active,
  as expected from their association with the
  leading edge of a drifting lithospheric plate.
  Sea cliffs are formed along a coastline by the
  undercutting action of the sea. As indentations
  are produced at water level, such a cliff becomes
  notched, leading to subsequent collapse and
  retreat of the cliff. Other erosional forms
  evolve along cliff-dominated coastlines,
  including sea caves, sea arches, and sea stacks.
  As erosion continues, arches may collapse,
  leaving isolated stacks out in the water. The
  coasts of southern England and Oregon are prime
  examples of such erosional landscapes. (Fig.
  16-12).

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14. What is meant by a depositional coast? What
are the features?

• Depositional coasts generally are located
  near onshore plains of gentle relief, where
  sediments are available from many sources. Such
  is the case with the Atlantic and Gulf coastal
  plains of the United States, which lie along the
  relatively passive, trailing edge of the North
  American lithospheric plate. A spit consists of
  sand deposited in a long ridge extending out from
  a coast it partially crosses and blocks the
  mouth of a bay. A classic example is Cape Cod,
  Mass. The spit becomes a bay barrier if it
  completely cuts the bay off from the ocean and
  forms an inland lagoon. Spits are made up of
  materials that have been eroded and transported
  by drift for much sand to accumulate, offshore
  currents must be weak. A tombolo occurs when
  sand deposits connect the shoreline with an
  offshore island. (Fig 16-13).

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15. How do people modify littoral drift?

• Figure 16-14, illustrates several of the common
  approaches jetties to block material from harbor
  entrances, groins to slow drift action along the
  coast, and a breakwater to create a zone of still
  water near the coastline. However, interrupting
  the coastal drift that is the natural
  replenishment for beaches may lead to unwanted
  changes in sand distribution downcurrent. In
  addition, enormous energy and materials must be
  committed to counteract the enormous and
  relentless energy that nature invests along the
  coast.

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16. Describe a beachits form and composition.

• A beach is that place along a coast where
  sediment is in motion. Material from the land
  temporarily resides there while it is in active
  transit along the shore. The beach zone ranges,
  on average, from 5 m above high tide to 10 m
  below low tide, although specific definition
  varies greatly along individual shorelines.
  Beaches are dominated by quartz (SiO2) because it
  is the most abundant mineral on Earth, resists
  weathering, and therefore remains after other
  minerals are removed. A beach acts to stabilize
  a shoreline by absorbing wave energy, as is
  evident by the amount of material that is in
  almost constant motion.

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17. On the basis of the information in the text,
do you think barrier islands and beaches should
be used for development? If so, under what
conditions? If not, why not?
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Answer

• Offshore sand bars gradually migrate toward shore
  as the sea level rises. Because many barrier
  beaches evidence this landward migration today,
  they are an unwise choice for a home site or
  commercial building. Nonetheless, they are a
  common choice, even though they take the brunt of
  storm energy and actually act as protection for
  the mainland. The hazard represented by the
  settlement of barrier islands was made
  graphically clear when Hurricane Hugo (1989)
  assaulted South Carolina. Beachfront houses,
  barrier beach developments, and millions of tons
  of sand were swept away up to 95 of the
  single-family homes in Garden City alone were
  destroyed.

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Next Topic Living Coastal Environments Corals,
wetlands, salt marshes, and mangroves.

• 20. How are corals able to construct reefs and
  islands?

• A coral is a simple marine animal with a
  cylindrical, saclike body it is related to other
  marine invertebrates, such as anemones and
  jellyfish. Corals secrete calcium carbonate
  (CaCO3) from the lower half of their bodies,
  forming a hard external skeleton. Although both
  solitary and colonial corals exist, it is the
  colonial forms that produce enormous structures,
  varying from treelike and branching forms to
  round and flat shapes. Through many generations,
  live corals near the ocean's surface build on the
  foundation of older corals below, which in turn
  may rest upon a volcanic seamount or some other
  submarine feature built up from the ocean floor.
  An organically derived sedimentary formation of
  coral rock is called a reef and can assume one of
  several shapes, principally, a fringing reef, a
  barrier reef, or an atoll.

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21. A trend in corals that is troubling
scientists, some possible causes.

• Scientists are tracking an unprecedented
  bleaching and dying-off of corals worldwide. The
  Caribbean, Australia, Japan, Indonesia, Kenya,
  Florida, Texas, and Hawaii are experiencing this
  phenomenon. The bleaching is due to a loss of
  colorful algae from within and upon the coral
  itself. Normally colorful corals have turned
  stark white as the host coral expels
  nutrient-supplying algae. Exactly why the coral
  ejects its living partner is unknown.
  Possibilities include local pollution, disease,
  sedimentation, and changes in salinity.
• Another probable cause is the 1 to 2C warming of
  sea-surface temperatures, as stimulated by
  greenhouse warming of the atmosphere. During the
  1982-1983 areas of the Pacific Ocean were warmer
  than normal and widespread coral bleaching
  occurred. Coral bleaching worldwide is continuing
  as average ocean temperatures climb higher. (see
  Figure 16.17 for coral riff distribution).

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Figure 16.17. Worldwide Distribution of Living
Coral Formations. Yellow patches are areas of
prolific reef growth. The red dotted line marks
the limits of coral activity.
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22. Why are coastal wetlands poleward of 30 N
and S latitude different from those that are
equatorward?

• In terms of wetland distribution, salt marshes
  tend to form north of the 30th parallel, whereas
  mangrove swamps form equatorward of that point.
  This is dictated by the occurrence of freezing
  conditions, which control the survival of
  mangrove seedlings. Roughly the same latitudinal
  limits apply in the Southern Hemisphere. Salt
  marshes usually form in estuaries and behind
  barrier beaches and sand spits. An accumulation
  of mud produces a site for the growth of
  halophytic (salt-tolerant) plants. Plant growth
  then traps additional alluvial sediments and adds
  to the salt marsh area. Sediment accumulation on
  tropical coastlines provides the site for growth
  of mangrove trees, shrubs, and other small
  trees. The adventurous prop roots of the
  mangrove are constantly finding new anchorages
  and are visible above the water line but reach
  below the water surface, providing a habitat for
  a multitude of specialized life forms. Mangrove
  swamps often secure and fix enough material to
  form islands.

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Movie Waves, Beaches and Coasts

• This program shows the dynamic interaction of two
  geologic agents rocky landmasses and the energy
  of the ocean. Aspects of waves their types,
  parts, movement, and impact on the shore are
  illustrated. The program also covers shoreline
  characteristics, currents, sea barriers, tides,
  and how the greenhouse effect could impact sea
  level and coastal lands.
• http//www.learner.org/resources/series78.html

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End of Chapter 16

• Geosystems 5e
• An Introduction to Physical Geography

Robert W. Christopherson Charlie Thomsen