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Chapter 16 The Oceans, Coastal Processes and Landforms

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Title: Chapter 16 The Oceans, Coastal Processes and Landforms


1
Chapter 16The Oceans, Coastal Processes and
Landforms
  • Geosystems 5e
  • An Introduction to Physical Geography

Robert W. Christopherson Charlie Thomsen
2
Homework
  • Assignment 3 (book review) is due on March 16.

3
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.

4
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.

5
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).

6
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).
7
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.

8
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).

9
Figure 16.3 The Oceans Physical Structure
10
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).

11
Figure 16.4 The Littoral Zone. The littoral
zone includes the coast, beach, and nearshore
environments.
12
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.

13
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.

14
  • 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.

15
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.

16
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.

17
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.

18
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).

19
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).
20
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.

21
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.
22
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.

23
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.
24
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).

25
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).

26
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.

27
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.

28
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?
29
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.

30
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.

31
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).

32
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.
33
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.

34
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

35
End of Chapter 16
  • Geosystems 5e
  • An Introduction to Physical Geography

Robert W. Christopherson Charlie Thomsen
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