Chapter%2018:%20The%20Oceans%20And%20Their%20Margins

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Chapter 18 The Oceans And Their Margins
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Introduction The Worlds Oceans

• Seawater covers 70.8 percent of Earths surface,
  in three huge interconnected basins
• The Pacific Ocean.
• The Atlantic Ocean.
• The Indian Ocean.

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The Oceans Characteristics

• The greatest ocean depth yet measured (11,035 m)
  lies in the Mariana Trench.
• The average depth of the oceans, is about 3.8 km.
• The present volume of seawater is about 1.35
  billion cubic kilometers.
• More than half this volume resides in the Pacific
  Ocean.

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Figure 18.1
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Figure 18.2
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Ocean Salinity (1)

• Salinity is the measure of the seas saltiness,
  expressed in parts per mil ( parts per
  thousand).
• The salinity of seawater normally ranges between
  33 and 37.
• The principal elements that contribute to this
  salinity are sodium and chlorine.

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Ocean Salinity (2)

• More than 99.9 percent of the oceans salinity
  reflects the presence of only eight ions
• Chloride.
• Sodium.
• Sulfate.
• Magnesium.
• Calcium.
• Potassium.
• Bicarbonate.
• Bromine.

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Ocean Salinity (3)

• Cations are released by chemical weathering
  processes on land.
• Each year streams carry 2.5 billion tons of
  dissolved substances to the sea.
• The principal anions found in seawater are
  believed to have come from the mantle. Chemical
  analyses of gases released during volcanic
  eruptions show that the most important volatiles
  are water vapor (steam), carbon dioxide (CO2),
  and the chloride (CI1-) and sulfate (SO42-)
  anions.

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Ocean Salinity (4)

• Chloride and sulfate anions dissolve in
  atmospheric water vapor and return to Earth in
  precipitation, much of which falls directly into
  the ocean.
• Another source of ions is dust eroded from desert
  regions and blown out to sea.

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Figure 18.3A
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Temperature And Heat Capacity of the Ocean (1)

• Global summer sea-surface temperature is
  displayed with isotherms that lie approximately
  parallel to the equator.
• The warmest waters during August (gt280C) occur in
  a discontinuous belt between about 300 N and 100
  S latitude.
• In winter, the belt of warm water moves south
  until it is largely below the equator.

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Temperature And Heat Capacity of the Ocean (2)

• Waters become progressively cooler both north and
  south of this belt.
• Both the total range and the seasonal changes in
  ocean temperatures are much less than what we
  find on land.

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Temperature And Heat Capacity of the Ocean (3)

• The range of temperature on land is 1460 C.
• The highest recorded land temperature is 580C
  (Libyan Desert).
• The lowest is 880C (Vostok Station in central
  Antarctica).
• The range of temperature at the oceans surface
  is only 380 C.
• The highest recorded ocean temperature is 360 C
  (Persian Gulf).
• The coldest is 20 C (Polar Sea).

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Temperature And Heat Capacity of the Ocean (4)

• Coastal inhabitants benefit from the mild climate
  resulting from this natural ocean thermostat.
• In the interior of a continent, summer
  temperatures may exceed 400 C, whereas along the
  ocean margin they typically remain below 250 C.

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Figure 18.3B
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Vertical Stratification (1)

• Temperature and other physical properties of
  seawater vary with depth.
• When fresh river water meets salty ocean water at
  a coast, the fresh water, being less dense, flows
  over the denser saltwater, resulting in
  stratified water bodies.

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Vertical Stratification (2)

• The oceans also are vertically stratified as a
  result of variation in the density of seawater.
• Seawater become denser as
• Its temperature decreases.
• Its salinity increases.
• Gravity pulls dense water downward until it
  reaches a level where the surrounding water has
  the same density.
• These density-driven movements lead both to
  stratification of the oceans and to circulation
  in the deep ocean.

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Ocean Circulation

• Surface ocean currents are broad, slow drifts of
  surface water set in motion by the prevailing
  surface winds.
• A current of water is rarely more than 50 to 100
  m deep.
• The direction taken by ocean currents is also
  influenced by the Coriolis effect.

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Current Systems

• Low-latitude regions in the tradewind belts are
  dominated by the warm North and South Equatorial
  currents.
• Each major current is part of a large subcircular
  current system called a gyre.
• The Earth has five major ocean gyres.
• Two are in the Pacific Ocean.
• Two are in the Atlantic Ocean.
• One is in the Indian Ocean.

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Figure 18.4
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Major Water Masses (1)

• Ocean waters also circulate on a large scale
  within the deep ocean, driven by differences in
  water density.
• The water of the oceans is organized into major
  water masses, each having a characteristic range
  of
• Temperature.
• Salinity.

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Major Water Masses (2)

• The water masses are stratified based on their
  relative densities.
• Cold water is denser than warm water
• Salty water is denser than less salty water.

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Figure 18.5
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The Global Ocean Conveyor System (1)

• Dense, cold, and/or salty surface waters that
  flow toward adjacent warmer, less-salty waters
  will sink until they reach the level of water
  masses of equal density.
• The resulting stratification of water masses is
  thus based on relative density.

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The Global Ocean Conveyor System (2)

• The sinking dense water in the North Atlantic
  propels a global thermohaline circulation system,
  so called because it involves both the
  temperature (thermo) and salinity (haline)
  characteristics of the ocean waters.

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The Global Ocean Conveyor System (3)

• The Atlantic thermohaline circulation acts like a
  great conveyor belt, transporting low-density
  surface water northward and denser deep-ocean
  water southward.
• Heat lost to the atmosphere by this warm surface
  water, together with heat from the warm Gulf
  Stream, maintains a relatively mild climate in
  northwestern Europe.

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Figure 18.6A
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Figure 18.6B
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Ocean Tides (1)

• Tides
• Twice-daily rise and fall of ocean waters.
• Caused by the gravitational attraction between
  the Moon (and, to lesser degree, the sun) and the
  Earth.
• The Moon exerts a gravitational pull on the solid
  Earth.

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Ocean Tides (2)

• A water particle in the ocean on the side facing
  the Moon is attracted more strongly by the Moons
  gravitation than it would be if it were at
  Earths center, which lies at a greater distance.
• This creates a bulge on the ocean surface due to
  the excess inertial force (called the
  tide-raising force).

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Ocean Tides (3)

• On the opposite side of Earth, the inertial force
  exceeds the Moons gravitational attraction, and
  the tide-raising force is directed away from
  Earth.
• These unbalanced forces generate opposing tidal
  bulges.

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Figure 18.7
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Ocean Tides (4)

• At most places on the ocean margins, two high
  tides and two low tides are observed each day as
  a coast encounters both tidal bulges.
• Twice during each lunar month, Earth is directly
  aligned with the Sun and the Moon, whose
  gravitational effects are thereby reinforced,
  producing higher high tides and lower low tides.

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Figure 18.8
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Ocean Tides (5)

• At position halfway between these extremes, the
  gravitational pull of the Sun partially cancels
  that of the Moon, thus reducing the tidal range.
• In the open sea tides are small (less than 1 m).
• Along most coasts the tidal range commonly is no
  more than 2 m.

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Ocean Tides (6)

• In bays, straits, estuaries, and other narrow
  places along coasts, tidal fluctuations are
  amplified and may reach 16 m or more.
• Associated currents are often rapid and may
  approach 25 km/h.
• The incoming tide locally can create a wall of
  water a meter or more high (called a tidal bore).

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Tidal Power

• Energy obtained from the tides is renewable
  energy.
• One important difference between hydroelectric
  power from rivers and that from tidal power is
  that rivers flow continuously whereas tides can
  be exploited only twice a day.

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Ocean Waves (1)

• Ocean waves receive their energy from winds that
  blow across the water surface.
• The size of a wave depends on how fast, how far,
  and how low the wind blows.

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Ocean Waves (2)

• Because waveform is created by a loop-like motion
  of water parcels, the diameters of the loops at
  the water surface exactly equal wave height.
• Downward from the surface, a progressive loss of
  energy occurs, resulting in a decrease in loop
  diameter.

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Ocean Waves (3)

• L is used to represent wavelength, the distance
  between successive wave crests or troughs.
• At a depth equal to half the wavelength (L/2),
  the diameters of the loops have become so small
  that motion of the water is negligible.

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Ocean Waves (4)

• The depth L/2 is referred to as wave base.
• Landward of depth L/2, as the water depth
  decreases, the orbits of the water parcels become
  flatter until the movement of water at the
  seafloor in the shallow water zone is limited to
  a back-and-forth motion.

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Figure 18.10
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Ocean Waves (5)

• When the wave reaches depth L/2, its base
  encounters frictional resistance exerted by the
  seafloor.
• This causes the wave height to increase and the
  wave length to decrease.
• Eventually, the front becomes too steep to
  support the advancing wave and the wave
  collapses, or breaks.

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Ocean Waves (6)

• Such broken water is called surf
• The geologic work of waves is mainly accomplished
  by the direct action of surf.

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Figure 18.11
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Wave Refraction (1)

• A wave approaching a coast generally does not
  encounter the bottom simultaneously all along its
  length.
• As any segment of the wave touches the seafloor
• That part slows down.
• The wave length begins to decrease.
• The wave height increases.

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Wave Refraction (2)

• This process is called wave refraction.
• Wave refraction affects various sectors of a
  coastline differently.
• Waves converge on headlands, which are vigorously
  eroded.
• Refraction of waves approaching a bay will make
  them diverge, diffusing their energy at the
  shore.
• In the course of time, irregular coasts become
  smoother and less indented.

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Figure 18.13
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Coastal Erosion And Sediment Transport (1)

• Erosion by waves.
• Erosion below sea level
• Ocean waves rarely erode to depths of more than 7
  m.
• The lower limit of wave motion is half the
  wavelength of ocean waves, which is the lower
  limit of erosion of the ocean floor by waves.

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Coastal Erosion And Sediment Transport (2)

• Abrasion in the surf zone
• An important kind of erosion in the surf zone is
  the wearing down of rock by wave-transported rock
  particles,
• The surf is like an erosional knife edge or saw
  cutting horizontally into the land.
• Erosion above sea level
• Waves pounding against a cliff compress the air
  trapped in fissures.
• Nearly all the energy expended by waves in
  coastal erosion is confined to a zone that lies
  between 10 m above and 10 m below mean sea level.

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Coastal Erosion And Sediment Transport (3)

• Sediment transport by waves and currents.
• Longshore currents
• Longshore currents flow parallel to the shore.
• The direction of longshore currents may change
  seasonally.
• The longshore current moves the sediment along
  the coast.

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Figure 18.14
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Coastal Erosion And Sediment Transport (4)

• Beach drift
• The swash (uprushing water) of each wave travels
  obliquely up the beach before gravity pulls the
  water back directly down the slope of the beach.
• This zigzag movement of water carries sand and
  pebbles first up, then down the beach slope in a
  process known as beach drift.
• Beach drift can reach a rate of more than 800
  m/day.

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Figure 18.15
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Coastal Erosion And Sediment Transport (5)

• Beach placers
• Gold, diamond, and several other heavy minerals
  have been concentrated in beach sands by surf and
  longshore currents (Namibia, Alaska).
• Ilmenite, a primary source of titanium, is highly
  concentrated along several beaches in India.
• Magnetite-rich sands occur in Oregon, California,
  Brazil, and New Zealand.
• Chrome-rich sands are mined in Japan.

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Coastal Erosion And Sediment Transport (6)

• Offshore transport and sorting
• Far from shore only fine grains can be moved.
• Sediments grade seaward from sand into mud.

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Figure 18.16
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Coastal Deposits And Landforms

• Waves dash against firm rock, erode it, and move
  the eroded rock particles.
• The three important features of the shore profile
  are
• Beaches.
• Wave-cut cliffs.
• Wave-cut benches.

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Beaches (1)

• Beach is
• The sandy surface above the water along a shore.
• A wave-washed sediment along a coast, including
  sediment in the surf zone (sediment is
  continually in motion).
• Sediment of a beach may derived from
• Erosion of adjacent cliffs or cliffs elsewhere
  along the coast.
• Alluvium brought to the shore by rivers.

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Beaches (2)

• On low, open shores an exposed beach typically
  has several distinct elements
• A rather gently sloping foreshore (lowest tide to
  the average high-tide level).
• A berm (bench formed of sediment deposited by
  waves).
• The backshore (from the berm to the farthest
  point reached by surf).

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Figure 18.17
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Rocky (Cliffed) Coasts

• The usual elements of a cliffed coast due to
  erosion are
• A wave-cut cliff, which may have a well-developed
  notch at its base.
• A wave-cut bench, a platform cut across bedrock
  by surf.
• A beach, the result of deposition.
• Other erosional features associated with cliffed
  coasts are sea caves, sea arches, and stacks.

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Figure 18.18
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Factors Affecting The Shore Profile (1)

• Through erosion and the creation, transport, and
  deposition of sediment, the form of a coast
  changes, often slowly but sometimes very rapidly.
• During storms, the increased energy in the surf
  erodes the exposed part of a beach and makes it
  narrower.

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Factors Affecting The Shore Profile (2)

• In calm weather, the exposed beach is likely to
  receive more sediment than it loses and therefore
  becomes wider.
• Storminess may be seasonal, resulting in seasonal
  changes in beach profiles.
• Winter storm surf tends to carry away fine
  sediment, and the remaining coarse fraction
  assumes a steep profile.

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Major Coastal Deposits And Landforms

• Marine deltas are a compromise between the rate
  at which a river delivers sediment at its mouth
  and the ability of currents and waves to erode
  sediment along the delta front.

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Figure 18.21
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Major Coastal Deposits And Landforms (2)

• A spit is an elongated ridge of sand or gravel
  that projects from land and ends in open water.
• It is merely a continuation of a beach.
• It is built of sediment moved by longshore drift
  and dropped at the mouth of a bay.

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Major Coastal Deposits And Landforms (3)

• The free end curves landward in response to
  currents created by refraction as waves enter the
  bay.
• A spit-like ridge of sand or gravel that connects
  an island to the mainland or to another island,
  called a tombolo.
• A ridge of sand or gravel may be built across the
  mouth of a bay to form a bay barrier.

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Figure 18.22
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Major Coastal Deposits And Landforms (4)

• Beach ridges are low sandy bars parallel to the
  coast.
• A barrier islands is a long narrow sandy island
  lying offshore and parallel to a coast.
• An elongate bay lying inshore from a barrier
  island or strip of land such as coral reef is
  called a lagoon.

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Figure 18.24B
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Major Coastal Deposits And Landforms (5)

• Organic reefs and atolls
• A fringing reef is either attached to or closely
  borders the adjacent land (no lagoon).
• A barrier reef is separated from the land by a
  lagoon that may be of considerable length and
  width.
• Great Barrier Reef off Queensland, Australia.
• An atoll, a roughly circular coral reef enclosing
  a shallow lagoon, is formed when a tropical
  volcanic island with a fringing reef slowly
  subsides.

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Figure 18.26
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How Coasts Evolve (1)

• The configuration of coasts depends largely on
• The structure and erodibility of coastal rocks.
• The active geologic processes at work.
• The length of time over which these processes
  have operated.
• The history of world sea-level fluctuations.

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How Coasts Evolve (2)

• Types of coasts
• Most of the Pacific coast of North America is
  steep and rocky.
• The Atlantic and Gulf coasts traverse a broad
  coastal plain that slopes gently seaward and are
  festooned with barrier islands.
• The result is an embayed, rocky, coastline that
  shows the effects of both
• Differential glacial erosion.
• Drowning of the land by the most recent sea-level
  rise.

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How Coasts Evolve (3)

• Where rocks of different erodibilities are
  exposed along a coast, marine erosion is strongly
  controlled by rock type and structure.
• Coasts of Norway, Ireland, and Croatia.

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Geographic Influences on Coastal Processes

• Coasts lying at latitudes between about 45 and
  600 are subjected to higher-than-average storm
  waves generated by strong westerly winds.
• Subtropical east-facing coasts are subjected to
  infrequent but often disastrous hurricanes
  (called typhoons west of the 180th meridian).
• Sea ice is an effective agent of coastal erosion
  in the polar regions.

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Changing Sea Level

• Sea level fluctuates
• Daily as a result of tidal forces.
• Over much longer time scales as a result of
• Changes in the volume of water in the oceans as
  continental glaciers wax and wane.
• The motions of lithospheric plates that cause the
  volume of the ocean basins to change.
• Sea level fluctuations, on geologic time scales,
  contribute importantly to the evolution of the
  worlds coasts.

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Figure 18.27
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Submergence Relative Rise of Sea Level

• Nearly all coasts have experienced submergence, a
  rise of water level that accompanies the most
  recent deglaciation.
• Most larges estuaries, for example , are former
  river valleys that were drowned by the recent
  sea-level rise.

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Figure 18.28
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Emergence Relative Fall of Sea Level

• Many marine beaches, spits, and barriers exist
  from Virginia to Florida.
• The highest reaches an altitude of more than 30
  m.
• These landforms are related to a combination of
  broad up-arching of the crust, as well as
  submergence.

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Sea-Level Cycles (1)

• Many coastal and off-shore features date to times
  when relative sea level was either higher or
  lower than now.
• The major rises and falls of sea level are global
  movements.
• By contrast, uplift and subsidence of the land,
  which cause emergence or submergence along a
  coast, involve only parts of landmasses.

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Figure 18.29
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Sea-Level Cycles (2)

• Movements of land and sea level may occur
  simultaneously, in either the same or opposite
  directions.
• Unraveling the history of sea-level fluctuations
  along a coast can be difficult and challenging.

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Coastal Hazards Storms

• Storms cause infrequent bursts of rapid erosion.
• Atlantic hurricanes can be exceptionally
  devastating.

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Coastal Hazards Tsunamis

• A strong earthquake, landslide, or volcanic
  eruption can generate a potentially dangerous
  tsunami (a seismic sea wave).
• It can travel at a rate as high as 950 km/h.
• It has long wavelength up to 200 km.
• It can pile up rapidly to heights of 30 m.

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Figure 18.31
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Figure 18.32
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Coastal Hazards Landslides

• Cliffed shorelines are susceptible to frequent
  landsliding as erosion eats away at the base of a
  seacliff.
• Sometimes landslides on cliffed shorelines give
  rise to giant waves that are even more
  destructive than the slides themselves.
• Very large waves have also been produced by
  massive coastal landslides during earthquakes.

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Protection Against Shoreline Erosion (1)

• Seacliffs can be protected by
• An armor consisting of tightly packed boulders so
  large that they can withstand the onslaught of
  storm waves.
• A strong seawall built parallel to the shore.

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Protection Against Shoreline Erosion (2)

• Protection of beaches
• A breakwater is an offshore barrier designed to
  protect a beach or boat anchorages from incoming
  waves.
• A groin is a low wall built out into the water at
  a right angle to the shoreline.
• Another way of protecting an eroding beach is to
  haul in sand and pile it on the beach at the
  updrift end.

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Effects of Human Interference

• Dams trap the sand and gravel carried by the
  streams, thus preventing the sediment from
  reaching the sea.
• Large resort developments may interfere with the
  steady state that had existed among the supply of
  sediment to the coast, longshore current and
  beach drift, and deposition of sediment on
  beaches.

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Ocean Circulation And The Carbon Cycle (1)

• Photosynthesizing marine organisms exchange
  dissolved CO2 for dissolved O2 in surface waters.
• A wide variety of organisms draw bicarbonate
  anions out of seawater to form calcium carbonate
  shells.
• Calcium carbonate accumulates on the seafloor if
  it is shallower than about 4 kilometers.
• In greater depths, the calcite tends to dissolve.

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Ocean Circulation And The Carbon Cycle (2)

• Cold O2-rich water sinks into the deep ocean from
  the surface waters of the North Atlantic and
  offshore Antarctica.
• Unusual depositional conditions are common when
  an ocean basin initially opens, and in its last
  stages of closure.

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Ocean Circulation And The Carbon Cycle (3)

• If evaporation dominates the regional climate,
  salinity increases in small semi-isolated ocean
  basins.
• Evaporite deposits can form if the connection to
  the worlds oceans is broken by tectonic activity
  or by a drop in sea level.

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Ocean Circulation And The Carbon Cycle (4)

• Geologists have estimated that the Mediterranean
  would evaporate completely in only 1000 years if
  the Straits of Gibraltar were blocked.
• Thick salt deposits beneath the Mediterranean
  seafloor tell us that it dried out as many as 40
  times between 5 and 7 million years ago.