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15. Oceans and coastal processes

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Oceans &
Coastlines
Introduction
Depth of the Ocean Floor
Salinity & Temperature of the Oceans
Oceanic Circulation
Coastlines
Wave Action
Shorelines & the Sediment Budget
Shoreline Protection
Summary
The oceans are the planet's last great living wilderness, man's only
remaining frontier on earth, and perhaps his last chance to prove himself a
rational species.
John L.Culliney
[The coastal zone is] rich in a variety of natural, commercial, recreational,
industrial, and aesthetic resources of immediate and potential value to the
present and future well-being of the nation.
Coastal Zone Management Act
Introduction
•
•
•
Over two-thirds of Earth’s surface is ocean.
Global climate patterns are influenced by oceanic
circulation which in turn is controlled by the physical
characteristics of the oceans.
The evolution of coastal landforms depends on the
interaction of wave action with processes in the continental
interior and human actions.
Over two-thirds (~71%) of the planet is covered in seawater.
The world’s oceans are unevenly distributed and lie mostly in
the Southern Hemisphere (88% seawater). The three major
oceans (Pacific, Atlantic, Indian Oceans) are connected
together along their southern margins by the Southern Ocean
that encircles Antarctica. Smaller, enclosed or partially
enclosed water bodies are termed seas or gulfs (Fig. 1). All of
these features are created by plate tectonic processes and
many continue to increase or decrease in size as plates diverge
or converge (see the Plate Tectonics chapter for more on plate
tectonics and the seafloor).
It is in the various small seas that we can observe the most
obvious negative consequences of human activity on the ocean
realm in the form of overfishing, pollution, eutrophication, and
habitat destruction. These same consequences are visited on the
margins of the major oceans.
Figure 1. The
distribution of the
world's oceans
and selected seas.
Nearly 90% of the
Southern
Hemisphere is
covered by
oceans.
2
Today we appreciate the oceans for their direct and indirect
impacts on human activity. This chapter is divided into two
halves. The first part considers the physical characteristics of
the oceans and begins with the depth of the ocean floor. The
floor of the ocean is a few hundred meters below sea level
along the margins of the continents but averages over 3 km
depth between the major oceans and reaches a maximum depth
of 11 km in the western Pacific. The ocean floor has the
potential to be a great source of mineral wealth in the future
and mining companies are already staking claims to potentially
lucrative sections of subsea real estate.
Average ocean
area and depth:
Pacific Ocean
165,250,000 km2
4.28 km
Indian Ocean
73,440,000 km2
3.89 km
Atlantic Ocean
82,440,000 km2
3.33 km
Figure 2. Damage
from Hurricane
Fran along the
coast of North
Carolina. Note
change in position
of house indicated
by arrow. Images
Ocean currents are influenced not only by the extent and depth
of the oceans but also the salinity and temperature of ocean
waters. Ocean waters are thought to have originated from
meteorites and icy comets colliding with the early Earth. Both
temperature and salinity change with depth and latitude and
each is influenced by atmospheric weather patterns. Each
factor is examined in the section titled salinity and
temperature of the oceans.
The final section of the first half of the chapter examines how
depth, salinity, and temperature combine to generate
characteristic patterns of oceanic circulation. Surface currents
are controlled by dominant wind patterns that are in turn linked
to the rotation of the planet. Deepwater circulation patterns are
controlled by the density of ocean waters and the distribution
of landmasses. Both sets of currents redistribute the Earth's
heat budget and play a crucial role in controlling climate.
In the second half of the chapter we focus on how human
activity impacts the ocean margins and enclosed seas.
Coastlines represent the fragile strip of land that borders the
ocean. Developed coastal areas are threatened with potential
loss of life and billions of dollars in property damage as a result
of storm impacts and long-term erosion. For example
Hurricane Fran (Fig. 2) devastated parts of several eastern
courtesy of USGS
Recent HighlightsHazards.
3
states and was especially destructive in North Carolina where
winds of over 100 mph generated more than $5 billion in
damages.
The evolution of landforms along a coastline depends upon the
interaction of wave action with the shore with processes in the
continental interior and human actions. The coastline is a
dynamic environment that advances or retreats depending upon
the balance between the supply of sediment and the material
removed by erosion. The section on shorelines and the
sediment budget examines this balance.
The National Park Service completed the tricky task of
relocating the historical Cape Hatteras lighthouse to a site
further inland in July 1999, to protect the light from erosion
that threatened to topple the structure. Twenty-six of the thirty
states bordering an ocean or Great Lake are presently
experiencing net loss of their shorelines. Well-intentioned
efforts at shoreline protection often resulted in the
construction of coastal structures such as jetties, groins, or
breakwaters that altered the natural movement of sediment
along the coastline and simply exacerbated existing erosion
problems. We finish by examining the interaction of shoreline
erosion and shoreline protection efforts along the southern
shore of Lake Erie.
Depth of the Ocean Floor
•
•
•
The average depth of the ocean floor is nearly 4 km and the
maximum depth is a little over 11 km along the Mariana
Trench.
Four principal depth zones can be identified in the oceans;
continental shelf (and rise), abyssal plain, oceanic ridge,
and oceanic trench.
The oceanic ridge is a submarine mountain range that
occupies much of the floor of the Atlantic Ocean.
The depth of the ocean floor varies from sea level to a
maximum of over 11 km along the Mariana Trench in the
western Pacific Ocean. In contrast, the highest landform on the
continents is Mt. Everest, which is approximately 9 km (5.6
4
miles) in elevation. In addition, the average elevation of the
land surface is less than a kilometer but the average depth of
the oceans is approximately 3.8 km (2.3 miles). We could
dump the continents in the ocean basins and still have plenty of
room to spare.
Recent analysis of originally classified satellite data at the
National Oceanographic and Atmospheric Administration's
(NOAA) National Geophysical Data Center (NGDC) has
allowed scientists to use slight variations in the elevation of the
ocean surface to determine the topography of the seafloor (Fig.
3).
Figure 3.
Characteristics of
the seafloor for the
eastern Pacific and
northwestern
Atlantic Ocean
basins. Map of
seafloor topography
from satellite
altimetry from
NOAA's National
Geophysical Data
Center.
Topography of the Ocean Floor
Beginning at the edge of the continents we can recognize four
principal depth zones in the oceans (Fig. 4). The first depth
level is the continental shelf, the shallow ocean floor (0-150
meters) immediately adjacent to continental land masses. The
shelf slopes gently toward the ocean from the coast with
maximum depths of a few hundred meters. The shelf may be a
relatively wide zone (hundreds of kilometers) adjacent to
passive margins (e.g., U.S. Atlantic Coast) or a narrow strip
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(tens of kilometers) landward of subduction zones (e.g., west
coast of South America).
The width of the shelf increases when sea level rises and
decreases during times of sea level decline. Rapid seafloor
spreading associated during the Early Cretaceous period (138100 million years ago) caused an expansion of the oceanic
ridge system, displacing water and raising sea level. In
contrast, sea level declined during the most recent Ice Age (2
million -10,000 years ago) when substantial volume of ocean
water was locked up in ice sheets. The Atlantic shore of the
North American continent was located near the present edge of
the continental shelf during the Ice Age.
Beyond the shelf break the ocean floor steps down across the
continental slope and continental rise oceanward of the shelf.
The slope and rise mark the transition from the relatively
shallow shelf to the second depth level, the deep ocean floor
known as the abyssal plain (Figs. 4, 5). As its name suggests,
the continental slope represents an increase in the gradient of
the ocean floor from the adjoining shelf. The slope is often
dissected by submarine canyons formed during times of lower
sea level. The canyons transported sediment from the shelf to
the continental rise. The rise is that section of the slope that has
been a site of sediment accumulation, resulting in an decrease
in slope gradient toward the abyssal plain. There is an
unbroken transition from slope to rise, to abyssal plain along
passive margins where the continental and oceanic crust make
up part of the same plate. These margins are not characterized
by the volcanism and earthquake activity that distinguishes
active margins located along plate boundaries.
Much of the abyssal plain lies over four kilometers below the
ocean surface and represents the flattest portions of the Earth's
surface. The plains are covered by layers of sediment
precipitated in the oceans and are dotted with submarine
volcanoes (seamounts). The ocean floor rises to a third level
approaching the oceanic ridge system, a submarine mountain
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Figure 4.
Diagrammatic view
of the principal
features of the
ocean floor include
an elevated
oceanic ridge;
deep, narrow
trenches; and a
gradual rise of the
ocean floor to the
continents along
passive margins.
Note narrow shelf
along active
margin, broad
shelf along passive
margin. Vertical
and horizontal
dimensions of
some elements
have been
exaggerated.
Figure 5. Principal
topographic
features of the
floor of the
southern Atlantic
Ocean. The
oceanic ridge
occupies more
than half the width
of the ocean floor.
Image modified from
original at NOAA's
National
Geophysical Data
Center.
chain that can be traced around the world. The ocean floor is
relatively shallow (less than 3 km) along the ridge system. The
ocean ridge system dominates the floor of the Atlantic Ocean,
occupying over half its width.
The final depth level is apparent in the narrow oceanic
trenches found along active margins marked by the boundary
between two plates (see Plate Tectonics chapter). The trenches
mark the locations of subduction zones where oceanic
lithosphere descends into the mantle. Trenches, the deepest
areas on the ocean floor, record depths of 7 to 11 kilometers (47 miles).
Think about it . . .
1. Examine the maps and diagrams of ocean floor
topography at the end of the chapter and answer the
related questions.
2. Label as many features as you can in the image of the
ocean floor adjacent to Monterey Bay, California, found
at the end of the chapter.
Salinity and Temperature of the Oceans
•
•
•
Salinity in surface waters is controlled by currents and
temperature and averages approximately 35 parts per
thousand.
Salinity values are relatively uniform in well-mixed surface
waters of the open oceans but are more extreme in
restricted waters of coastal seas.
Salinity and temperature change with depth with the most
rapid change occurring in a depth zone labeled the
halocline.
7
•
•
Ocean temperatures of 27oC are typical of tropical surface
waters and temperatures of 2oC are typical for deep ocean
waters.
Cold water is more dense than warm water, but ice (frozen
solid water) is less dense than liquid water.
Salinity and Latitude
Seawater contains dissolved salts. The concentration of salt in
seawater is salinity. Salinity varies around the world's oceans
depending on temperature and the mixing action of ocean
currents. Salinity is measured in parts per thousand (ppt; 10
ppt = 1%) of salt in water. The salinity of the warm, wellmixed surface waters over much of the world's major ocean
basins ranges from 33 to 37 parts per thousand (Fig. 6).
Figure 6. Map of
salinity at the
ocean surface.
Numbers
represent salinity
values in parts per
thousand. Salinity
in the open ocean
is greatest in
tropical regions
and decreases in
the isolated Arctic
Ocean. Map
generated at
University of Tokyo
website.
Higher- and lower-salinity values are observed in smaller,
restricted ocean basins and seas (Fig. 7). For example, salinity
values of 20 to 30 ppt are recorded for the high-latitude Arctic
Ocean and values of over 40 ppt occur in the narrow tropical
Red Sea basin between north Africa and the Arabian
peninsula. Salinity is higher at low latitudes because high
temperatures at these locations promote evaporation which
removes water but leaves the salt it contains behind. Salinity
values are lower at high latitudes because of the lack of
evaporation, high precipitation, and the influx of freshwater
from melting ice sheets. The isolated Baltic Sea between
Sweden and Finland has salinity values that approach
freshwater along its northern shore.
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Figure 7. High and
low salinity in
restricted seas in
low (Red Sea) and
high (Baltic Sea)
latitudes. Maps
generated at
University of Tokyo
website.
Salinity and Depth
Salinity values are variable in the shallow (e.g. 0-200 m) ocean
but are much more uniform in deeper waters below 2,000
meters (6,600 feet; Fig. 8). Salinity may decrease with depth in
the tropics but increases with depth at high latitudes (+60oN/S).
The salinity in the Arctic Ocean (north of 70oN latitude)
increases with depth from 30 to 35 ppt. Salinity in this
relatively isolated ocean basin remains uniform below a depth
of approximately 300 meters (1,000 feet).
The change of salinity occurs over a depth zone known as the
halocline. The depth range for the halocline is from
approximately 200 to 1,000 meters (660-3,300 feet) but will
show some variation with location. Salinity is uniform with a
value of 34 to 35 ppt below the halocline.
Temperature and Latitude
Solar radiation strikes Earth more directly at the equator and
tropics than in polar regions (Fig. 9). Radiation strikes Earth at
a lower angle near the poles and the Sun’s rays must therefore
penetrate a greater thickness of atmosphere. Some of the solar
radiation is scattered in the atmosphere and more heat energy is
lost near the poles as a result of scattering. Earth's surface at
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Figure 8. Northsouth profile
through the Pacific
Ocean along the
155.5 meridian
illustrating the
range of salinity
with depth and
latitude. Numbers
represent salinity
values in parts per
thousand. Cross
section generated at
University of Tokyo
website.
the equator receives 2.5 times more insolation, incoming solar
radiation, than the atmosphere above the poles. The highest
average annual ocean temperatures (~27oC) are present along
the equator and temperatures decrease symmetrically to the
north and south approaching 0oC at high latitudes (Fig. 10).
Water has two relatively unusual thermal properties that
make the oceans a great storage reservoir for heat energy and
contribute to global oceanic circulation patterns. First, the heat
capacity of a material is measured as the amount of heat
required (in calories) to raise the temperature of 1 gram of the
substance by 1oC. Materials with high heat capacity, such as
water, can absorb substantial quantities of heat without any
significant change in temperature. The ability of the oceans to
store heat plays a crucial role in controlling global climate
patterns.
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Figure 9. Solar
radiation is
distributed over a
wider area and
must penetrate a
greater thickness
of atmosphere at
the poles, reducing
the amount of
solar energy
reaching Earth's
surface.
Consequently,
ocean
temperatures are
greater near the
equator.
Second, cold water can be both less dense and more dense than
warm water. Water density increases as water temperature
decreases down to approximately 4oC. Below that temperature
water density decreases, especially when water changes state
from a liquid to solid (ice) form. Consequently, dense cold
water can sink below less dense warm water but ice will float
on the ocean's surface.
Figure 10. Map of
world's oceans
illustrating the
average annual
range of
temperature with
latitude. Numbers
represent
temperature in
degrees
Celsius. Map
generated at
University of Tokyo
website.
Temperature and Depth
The major oceans can be divided into layers of relatively warm
waters at shallow depths and cold waters at greater depths (Fig.
11). Surface waters are warmed by solar radiation and currents
cause thermal mixing that results in relatively uniform
temperature distributions by latitude. Sunlight doesn't penetrate
more than a few hundred meters below the ocean surface and
the impact of current activity diminishes with depth.
Temperatures exceed 20oC over much of the tropical ocean's
surface but decline to a chilly 2oC below 2,000 meters (6,600
feet) depth. The depth zone in which temperature decreases
rapidly is known as a thermocline. The base of the
thermocline is at a depth of approximately 1,000 meters (3,300
feet).
11
Figure 11. Northsouth profile
through the Pacific
Ocean along the
155.5 meridian
illustrating the
range of
temperature with
depth and
latitude. Cross
section generated at
University of Tokyo
website.
Think about it . . .
Use the data in the thermocline exercise at the end of the
chapter to plot two ocean temperature profiles and answer
the questions that follow.
Oceanic Circulation
•
•
•
•
12
Surface ocean currents are driven by winds and involve
only 10% of ocean waters.
Oceanic circulation patterns generate current systems
known as gyres.
Fast-flowing western boundary currents redistribute heat
from the relatively warm tropics to cooler high latitudes
The Coriolis effect is the name of the apparent deflection of
ocean currents or winds to the right of their course in the
Northern Hemisphere and to the left of their course in the
Southern Hemisphere.
•
The global conveyer belt moves heat energy from the
tropics to the poles in surface waters and transports cold
waters to warmer location by deep ocean circulation.
Ocean Currents
Ocean surface currents are mainly controlled by climate
(temperature, winds) but are also influenced by the distribution
of continents and Earth's rotation. Surface currents involve
approximately 10% of the world's ocean waters. Sea level is
higher at the equator because of thermal expansion of warm
waters and diminishes toward the poles. The contrast in the
elevation of the ocean surface is about 15 cm (6 inches). In the
absence of winds, water would simply flow away from the
equator ("downhill") under the influence of gravity. Winds
blowing over the ocean exert a frictional drag on surface waters
and are the principal force in controlling oceanic circulation.
Ocean currents follow wind directions except where wind
blows onland. The continents represent barriers to currents,
deflecting them to the north or south of their course (Fig. 12).
Figure 12.
Distribution of
ocean currents.
Note circular
patterns (gyres)
with clockwise
pattern north of
equator and
counterclockwise
pattern south of
equator.
Global atmospheric circulation patterns generate circular ocean
current systems known as gyres that are centered on 30 degrees
latitude in each of the major ocean basins (Fig. 12). Circulation
of the gyres is clockwise in the Northern Hemisphere and
counterclockwise in the Southern Hemisphere. Surface water
might take several months to a few years to complete the
circuit of a gyre.
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Westerly winds cause water to pile up along the western sides
of major oceans. These concentrations of surface water
generate fast-flowing western boundary currents that
redistribute warm tropical waters toward the poles (Fig. 12).
These currents (e.g., Gulf Stream, Kuroshio, Brazil) can be
thought of as marine rivers, relatively narrow (less than 100 km
across) water masses that flow at speeds of 100 to 200 km/day
for thousands of kilometers. The Gulf Stream can transport
over 50 million cubic meters of water per second, hundreds of
times more water than the Amazon, the world's largest river.
In contrast, the eastern boundary currents (e.g., Canary,
California, Peru) that complete the eastern leg of each gyre are
wider, carry less water, and move more slowly. The Canary
current, nearly 1,000 km (625 miles) wide, carries just a third
of the volume of water in the Gulf Stream and travels at tens of
kilometers per day.
Coriolis Effect
Currents are deflected to the right of their course in the
Northern Hemisphere and to the left of their course in the
Southern Hemisphere: this pattern is termed the Coriolis effect
(Fig. 13).
To an observer on earth, the path of a north- or south-directed
wind or ocean current will appear to be deflected. Note that the
wind or current doesn’t actually change direction, but the
planet beneath it has changed position. An object (rocket, air
mass, ocean current, etc.) that travels directly north or south in
the Northern Hemisphere appears to be deflected to the
right of its course when viewed from a location on the solid
14
Figure 13. Objects
on Earth's equator
travel further (and
faster) than objects
at higher latitudes.
It is this contrast in
velocity that results
in the Coriolis
effect. Objects
moving north from
the equator have a
greater component
of eastward motion
than objects at
higher latitudes
and thus appear to
deflect to the right
of their course.
Earth's surface. Objects are deflected to the left of their course
in the Southern Hemisphere. The net result of these
deflections is the circular path of ocean currents.
Global Ocean Conveyer Belt
Surface ocean currents carry warm water away from the
equator and toward the poles. Deeper currents are driven by
contrasts in water density and are dependent upon temperature
and salinity contrasts below 1,000 meters. The pattern of deep
currents is termed thermohaline circulation.
Currents in the North Atlantic cool as they approach the
northern latitudes. Cold, salty (dense) water sinks in the North
Atlantic Ocean south of Greenland and moves southward as the
North Atlantic Deep Water (NADW) current at depths of 2 to 4
km (1-2.5 miles; Fig. 14). When the NADW reaches Antarctica
it is diverted to the Indian and Pacific Oceans by the Antarctic
circumpolar current. The deep water current eventually comes
to the surface (upwelling) in the northern Indian and Pacific
Oceans before returning to the Atlantic Ocean by a series of
surface currents (Fig. 14). A complete loop may take 1,000
years.
Figure 14. Global
oceanic circulation.
Cold water sinks in
northern Atlantic
Ocean and travels
southward in deep
water before
upwelling in the
Indian and Pacific
Oceans. Surface
currents return
warm water to
Atlantic Ocean.
The sinking of this cold, dense water in the North Atlantic is a
key step in the global conveyer belt. This system moves
energy from the tropics to the poles and back again and serves
to moderate Earth's climate.
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Think about it . . .
1. A shipment of rubber elephants falls overboard in the
northern Pacific Ocean at location A on the map below.
What path do the elephants subsequently follow?
a) A to G to B to F to E to A c) A to G to C to E to A
b) A to E to C to G to A
d) A to E to F to B to G to A
2. How would the deflection of ocean currents be altered in
the Northern Hemisphere if Earth’s rotation changed
direction from west to east to east to west? Complete the
statement below using one of the choices that follow.
Ocean current directions would ____________ because
currents would be deflected to the _______________.
a) stay the same; right of their course
b) stay the same; left of their course
c) switch direction; right of their course
d) switch direction; left of their course
Coastlines
•
•
16
Over a quarter of the U.S. population lives in counties
along the Atlantic or Gulf Coasts.
Ten hurricanes have inflicted over a billion dollars of
damages on sites along the East Coast since 1980 and
Hurricane Andrew was the most expensive natural disaster
in U.S. history.
•
•
Coastal landforms are dependent on the interaction of wave
action with fluvial processes in the continental interior and
human activity.
Long-term processes such as climate cycles (hundreds of
years) and tectonic history (thousands to millions of years)
can raise or lower sea level or coastlines, respectively.
Author Henry Beston considered the sound of the ocean on a
shore to be one of the three great elemental sounds of nature
(the other two where the rain, and the wind in woods). Beston
should have known, he built a small cabin and lived alone on
Cape Cod, Massachusetts, for a year, recording his
observations of the changing coastline in a highly regarded
book, The Outermost House (1928).
Our culture places a premium on living along the fringe of the
continent facing the ocean. Over 70 million people live in
counties along the Atlantic and Gulf Coasts and coastal
property values in Florida rose by over 50% in the last 10
years. In an effort to ensure shoreline access to the public,
Cape Hatteras, North Carolina, was designated (August 17,
1937) the nation’s first national seashore. Other public
seashores followed, including Cape Cod (August 7, 1961). The
establishment of Cape Cod represented an early effort to create
parklands that were within easy access of millions of
Americans.
Figure 15.
Hurricane Fran
approaches the
U.S. coastline.
Image from Goddard
Space Flight
Center's Public
Photographic Image
Retrieval System.
Developed coastal areas are threatened with potential loss of
life and billions of dollars in property damage as a result of
storm impacts and long-term erosion. The weather patterns that
bring balmy breezes on warm summer days can also generate
devastating hurricanes (Fig. 15). There were 30 U.S. weather
disasters that resulted in over a billion dollars in damages
between 1980 to 1997. A third of these events were hurricanes
or tropical storms that battered the Atlantic or Gulf Coast
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states. Hurricane Andrew (August 24, 1992) decimated
southern Florida and was the most expensive natural disaster in
U.S. history (58 deaths, $30 billion damages). Many small
insurance companies went bust following Andrew. State Farm,
the nation’s largest insurance company, shelled out $3.7 billion
in claims. Many in the insurance industry are nervous of a
financial meltdown following a cataclysmic hurricane that
generates damages similar to those of an unnamed storm that
killed over 8,000 people in Galveston, Texas, September 8,
1900.
Figure 16. Rocky
coastline with
headlands and
beaches
characteristic of
U.S. West Coast
(top) and sandy
coastline with lowlying beaches
typical of the East
Coast (bottom).
Images courtesy of
NOAA photo
collection.
The short-term evolution of landforms along a coastline
represents the interaction of wave action with fluvial
processes in the continental interior and human activity. The
coastline is a dynamic environment that advances or retreats
depending upon the balance between the supply of sediment
and the material removed by wave erosion. This balance may
be upset by geologic processes that act at a variety of time
scales. Seasonal variations in stream flow and storm activity
affect the volume of sediment supplied to the coast and the rate
of erosion. Climate cycles that result in increasing or
decreasing sea levels will have long-term effects measured in
decades or centuries. Finally, tectonic cycles measured in
hundreds or thousands of years may continually revitalize
rugged coastlines by periodic uplifts. The role plate tectonics
plays in influencing the physical character of the coastline is
exemplified by the contrast between the sandy beaches of the
Atlantic shore (passive margin) and the rocky headlands of the
active margin represented by the Pacific Coast (Fig. 16).
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Some areas in the Gulf of Mexico coastline are actively
subsiding. Sediment deposited in a delta at the mouth of the
Mississippi River is submerged below sea level during
compaction. Subsidence rates are approximately 1 meter per
century. In the past this subsidence was compensated by
additional sediment supplied during flood events. However, the
construction of levees along the river’s channel prevents the
redistribution of sediment during flooding.
Figure 17. High
and low locations
of the Atlantic
coastline over the
last 5 million years.
Image from USGS
Coasts in Crisis.
Sea level has fluctuated considerably during the geological
past. It was ~100 m (330 feet) lower during the last ice age
when some of the water present in today’s oceans was locked
up in ice sheets (Fig. 17). Today sea level is believed to be
increasing at a rate of 10 to 15 cm (4-6 inches) per century in
response to the global warming which causes melting of ice
sheets and thermal expansion of ocean waters. Increasing sea
levels will have a much more significant impact on the lowlying East Coast than the rugged West Coast. Some islands in
Chesapeake Bay, between parts of Virginia and Maryland, that
were inhabited by colonists four centuries ago are now
submerged below the waters of the estuary.
19
Wave Action
•
•
•
•
•
Wave size, speed, and direction are controlled by winds.
The wave shape moves but the water within it does not
travel.
Wavelength is the distance between adjacent waves.
Waves increase height and break along the shore when the
depth of water diminishes to less than the wave base.
Waves are refracted toward headlands.
Wave Motion
Shoreline erosion is tied to the interaction between waves and
the coastline. Wave action erodes, transports, and redistributes
sediment along the shoreline. Wave size, speed, and direction
are controlled by winds. Water does not travel with waves but
simply moves vertically, tracing a circular path as a wave
passes. It is the shape of the wave (the waveform) that moves
across the ocean surface, not the water itself (Fig. 18).
Consider the "wave" performed by a crowd at a sporting event.
The wave passes around the stadium as each individual in turn
stands up and sits down. The people in the stadium play the
role of the water particles in ocean waves.
Figure 18.
Sequential wave
motion in open
water. The
waveform moves
while water
particles follow a
circular path and
remain in place.
The distance between adjacent wave crests is termed the
wavelength. Wave motion only affects the surface waters.
Wave motion decreases downward, with increasing distance
from the winds, to the wave base. The depth of the wave base
is approximately half the wavelength (Fig. 19).
20
Figure 19. Water
particle motion
decreases
downward from
waves on the
ocean surface
ending at the
wave base.
Waves and Coastlines
Water piles up as waves approach shore because of the effect
of friction between the wave and the seafloor above the wave
base (Fig. 20). Water in contact with the seafloor is slowed by
friction but water on the surface is unaffected and moves
forward more rapidly, encroaching on preceding waves and
piling up to form taller, steeper waves.
Figure 20. Waves
steepen and
wavelength
decreases as
waves approach
shore. Steepened
waves eventually
collapse (break)
forming surf that
surges up the
slope of the shore.
The wave eventually collapses (breaks) forming surf that
washes up the shore before flowing back down to sea.
Turbulent flow in the surf zone, between the line of breaking
waves and the shore, can cause erosion by washing sand
particles from beaches and breaking rocks from headlands.
Some of this material may also be transported along the shore
by currents in the surf zone.
Waves approaching a coastline are reoriented to follow the
slope of the seafloor, this process is termed wave refraction.
The seafloor will shallow more rapidly toward headlands than
adjoining bays and waves approaching a rugged coastline will
be refracted toward the resistant headlands (Fig. 21). One result
of this pattern is that wave erosion is concentrated on
headlands while adjoining bays become areas of deposition.
Sediment eroded from the headlands are deposited in the
relatively quiet waters of the bays to form beaches. The
coastline is straightened as erosion wears away the headlands
and the bays are filled with sediment (Fig. 21).
21
Renewed tectonic activity may result in uplift of the coastline
and another cycle of erosion. The rugged coastline of Oregon
and Washington is constantly revitalized as part of an active
plate boundary separating North America and the small Juan de
Fuca Plate. The coastline is dominated by rocky headlands
separated by small (pocket) beaches.
Figure 21. Wave
erosion is
concentrated on
headlands (top)
and bays become
areas of deposition
(left). The coastline
is straightened
(bottom right) as
erosion continues
as the headlands
are eroded back
and the bays are
filled with
sediment.
Wave erosion associated with large storms can remove large
sections of beach in a single storm (Fig. 22). Recent hurricanes
on the East Coast and El Nino-induced storms along the West
Coast were responsible for substantial coastal erosion.
Figure 22. Beach
erosion near San
Diego, California,
following winter
storms (1997)
associated with the
El Nino weather
phenomenon. Top
image taken in
October 1997;
bottom image from
April 1998. Images
courtesy of USGS
Center for Coastal
Geology.
22
Shorelines and the Sediment Budget
•
•
•
•
Twenty-six of the thirty states bordering an ocean or Great
Lake are presently experiencing net loss of their shorelines.
Waves that strike a beach at an angle will generate a
longshore current that transports sediment parallel to the
beach in the surf zone.
The sediment budget is the balance between material added
to the shore by deposition and the material removed by
erosion.
Sediment entering the shoreline system is frequently
reduced by human activity (dams, development) and is also
subject to changing natural conditions (drought).
Shoreline Erosion
The short-term evolution of landforms along a coastline
represents the interaction of wave action with fluvial
processes in the continental interior, and human activity. The
coastline is a dynamic environment that advances or retreats
depending upon the balance between the supply of sediment
and the material removed by wave erosion. Twenty-six of the
thirty states bordering an ocean or Great Lake are presently
experiencing net loss of their shorelines (Fig. 23). The most
Figure 23. Coastal
erosion rates along
the Atlantic, Gulf,
and Great Lakes
shorelines. Image
modified from
original in Coasts in
Crisis, an on-line
USGS publication.
23
rapid erosion rates are along the Gulf Coast (Louisiana) and
Atlantic shore (South Carolina, Maryland, New Jersey).
Erosion rates in Louisiana are as high 20 meters per year.
Much of the coastline of Louisiana is represented by sediment
deposited in a delta at the mouth of the Mississippi River. The
delta is submerged below sea level as the sediment becomes
compacted. Subsidence rates are approximately 1 meter per
century. In the past this subsidence was compensated by
additional sediment supplied during flood events. However, the
construction of flood control levees along the river’s channel
across the delta has robbed the delta of its primary source of
sediment.
Erosion is accelerated by the actions of storm surges, violent
waves associated with large storm events. Erosion is most
effective on unconsolidated sediments of beaches or dunes.
Rocky shorelines are less susceptible to erosion. Hurricanes
generate waves that can destroy eastern beaches in a matter of
hours. West Coast erosion (Fig. 24) is particularly severe in
association with winter storms, especially those occurring
during an El Nino year when sea level is higher than normal.
Typical erosion rates for part of the California coastline near
San Francisco are 0.2 meters per year. Yet, winter storms
during 1997-1998 caused some cliff lines to recede over 10
meters (33 feet), more than 50 times faster than normal.
24
Figure 24.
Retreating
shorelines at
Pacifica (top) and
Monterey Bay
(bottom left),
California. Twelve
homes were
condemned when
the cliff at Pacifica
retreated 10 meters
because of winter
storms (1997-1998).
The graph illustrates
the magnitude of
shoreline retreat (15
meters) at the
Monterey Bay
example. Lower
image and original
graph courtesy of
USGS Center for
Coastal Geology;
upper image from
NASA Airborne
Topographic Mapper
site describing West
Coast projects.
Sediment Transport
Wave action causes the erosion, transportation and redeposition of sand along the Atlantic shore. In this
environment, the relative orientation of the waves to the beach
controls the distribution of erosion and deposition. Waves that
strike a beach at an angle (Fig. 25) will generate a longshore
current that transports sediment parallel to the beach in the
surf zone (Fig. 26). In addition, sediment on the beach is also
transported laterally parallel to the shoreline. Sediment is
carried up the beach parallel to the direction of wave motion.
Water washes back down slope carrying the sediment parallel
to the slope of the beach. If the waves strike the beach
obliquely the sediment is transported along the beach in a
zigzag pattern. Both mechanisms ensure that sediment is
transported along the length of the beach (Figs. 26, 27).
Figure 25. Waves
strike a
Washington beach
obliquely,
generating a
longshore current
that transfers
sediment from left
to right along the
shore. Image
courtesy of USGS
Center for Coastal
Geology.
Figure 26.
Longshore
currents are
generated in the
surf zone where
waves strike
obliquely against
the shoreline.
Sand particles are
moved along the
beach in a zigzag
pattern and
sediment is
transported along
shore.
25
Sediment transported along the beach can give rise to some
characteristic landforms when it is eventually deposited in calm
waters of adjoining bays (Fig. 28). Sediment may block the
entrance to the mouth of a bay to form a baymouth bar or may
only partially block a channel to form a landform termed a
spit.
Figure 27. Addition of
sand to a beach as a
result of longshore
currents generated
during winter storms,
Tomales Bay, north of
Point Reyes,
California. Left: Prior
to storms. Right: After
storms, sand was
transported along the
shoreline from the left
side of the image.
Images courtesy of
USGS Center for
Coastal Geology.
Figure 28. Matagorda
Bay, Texas. The bay
has been almost
cutoff from the Gulf of
Mexico by narrow
baymouth bars
formed from sediment
transported along the
coastline by
longshore currents.
Image courtesy of
NASA's Earth from
Space.
Sediment Budget
The beach (or shoreline) is not the final resting place for the
sediment. It is an intermediate stop on a longer journey.
Sediment is transported to the coast by streams, redistributed
along the coast by longshore currents, and eventually deposited
offshore.
Shoreline processes are influenced by the sediment budget:
the balance between material added to the shore by deposition
and the material removed by erosion. Sediment added to the
shore comes from headland erosion or is delivered to the coast
by stream flow (Fig. 29). Much like a financial balance sheet,
the sediment budget remains in a state of equilibrium as long as
the sediment coming in is equal to the material that is lost.
However, the material entering the shoreline system is
26
Figure 29.
Sediment supply
along a coastline
can be disrupted
by human activity
such as dredging
or building jetties
to prevent in-filling
of bays or stream
channels; or by
shoreline
development that
results in the
construction of
seawalls or
breakwaters to
prevent erosion
(and sediment
production) that
threatens homes
and other
buildings.
frequently reduced by human activity and is also subject to
changing natural conditions (Fig. 29).
The construction of dams on major rivers will reduce the
volume of sediment reaching the coast, resulting in sediment
starvation. Sediment that would once have been deposited
along the shoreline is trapped in upstream reservoirs. Dams
within the Mississippi River drainage basin have reduced
sediment supply to the Mississippi delta by approximately half.
Drought conditions may also reduce streamflow and thus
diminish sediment transported to the shore by streams. Coastal
development may result in the construction of structures
designed to reduce erosion (e.g., breakwaters, seawalls) or to
control the local depositional patterns to prevent infilling of
navigation channels (Fig. 29).
The loss of sediment from any of these sources may cause
longshore currents to cannibalize existing beaches and cause a
short circuit in the natural cycle of shoreline erosion, transport
and deposition. The net result is the loss of beaches and
increased erosion of the shoreline, especially during large
storms.
Think about it . . .
Scientists surveyed the Californian coastline to evaluate
erosion and/or accretion associated with winter storms of
1997-1998. Two images of the coastline around Ventura,
California, are presented at the end of the chapter that
show the coastline before and after the storms. Answer the
related questions.
27
Shoreline Protection
•
•
•
•
•
•
Structures built to protect coastlines may prevent erosion of
part of the shoreline but can result in accelerated erosion
elsewhere.
Seawalls, groins, and breakwaters differ in their locations
and orientations relative to the shoreline but all act to
prevent erosion and/or encourage deposition.
Artificial beach nourishment occurs when sand is dredged
and pumped onto the beach from offshore.
Lake Erie is the shallowest of the Great Lakes and is
surrounded by the large population centers along U.S. and
Canadian shores.
Most of Lake Erie's shoreline is eroding and erosion rates
are greatest where glacial deposits form the shoreline and
are least where bedrock forms the coast.
Presque Isle in the east basin of Lake Erie is eroding
because of shoreline protection measures in the central and
west basins.
Techniques that attempt to prevent beach erosion revolve
around methods to limit the removal of sediment along specific
areas of the coast or involve adding material to areas
undergoing erosion. Unfortunately, nearly all these methods
have shortcomings. Most of these methods aim to prevent
erosion but some recent regulations have recognized that
erosion will inevitably occur and have instead focused on
controlling construction adjacent to eroding coasts. Florida
introduced strict regulations that required buildings constructed
near the shoreline to meet rigorous standards to prevent
destruction from storm surges or high winds. No buildings
constructed to these standards failed when Hurricane Opel
struck southern Florida in 1995. In contrast, 56% of all other
habitable buildings in the storm's path were heavily damaged.
Seawalls
Sea walls are built to protect shoreline property owners from
receding shorelines (Fig. 30). As such, they represent a barrier
between waves and the shoreline. Waves are reflected back
from the walls onto the adjoining beach and may promote
beach erosion. Unfortunately, erosion is often exaggerated
where the seawall ends, causing the shoreline to recede more
rapidly on either side of the structure.
28
Figure 30. Seawall
(left) at base of
eroding cliff, north
of Monterey,
California. Note
how erosion is
exaggerated
where the seawall
ends. Image
courtesy of USGS
Center for Coastal
Geology.
Groins
Groins are wall-like structures built along beaches to act as
barriers to longshore currents (Fig. 31). A longshore current
will lose velocity as it meets the groins, causing the current to
deposit part of its sediment load on the upcurrent side of the
groin, thus building up the adjacent beach. However, as the
current passes the groin it picks up additional sediment on the
downcurrent side of the structure causing local erosion.
Figure 31. A groin
adjacent to Cape
Hatteras
lighthouse, North
Carolina, prior to
relocation of
lighthouse (1999).
Image courtesy of
USGS Center for
Coastal Geology.
Breakwaters
Breakwaters are barriers built offshore to protect part of the
shoreline (Fig. 32). They act as obstacles to waves, preventing
erosion and allowing the beach to grow behind the structure.
However, the beach behind the breakwater often grows at the
expense of the adjacent unprotected shoreline.
Artificial Beach Nourishment
Artificial beach nourishment occurs when sand is dredged and
pumped onto the beach from offshore (Fig. 33). The beach will
29
Figure 32.
Breakwaters,
south shore of
Lake Erie,
Maumee Bay State
Park, Ohio. Image
courtesy of the U.S.
Army Corps of
Engineers.
grow if material is added to the beach faster than natural
processes remove it. This is a temporary fix because the sand is
eroded again and must be replaced. Material added to many
East Coast beaches remained for less than two years before the
beach returned to its prenourishment state. One successful
effort was for Miami Beach, Florida, which spent $64 million
in the 1970s to stabilize and expand its beaches to meet the
needs of the booming tourism industry.
Figure 33. Beach
nourishment
project on Ocean
City Beach,
Maryland. Note
wider beach near
bottom of image.
Pipeline pumps
sand collected
offshore by a
dredge onto
beach. Images
courtesy of the U.S.
Army Corps of
Engineers Digital
Images Library.
Example: Lake Erie
Lake Erie is the shallowest of the Great Lakes and was formed
when glaciers scoured out a depression in the bedrock during
the last Ice Age. The lake is the 11th largest in the world and
represents the "North coast" of Ohio, Pennsylvania, and
western New York. It covers over 26,000 km2 and has a
maximum depth of 64 meters (Fig. 34). The lake is divisible
into three separate basins that increase in depth from west to
30
east. Regional flow in the lake carries water from west to east
but local currents may reverse that direction.
Figure 34. Map of
Lake Erie. The map
was modified from a
bathymmetry map at
the Great Lakes
Forecasting System
website.
Figure 35. Below
right: Erosion of
the southern shore
of Lake Erie at
Painesville-on-theLake, Ohio. Note
how roads end at
top of receding
cliff-line. Below
left: The view from
the lake (north)
toward the
shoreline (south).
Image courtesy of
Dr. Charles Carter.
Ninety-five percent of the lake's shoreline in Ohio is eroding
(Fig. 35). Average erosion rates are 10 to 80 cm per year (0.42.7 feet per year) but rates of up to 33 meters (100 feet) per
year have been recorded. Rates are largely controlled by the
geology of the coastline. More resistant rocks such as
sandstone erode slowly whereas glacial sediments and weaker
rocks erode more rapidly. Economic losses from damages to
structures are estimated to be millions of dollars per year.
Erosion of the shorelines of Lake Erie is evident in the image
below that shows how the coastline receded southward,
eroding the land along the northern edge of this subdivision.
The white lines are roads. Notice how the east-west trending
road near the center of the image is truncated by the cliff.
Further information on the costs and effects of coastal erosion
in Ohio are provided at the Ohio Geological Survey website.
Development around the Great Lakes has covered much of the
land area, reducing sediment sources. Population around Lake
31
Erie alone has climbed from 3 million to over 14 million
people today.
Erosion control measures in the western half of the lake have
reduced sediment supply and resulted in increased erosion rates
along the shoreline in the east basin. Presque Isle is an unusual
sand deposit that built outward from the Lake Erie shoreline
near Erie, Pennsylvania (Fig. 36). The construction of coastal
structures in Ohio to the west blocked the eastward flow of
sediment needed to replenish the deposit. The narrow neck that
connects the island to the mainland is eroding as fast as 2.5
meters per year. The U.S. Army Corps of Engineers has the
responsibility of coming up with a plan to protect Presque Isle.
Figure 36. Top:
View of Presque
Isle, Pennsylvania,
from the southwest
looking along the
shore of Lake Erie
to the
northeast. Erie,
Pennsylvania, is to
the right of the
image. Current
directions along
the shoreline are
from the bottom of
the image toward
the top. Below:
Breakwaters and
groins, Presque
Isle. Deposition
occurs behind
breakwaters that
form barriers to
onshore currents.
Think about it . . .
Create a concept map that illustrates the characteristics of
sediment erosion, deposition, and transport along the
shoreline and the factors that affect these processes.
32
Summary
1. How much of Earth's surface is covered by oceans?
Approximately 71% of the planet is covered by oceans. There
are three major oceans (Indian, Pacific, Atlantic) that are
connected along their southern margins by the Southern Ocean
circling Antarctica.
2. How does the depth of the oceans vary?
The average depth of the ocean floor is nearly 4 km and a
maximum depth of a little over 11 km has been recorded along
the Mariana Trench in the western Pacific Ocean. Four
principal depth zones can be identified ranging from the
shallow shelf along the continental margins (~100s meters), to
the near horizontal floor of the abyssal plain (4-5 km), rising to
the crest of the oceanic ridge (~3 km), and descending to the
narrow depths of the oceanic trenches (7-11 km).
3. What factors control variations in the salinity of the
oceans?
Seawater contains dissolved salts. The concentration of salt in
seawater is salinity. Salinity is measured in parts per thousand
(ppt; 1 ppt = 0.1%, 10 ppt = 1%) of salt in water. Salinity
varies depending on temperature and the mixing action of
ocean currents. Salinity is higher at low latitudes because high
temperatures at these locations promote evaporation which
removes water but leaves the salt it contains behind. However,
the mixing action of ocean currents ensures a consistent
salinity range of 33 to 37 parts per thousand for much of the
open ocean.
4. Where are salinity values highest and lowest?
Salinity values are most extreme in restricted ocean basins
where the effects of evaporation or stream inflow are
exaggerated. Salinity values of over 40 ppt occur in the narrow
tropical Red Sea basin between north Africa and the Arabian
peninsula. Salinity is lower at high latitudes because of the lack
of evaporation, high precipitation, and the influx of freshwater
from melting ice sheets. Salinity values of less than 10 ppt are
recorded from the Baltic Sea in northern Europe.
5. How does salinity change with depth?
Salinity increases with depth in the restricted northern ocean
waters (Arctic Ocean) but decreases slightly with depth in the
tropical open ocean. Salinity is much more consistent at depth
(2 km) below the halocline.
33
6. What factors control the temperature of the oceans?
Solar radiation is distributed over a wider area and must
penetrate a greater thickness of atmosphere at the poles,
reducing the amount of solar energy reaching Earth's surface.
Consequently, ocean temperatures are greater near the equator.
The highest ocean temperatures (~27oC) are present along the
equator and temperatures decrease symmetrically to the north
and south approaching 0oC at high latitudes.
7. How does temperature vary with depth?
Temperature decreases significantly with depth. The effects
insolation and the surface mixing of currents diminish with
depth. Temperature declines steadily to a depth of
approximately 1,000 meters. Deeper waters have a uniform
temperature of 1 to 2oC.
8. What controls the direction of ocean currents?
Winds generated by atmospheric circulation patterns represent
the principal control on ocean currents but the distribution of
continents and the Coriolis effect also affect currents.
Circulation patterns known as gyres control currents in the
open oceans. Currents form a clockwise pattern in gyres of the
Northern Hemisphere and a counterclockwise pattern south of
the equator.
9. Which currents are important in global climate?
Although all currents contribute to global climate patterns, the
western boundary currents such as the Gulf Stream and Brazil
currents have an especially significant role as they transport
warm tropical waters to higher latitudes.
10. What is the Coriolis effect?
The Coriolis effect represents the deflection of currents to the
right of their course in the Northern Hemisphere and to the left
of their course in the Southern Hemisphere. The Coriolis Effect
results from the contrast in the Earth's rotation velocity with
latitude.
11. What is thermohaline circulation?
Thermohaline circulation occurs in deeper ocean waters and is
driven by density contrasts related to differences in water
temperature and salinity. Thermohaline circulation drives the
global conveyer belt that causes surface waters to sink in the
northern Atlantic Ocean and sends cold, deep currents through
34
the world's oceans before upwelling in the northern Pacific and
Indian Oceans.
12. What factors influence the development of coastal
landforms?
The coastline is a dynamic environment that advances or
retreats depending upon the balance between the supply of
sediment and the material removed by wave erosion. Seasonal
variations in stream flow and storm activity affect the volume
of sediment supplied to the coast and the rate of erosion.
Climate cycles that result in increasing or decreasing sea levels
will have long-term effects measured in decades or centuries.
Finally, tectonic cycles measured in hundreds or thousands of
years may continually revitalize rugged coastlines by periodic
uplifts.
13. How does water move in waves?
The waveform is a shape that moves across the open ocean but
the water particles don't move with the wave but instead trace
out a circular path while remaining essentially in place.
14. What happens when waves approach the coast?
Material is eroded and redeposited by turbulent flow that
occurs in the surf zone as waves break along the shoreline.
Wave refraction results in wave action being concentrated on
headlands. Deposition occurs in the calmer waters of sheltered
bays.
15. Where does coastal erosion occur?
Coastal erosion occurs where erosion by wave action is not
balanced by local deposition of the eroded material and the
supply of sediment from streams. Erosion exceeds deposition,
resulting in a loss of shoreline, along most of the U.S.
coastline.
16. What is a longshore current?
A longshore current is generated when waves strike the coast at
an angle. The current forms in the surf zone and transports
sediment laterally along the shoreline. Longshore currents can
result in the formation of characteristic depositional landforms
parallel to the coastline.
17. What is the sediment budget?
The sediment budget is the balance between material added to
the shore by deposition and the material removed by erosion.
The construction of dams will reduce the volume of sediment
35
reaching the coast. Drought conditions may also reduce
streamflow and thus diminish sediment transported to the shore
by streams. Coastal development may result in the construction
of structures designed to reduce erosion (e.g., breakwaters,
seawalls) or to control the local depositional patterns to prevent
infilling of navigation channels (jetties).
18. Why are shoreline protection structures regarded as a
mixed blessing?
Seawalls, groins, and breakwaters act to prevent erosion and/or
encourage deposition. However, structures built to protect
coastlines may prevent erosion of part of the shoreline but can
result in accelerated erosion elsewhere. Artificial beach
nourishment occurs when sand is dredged and pumped onto the
beach from offshore but has a limited life span unless the
processes that caused the original erosion are stopped.
36
Image Analysis: Continental Margins
1. The image below illustrates part of South America. The
ocean floor is represented by the blue colors. The deeper
depths are shown as darker blues. Label the following
features on the image.
active margin
continental shelf
abyssal plain
Atlantic Ocean
passive margin
continental slope
oceanic trench
Pacific Ocean
2. Label three active continental margins on the map below
with the letter A and label three passive continental
margins with a P.
37
3. Which of the profile views below most accurately reflects
the topography of the ocean floor along an east-west line
from Australia to South America?
4. Sketch and label a profile of the ocean floor topography
between North America and north Africa in the space
below.
38
Image Analysis: Monterey Bay
The image below illustrates the topography of the ocean floor
adjacent to Monterey Bay, California. The landforms of
California are shown on the top right side of the image in the
green and gray colors. The blue colors represent the ocean
floor.
Label as many features as you can in the image.
39
Thermoclines
The thermocline marks a zone of relatively rapid temperature
change between the warm surface currents and deeper cold
waters. This exercise will attempt to identify the depth range of
this boundary by identifying the location of the thermocline.
1. Plot the data points from the table below on the graph on
the next page and sketch a best fit line for the data.
Depth (m)
0
100
300
500
1,000
1,500
2,000
3,000
Data 1
12
12
11
9
8
7
6
5
Data 2
30
24
21
17
13
10
9
8
2. These data come from subpolar and tropical oceans. Label
the lines as subpolar and tropical.
3. Identify the approximate range of depths for the
thermocline on each curve and label those parts of the
curves accordingly.
4. Circle the data points on the graph where you expect to find
the highest and lowest salinity values. Explain why you
chose those points.
40
41
Doing Science: Measuring Rates of Coastal
Erosion/Accretion
Scientists with the U.S. Geological Survey devised a plan to
evaluate erosion (removal of material along the shoreline) or
accretion (addition of material) over 1,200 km (750 miles) of
shoreline following the winter storms of 1997 to 1998. They
flew aerial surveys of the coastline before and after the storm
season and compared the images they acquired.
Two images of the coastline around Ventura, California, are
presented on page 43. One represents the coastline prior to the
winter storms and the other illustrates the state of the coastline
after the storms. The accompanying graph shows the relative
positions of the beach before (blue squares) and after (red
triangles) the storms.
1. Examine the graph. What happened to the beach as a result
of the winter storms?
2. How much did the width of the beach increase/decrease
over the winter?
3. Assuming that these changes occurred over a six-month
period, the rapid rate of coastal erosion/accretion
represented by the winter storms was approximately
________ m/yr? (Note: this was an exceptional cycle of
erosion/accretion and we shouldn’t consider such rates to
be “normal”).
a) 20
b) 50
c) 120
d) 200
4. Which photograph represents the beach in October 1997?
a) A
42
b) B
x
x marks
the
same
location
in each
image
A
x
B
43
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