Download Chapter 16 Oceans, Shorelines and Shoreline Processes Exploring

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Surface currents are controlled by
• Solar radiation
• Coriolis Force
Chapter 16
Oceans, Shorelines and
Shoreline Processes
Large scale, vertical currents controlled by:
• temperature differences
• density differences
Oceanic currents are complex
and controlled by water depths,
locations of continents, etc.
Exploring the Oceans
Scientific study of the ocean basins began in the late 1700’s.
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Scientists discovered that the ocean floor was not flat, but had varied topography like the land.
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Today, research ships investigate the sea floor by drilling, echo sounding, and seismic profiling. 1
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Sediments and Sedimentary Rocks of
the Ocean Floor
Seawater
 Seawater contains more than 70 elements in solution.
 Sodium and chloride make up 85.6% of the chemicals. Deep sea sediments are fine-grained.
Sediments and Sedimentary Rocks on the Seafloor
Sediments and Sedimentary
Rocks on the Seafloor
– Deep-ocean sediments mostly consist of:
• carbonate and siliceous skeletons of microscopic
organisms (calcareous and siliceous ooze)
• pelagic clays, which are derived from continents
and oceanic islands; mostly wind-blown dust and
volcanic ash
• Reefs are wave resistant structures built by the skeletons of corals and other marine organisms. Fig. 16.5, p. 391
Sediments and Sedimentary Rocks on the Seafloor
• Although commonly called coral reefs, they often contain clams, sponges and other marine organisms. • Most coral reefs grow in shallow tropical waters, where temperatures do not drop below 20oC. • Because corals depend on symbiotic algae that require photosynthesis, they rarely grow at depths greater than 50 m. Fig. 16.6b, p. 392
Sediments and Sedimentary Rocks on the Seafloor
– Three types of reefs: – Fringing: up to 1 km wide, attached to a landmass, such as a volcanic island
– Barrier: separated from the landmass by a lagoon. – Atoll: circular or oval reefs surrounding a lagoon
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Shorelines and Shoreline Processes
Tides
• A shoreline is the area from low tide to the highest level on land affected by storm waves.
• A coast is a much broader area. It includes the:
• Shoreline
• Nearshore sandbars and islands • Sand dunes
• Marshes
• Sea cliffs
Fig. 16.7, p. 393
Tides occur because of the gravitational attraction of the moon and
sun, and the rotational force on water provided by the rotation of
the Earth. The water bulges towards the moon due to the greater
gravitational force there than at the center of the Earth. The water
bulges away from the moon due to the greater rotational force away
from the moon than at the center of the Earth. Therefore, two high
tides and two low tides on opposite sides of the Earth!
Mt. Saint Michel, France
Spring tides occur when the Sun
and Moon are aligned. How often
does this occur?
Neap tides occur when the Sun
and Moon are not aligned.
Tides do not occur at the same time every day because the Moon
takes 24 hours and 50 minutes to get over the same part of the
Earth each day, hence the tides come at different times each day.
The cycle is predictable, however, and tide tables are readily available.
The height of tides varies depending on the shape of the coast, location
on the Earth, etc.
Waves are formed by the interaction of wind on the surface
of the water. The height of waves depends on three factors:
• Wind velocity
• Amount of time the wind blows
• The distance over which the wind blows (the fetch).
Wave motion decreases with depth.
New Brunswick, Canada
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Shorelines and Shoreline Processes
• Wave Terminology
– Crest – highest part of the wave
– Trough – lowest part of the wave
– Wavelength – distance from crest to adjacent crest
– Wave height – vertical from the trough to the crest
– Wave base ‐ a depth corresponding to one‐half wavelength
Shorelines and Shoreline Processes
• Celerity (C) is the speed of an advancing wave. – C= L/T
– L = wavelength
– T = wave period, time it takes for two successive wave crests to pass a given point
Fig. 16.10a, p. 395
Fig. 16.10a, p. 395
Shorelines and Shoreline Processes
• Waves are responsible for most erosion, sediment transport and deposition in coastal areas.
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Wave Generation
Most geologic modification
of shorelines is
accomplished by windgenerated waves, especially
storm waves.
Waves can also be
produced by earthquakes,
volcanic eruptions, and
landslides.
Shorelines and Shoreline Processes
• Waves
– Shallow‐Water Waves and Breakers
– As waves enter water shallower than their wave base (1/2 their wavelength), the waves hit the seafloor. – The wave shape changes and water is displaced in the direction of wave advance.
– Broad, deep water waves become sharp crested.
– The waves then become oversteepened and plunge forward as breakers. Shorelines and Shoreline Processes
• Waves
• Wave Generation
– Wave size in wind‐generated waves is controlled by fetch.
– Fetch is the distance the wind blows over a continuous water surface.
– Waves in the ocean have a larger fetch than waves in lakes and ponds.
Shorelines and Shoreline Processes
• Nearshore Zone – an area that extends from the upper limit of the shoreline to just beyond the area of breaking waves.
– Includes the breaker zone and the surf zone
Fig. 16.10a, p. 395
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Shorelines and Shoreline Processes
As waves approach the
shore, they bend or
refract. Why?
• Nearshore Zone – Includes:
• Longshore currents
• Rip currents
Fig. 16.10a, p. 395
Wave Refraction
Wave refraction also occurs at headlands, concentrating
wave energy at the headlands, leading to more erosion.
California
Wave Refraction
Over time, the
concentration of
wave energy at
the headlands
tends to erode
the coastline…
California
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Shorelines and Shoreline Processes
• Nearshore Zone – Includes:
… until the coastline
becomes straight.
• Longshore currents
• Rip currents
Fig. 16.10a, p. 395
Shorelines and Shoreline Processes
Shorelines and Shoreline Processes
• Longshore Currents
• Wave Refraction
– Waves that hit the shore at an angle generate a current along the shore in the same direction as the approaching waves.
– Longshore currents can produce significant erosion, transportation and deposition.
– Most waves approach a shoreline at some angle. – The forward end of the wave hits the shallow water first and slows down, while the trailing end races ahead, thereby bringing the wave more nearly parallel to the shoreline. Fig. 16.17a, p. 401
Fig. 16.11, p. 397
Shorelines and Shoreline Processes
Beach Profile
• Rip Currents ‐ Narrow, surface currents which rapidly carry water from the nearshore zone seaward through the breaker zone. Extremely dangerous to swimmers
• Created by longshore
currents
Fig. 16.12, p. 397
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Foreshore
Berm
Backshore
California
The Grand Strand,
Myrtle Beach, SC
Erosion and Deposition Along Shorelines
• Seasonal Changes in Beaches
– Summer beaches usually have a wide berm, gently sloping beach face, and smooth offshore profile. – Winter beaches tend to be steeper and made up of coarser materials than summer beaches because waves are more energetic during this season. Berms may be absent. Beaches, or
any shoreline,
will change over
time.
Winter
– The sand eroded from beaches during winter is stored in offshore bars until it is driven back shoreward by more gentle waves.
Summer
A longshore current is developed by some of the
wave motion remaining parallel to the coastline.
Longshore currents move sand along the shore,
Depositing sand in the form or baymouth bars or spits.
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A baymouth bar
A spit
Oregon
California
Theories of Barrier Island Formation
The shoreline
budget works
through a series
of inputs & outputs.
The Outer Banks, NC
North Carolina
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An erosional coastline has features such as these:
Near Pensacola, Florida
Australia
Australia
Wave-cut platform
Wave-cut platforms or marine terraces form when waves erode rock.
California
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A marine terrace and sea stacks.
Marine terrace
California
California
Barrier islands
migrate as sea
level rises
or falls.
The evolution of Mononoy Point,
Massachesetts, from 1830 to 1987.
1830 –
1850
1870 –
1890
1910 –
1930
The position of Galveston Island in Texas
has changed significantly over the past 3500 years.
An attempt at
controlling Nature.
Engineers have
constructed groins
to help preserve
the beach.
1950 –
1970
1987
How successful
was this effort?
Longshore
Current
direction
Cape May, NJ
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Jetties were constructed to keep the channel open for shipping.
What direction is the longshore current here?
The construction of
jetties at Ocean City,
Maryland to keep a
channel open in the
barrier island after a
storm in the 1930s has
led to an extreme case
of barrier island retreat.
New Jersey
The city of Miami Beach has opted to restore their beaches
by pumping millions of tons of sand near the shore to form
a new beach. What will happen to this sand with time?
New Beach
Old Beach
Predicted rises in
global sea levels
will cause barrier
islands to migrate
towards shore, and
the retreat of
cliffs along
emergent coastlines.
Massachusetts
California
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The big picture lesson for today:
Change along coastlines is inevitable.
Any changes we make are temporary at best.
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