10/19/2020 Assignment 5 Shorelines and Ocean Floor
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Assignment 5 Shorelines and Ocean Floor
Due Nov 1 by 11:59pm Points 28 Submitting a text entry box or a file upload
Available Aug 24 at 12:01am Nov 1 at 11:59pm 2 months
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Background
Shorelines at the intersections of land masses and oceans are dynamic regions shaped by wave energy.
Longshore drift refers to a net movement of sand parallel to a shoreline. Waves tend to approach
shorelines diagonally rather than precisely head on (see image below). This oblique approach results in
a longshore current, or movement of water in the surf zone (region of crashing waves). In addition to
moving water, the longshore current moves many tons of sand each year. Further landward, waves
swash up the beach face (swash zone) at an angle, but drain straight back down the beach. This action
entrains particles of sand in a zig-zag pattern, and over time they move along the swash zone in the
same direction as the longshore current in the surf zone.
The sand found on beaches and transported as longshore drift comes mainly from rivers, but may also
derive locally from rock cliffs, volcanoes, corals, or shells. When rivers are dammed for hydropower,
water supply, recreation, or flood control, beaches are deprived of an important source of sand.
Engineers attempt to stabilize shorelines with various structures, including groins, jetties, breakwaters,
and seawalls. Typically, groins, jetties, and breakwaters consist of large boulders accumulated into linear
ridges that rise above sea level; respectively, they maintain or widen beaches, keep the mouths of rivers
open, and protect ships or property from crashing waves. Seawalls are generally made of concrete or
large boulders and constructed on land rather than offshore. Their purpose is to protect the land behind
them from hurricanes and storm surges.
The map below shows a groin, jetty, and breakwater (shaded rectangles) in the sea near the shoreline.
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These structures can have both positive and adverse outcomes. By design, groins trap sand up current of
the structure; however, they often increase beach erosion down current of the structure. The longshore
current deflects around the groin, thus diminishing sand supply to the immediate down current area.
Jetties may induce similar patterns of deposition and erosion. Calm water behind breakwaters tends to fill
in with sand (the beach grows outward). Seawalls require heavy maintenance and, by protecting rock
exposures behind them, reduce the amount of sand supplied to beaches.
Rising sea level compounds coastal erosion problems. Sea levels fluctuate naturally over time, but can
also be influenced by human actions. Currently, we are in a warming period, and globally sea level is
rising.
Shorelines mark the beginnings of vast oceans, which cover about three-fourths of the earths surface.
We knew little about the topography of ocean floors until the 1940s and following decades, when echo
sounders (devices transmitting sound waves) were developed and deployed for military (detecting
submarines) and mapping applications.
Echo sounders transmit sound waves from a ship, measuring how long it takes for them to reach the
ocean floor and travel back. The travel time depends on the distance from sea level to the ocean floor. Let
D denote the distance from sea level to the ocean floor, t denote the two-way travel time of a sound wave,
and v denote the velocity of sound in water. It follows that 2D=vt, or D=vt/2.
By measuring the distance from sea level to the ocean floor at thousands of points, scientists were able
to construct topographic maps of the ocean floors (maps depicting the shape of the ocean floor). These
maps revealed complex forms rather than flat, featureless plains as previously thought. Oceans include
three major topographic units: continental margins, ocean basin floors, and mid-ocean ridges.
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Continental margins occupy the perimeters of ocean basins where they meet the continents. Many
continental margins exhibit three distinct segments: a shelf, slope, and rise. Shelves (most landward
segments) are flooded extensions of the continents. Slopes (middle segment) are steep as the name
implies, and represent the approximate boundary between oceanic and continental crust. The rise (most
seaward segment) is a gradual descent to the deep basin.
Ocean basin floors consist of flat plains (called abyssal plains) dotted with seamounts and also include
deep trenches. Seamounts are underwater volcanoes. Most seamounts are extinct. Often they form at
mid-ocean ridges (discussed below). Embedded in moving plates of lithosphere, seamounts migrate
away from mid-ocean ridges and gradually subside. Guyots are seamounts with flat tops, formed by
waves eroding a volcano that once rose above sea level.
Ocean trenches are long narrow features of the ocean floor, which form where moving plates plunge into
the mantle below. This setting is called a subduction zone. Trenches are the deepest parts of oceans; the
deepest trenches are approximately 11 km beneath sea level.
Extensive faulting and volcanic structures characterize mid-ocean ridges (sometimes called spreading
centers). Plates on either side of a mid-ocean ridge spread apart from one another. Mid-ocean ridges
average about 1,300 km wide, with a 20-80 km wide central depression called the rift valley, a site of
active volcanism. In some places, the volcanoes extend above sea level to form islands such as Iceland.
Assignment
1. Sketch and label areas of erosion and deposition resulting from the groin (top), jetty (middle), and
breakwater (bottom) in these three maps:
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2. Rising seas inundate land, compounding coastal erosion problems outlined above. Calculate how
much land would be inundated by water if sea level rose 1 foot over the next 30 years. Use three
scenarios for land surface slope: 1%, 50%, and 100%. For example, a 50% slope (0.5 in decimal form)
means that as you move inland, the land elevation increases 0.5 feet for every 1 foot of horizontal
distance. To solve this problem, express each slope in decimal form, set it equal to (change in
elevation)/(change in horizontal distance), and solve for change in horizontal distance.
1%:
50%:
100%:
3. The velocity of sound in water is approximately 1,500 m/s. Calculate the depth from sea level to the
ocean floor given the following two-way travel time measurements taken by an echo sounder (see
equation above, D=vt/2).
0.17 seconds:
3.60 seconds:
11.23 seconds:
4. (This exercise is from Physical Geology by Steven Earle and is used under a CC BY 4.0 license.) The
following map shows part of the sea floor near the southern tip of South America. North is toward the top
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Assignment 5 Rubric
of the map. Cooler colors (purples and blues) represent lower elevations, and warmer colors represent
higher elevations.
Identify the locations of the following features on the map:
continental shelf
continental slope
spreading ridge
subduction zone with a deep trench
an abyssal plain
some isolated seamounts
5. Worldwide, the oldest rocks at the ocean floor are about 180 million years old. In contrast, the oldest
continental rocks are approximately 4 billion years old.
Is this observation consistent with the observed tendency for oceanic rather than continental
lithosphere to descend into the mantle beneath ocean trenches?
Is it consistent with the observed density of oceanic (3.0 g/cm3) versus continental (2.8 g/cm3) crust?
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Total Points: 28.0
Criteria    Ratings    Pts
6.0 pts    Question 1
2 points for each sketch   
6.0 to >0.0 pts
Full Marks    0.0 pts
No Marks
6.0 pts    Question 2
2 points for each calculation   
6.0 to >0.0 pts
Full Marks    0.0 pts
No Marks
6.0 pts    Question 3
2 points for each calculation   
6.0 to >0.0 pts
Full Marks    0.0 pts
No Marks
6.0 pts    Question 4
1 point for each location   
6.0 to >0.0 pts
Full Marks    0.0 pts
No Marks
4.0 pts    Question 5
2 points for each interpretation   
4.0 to >0.0 pts
Full Marks    0.0 pts
No Marks