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Key Concept Review (Answers to in-text “Concept Checks”)
Chapter 12
1. The place where ocean meets land is usually called the shore. Coast refers to the
larger zone affected by the processes that occur at this boundary.
2. Sea level depends on the amount of water in the world ocean, the volume of the
ocean’s “container,” and the temperature of the water (water expands as it
warms). Tectonic forces of uplift and subsidence (along with isostatic
equilibrium) determine the position of the shore.
3. Erosional coasts are new coasts in which the dominant processes are those that
remove coastal material. Depositional coasts are usually older coasts that are
steady or growing because of their rate of sediment accumulation.
4. Erosional coasts are shaped and attacked from the land by stream erosion,
abrasion by wind-driven grit, glacial activity, rainfall, dissolution by acids from
soil, and slumping. From the sea, large storm surf routinely generates tremendous
pressures. Tiny pieces of sand, bits of gravel, or stones hurled by the waves are
effective at eroding the shore.
5. Common features include sea cliffs, sea caves, and wave-cut platforms just
offshore. Much of the debris removed from cliffs during the formation of these
structures is deposited in the quieter water farther offshore, but some can rest at
the bottom of the cliffs as exposed beaches.
6. Because of wave refraction, wave energy is focused onto headlands and away
from bays by wave refraction .Over time, coastal erosion tends to produce a
straight shoreline.
7. As we saw in Chapter 4, most islands that rise from the deep ocean are of
volcanic origin. If the volcanism has been recent, the coasts of a volcanic island
will consist of lobed lava flows extending seaward, common features in the
Hawaiian Islands. Craters at the coast may fill with seawater after volcanic
activity has slowed.
8. Over time, erosional shorelines can evolve into depositional ones.
9. The most familiar feature of a depositional coast is the beach.
10. Tidal range, pattern, and height – coupled with wave action – are the most
important factors determining beach profile.
11. The movement of sediment along the coast, driven by wave action, is referred to
as longshore drift. Longshore drift occurs in two ways: the wave-driven
movement of sand along the exposed beach, and the current-driven movement of
sand in the surf zone just offshore.
12. The natural sector of a coastline in which sand input and sand outflow are
balanced may be thought of as a coastal cell. Sand enters a cell from rivers or
streams, and exits as it falls into a submarine canyon.
13. A bay mouth bar forms when a sand spit closes off a bay by attaching to a
headland adjacent to the bay.
14. Barrier islands are narrow, exposed sandbars that are parallel to but separated
from land. Unlike barrier islands, sea islands contain a firm central core that was
part of the mainland when sea level was lower.
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15. A broad continental shelf must be present to provide a platform on which
sediment can accumulate to form a delta. Tidal range must be low, and waves and
currents generally mild. Deltas are most common on the low-energy shores of
enclosed seas (where the tidal range is not extreme) and along the tectonically
stable trailing edges of some continents.
16. Coral animals, some forms of cyanobacteria, and mangroves are effective at
modifying coastlines. The greatest of all biologically modified coasts is the Great
Queensland Barrier Reef in Australia.
17. Charles Darwin classified tropical coral reef structures into three types: Fringing
reefs, barrier reefs, and atolls.
18. An estuary is a body of water partially surrounded by land, where fresh water
from a river mixes with ocean water.
19. By origin, estuary types are drowned river mouths, fjords, bar-built, or tectonic.
20. By water circulation patterns, estuaries are classified as salt-wedge, well-mixed,
partially mixed, and reverse estuaries.
21. Estuaries often support a tremendous number of living organisms. The easy
availability of nutrients and sunlight, protection from wave shock, and the
presence of many habitats permit the growth of many species and individuals.
Estuaries are frequently nurseries for marine animals; several species of perch,
anchovy. Unfortunately for their inhabitants, the high demand for development is
incompatible with a healthy estuarine ecosystem.
22. The Pacific coast is an actively rising margin on which volcanoes, earthquakes,
and other indications of recent tectonic activity are easily observed. Most of the
sediments on the Pacific coast originated from erosion of relatively young granitic
or volcanic rocks of nearby mountains. The Atlantic coast is a passive margin,
tectonically calm and subsiding because of its trailing position on the North
American Plate. Subsidence along the coast has been considerable—3,000 meters
(10,000 feet) over the last 150 million years. A deep layer of sediment has built
up offshore, material that helped produce today’s barrier islands. The Gulf coast
experiences a smaller tidal range and—hurricanes excepted—a smaller average
wave size than either the Pacific or Atlantic coasts. Reduced longshore drift and
an absence of interrupting submarine canyons allow the great volume of
accumulated sediments from the Mississippi and other rivers to form large deltas,
barrier islands, and a long raised “super berm” that prevents the ocean from
inundating much of this sinking coast.
23. Steps taken to preserve or “improve” a stretch of coast may have the opposite
effect, and coastal residents do not always learn by example. Intervention in
coastal processes is almost always costly and temporary.
24. Because sediment flow into coastal cells has lessened due to dams and sediment
diversion, United States beaches are generally shrinking.
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