Chapter 11 coasts
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Chapter 11 COASTS 1. How is a shore different from a coast? 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. What factors affect sea level and the location of a coast? 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. How is an erosional coast different from a depositional coast? 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. What wears down erosional coasts? 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. What are some features common to erosional coasts? 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. Over time, coastal erosion tends to produce a straight shoreline. Why? 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. How might volcanic activity shape a coast? 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. Do erosional coasts tend to evolve into depositional coasts, or is it the other way around? Over time, erosional shorelines can evolve into depositional ones. 9. What is the most common feature of a depositional coast? The most familiar feature of a depositional coast is the beach. 10. What two marine factors are most important in shaping beaches? Tidal range, pattern, and height – coupled with wave action – are the most important factors determining beach profile. 11. How does sand move on a beach? 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. What is a coastal cell? Where does sand in a coastal cell come from? Where does it go? 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. Distinguish between sand spits and bay mouth bars. A bay mouth bar forms when a sand spit closes off a bay by attaching to a headland adjacent to the bay. 14. What is the difference between sea islands and barrier islands? 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. 15. Why don’t deltas form at every river mouth? 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. What organisms can affect coastal configuration? 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. What is an estuary? An estuary is a body of water partially surrounded by land, where fresh water from a river mixes with ocean water. 18. Estuaries are classified by their origins. What types of estuaries exist? By origin, estuary types are drowned river mouths, fjords, bar-built, or tectonic. 19. Estuaries are also classified by the type of water they contain and the flow characteristics of that water. How are estuaries classified by water circulation patterns? By water circulation patterns, estuaries are classified as salt-wedge, well-mixed, partially mixed, and reverse estuaries. 20. Of what value are estuaries? 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. 21. Briefly compare the U.S. Pacific, Atlantic, and Gulf coasts. What are the most important forces influencing these coasts? 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. 22. Generally speaking, would you say human intervention in coastal processes has been largely successful in achieving long-term goals of stabilization? 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. 23. Again, generally speaking, would you say beaches on U.S. coasts are growing, shrinking, or staying about the same size? Because sediment flow into coastal cells has lessened due to dams and sediment diversion, United States beaches are generally shrinking. 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