11: Coasts
- Page ID
- 45481
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CC2 Isostasy, Eustasy, and Sea Level: Earth’s crust floats on the plastic asthenosphere. Sections of crust rise and fall isostatically as temperature changes alter their density or as their mass loading changes. This, in turn, causes isostatic changes of sea level. Eustatic changes of sea level occur globally when the volume of water in the oceans changes or when the volume of the ocean basins themselves change. Sea level changes create new coasts. Oceanic crust cools progressively after it is formed and sinks because its density rises. Thus, the hot spot volcanic islands slowly sink after they move away from the hot spot.
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CC4 Particle Size, Sinking, Deposition, and Resuspension: Suspended particles in the ocean sink at rates primarily determined by particle size: large particles sink faster than small particles. Once deposited, particles can be resuspended if current speeds are high enough. Generally large particles are more difficult to resuspend, although some very fine particles may be cohesive and therefore, also difficult to resuspend. Sinking and resuspension rates are primary factors in determining the grain size characteristics of beach sands and sediments at any given location.
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CC8 Residence Time: The residence time of seawater in a given segment of the oceans is the average length of time the water spends in that segment. In restricted arms of the sea, such as lagoons behind barrier islands and fringing reefs, residence time can be long, in which case the nutrients may be depleted, limiting the growth of corals and other marine species
A large proportion of the world’s population lives within a few tens of kilometers of a coast. In 2020, 40% of the total U.S. population, or more than 129 million people, lived in counties directly on the shoreline, which account for less than 10% of the nation’s land mass. Tens of millions more Americans live in counties that, while not shoreline counties, are close to a river or estuary. The concentration of human populations on or near the coast reflects the importance of the coastal zone to civilization.
The term coast has various definitions, but it is generally considered to be the strip of land between the coastline (where water meets land) and the inland location where there is no longer any environmental influence of the ocean. We build houses, factories, piers, docks, and marinas in this zone, primarily because the ocean is valuable for transport, recreation, and disposal of wastes, particularly treated sewage. In a human lifetime, most coastlines seem fixed and permanent. In reality, they are in a continuous state of change and movement. This chapter examines different types of coasts and the processes that form and reshape them. It also examines human attempts to control and mold the coasts, and the futility of many such efforts.