Chapter 15: Restless Realm: Oceans and Coasts
Study Plan
Over 70% of Earth's surface is covered by the oceans, which thus play a key role in the Earth system.
Credit: USGS
Guide to Reading
Chapter 14 dealt with fresh water on Earth's surface; Chapter 15 deals with the larger realm of salt water on Earth: the oceans. It begins with the physical structure of ocean basins. Although these are difficult places to access, over the past century scientists have managed to collect and piece together information that gives quite a comprehensive picture of both the geology and geography of the ocean floor. They have gathered an impressive amount of data directly, starting with research cruises of HMS Challenger in the 1870s and continuing through sea-floor explorations using the deep-sea submersible Alvin a century later. In this section some of your reading is a review of plate tectonics activities and features (oceanic crust and lithosphere, mid-ocean ridges, fracture zones, trenches, and active and passive continental margins). There are also many new concepts concerning ocean depths and landscapes (bathymetry, continental shelves, slopes and rises, abyssal plains, submarine canyons, turbidites, submarine fans, pelagic sediment, oceanic plateaus, seamounts, and guyots). The composition and characteristics of ocean water are discussed next. You read about salinity, the halocline, the heat capacity of water, and the thermocline.
Ocean waters exhibit numerous patterns of movement. They include surface currents, deep currents, eddies, the Coriolis effect, the Ekman spiral, gyres, upwelling zones, downwelling zones, thermohaline circulation, and water masses. Tides, of course, are also water motion, but they are treated thoroughly enough in this chapter to warrant their own subcategory. Some special advice: This is a topic that seems so elementary it's easy to skim too lightly over the section. Everybody knows tides go in and out, but unless you live on the coast, you probably don't really know much about tidal reach, mean sea level, flood tide, ebb tide, tidal flats, tidal bores, intertidal zones, the tide-generating force, neap tides, spring tides, and the effect tides have on the rotational speed of Earth.
Waves are another topic that you should be careful not to skim over too lightly, for the author presents more than the average landlubber would believe there is to know about waves. You read about their causes and geometric shapes and nomenclature, including the terms wave base, strength and fetch of a wind, ripples, swells, amplitude and wavelength of a wave, interference and refraction of waves, breakers and surf zones, swash and backwash, effects on embayments and headlands, longshore currents, and rip currents.
Ocean study includes a look at ocean boundaries, that is, coastal areas and shorelines. One type of shoreline is a beach. Beaches may be composed of different types of sand, including silicic sands or carbonate sands, and they can have distinct areas, including a beach face, foreshore zone, intertidal zone, backshore zone, and berm. Beaches, geologically speaking, are here today, gone tomorrow, and change is constantly occurring. In connection with this you read about active and inactive sand layers, beach drift, sand spits, baymouth bars, offshore bars, barrier islands, lagoons, and sediment budgets of beaches.
A shoreline may be a rocky coast rather than a sandy one. Here wave erosion may produce wave-cut notches, cliff retreat, wave-cut platforms (benches), sea arches, sea stacks, and tombolos.
Coastal areas may be coastal wetlands that are flooded with shallow water but experience no wave action. The three basic types of wetlands you read about are swamps, marshes, and bogs.
Some coastal areas are flooded stream valleys called estuaries. Here a mix of fresh- and saltwater supports complex ecosystems inhabited by unique salt-tolerant organisms.
Fjords are dramatic coastlines that result from the flooding of glacial valleys.
Coral reefs are specialized communities found in shallow, warm, well-lit seawater. Their basic physical structures (limestone mounds) are created by colonial marine animals that live in a symbiotic (mutually beneficial) relationship with algae. In addition to these two creatures, the reef provides the environment for a complex community of marine organisms. Coral reefs are classified on the basis of their shapes, which are determined by their origins (fringing reefs, barrier reefs, and atolls).
Over time the ocean levels rise and fall, and these changes affect coastal areas. Coastal areas have always experienced change due to natural events; today the human population is large enough to add its influence. For example, people build groins, jetties, breakwaters, and seawalls to fight coastal erosion, and when this doesn't work well enough to suit them, they bring in their own sand (beach nourishment). The results of such actions are often unpredictable; quite often they benefit one area of a beach and harm an adjacent area. Thoughtless human activity has destroyed huge amounts of coastal wetlands and endangered coral reefs. The chapter ends with a section on hurricanes, a coastal calamity very familiar to readers due to the 2005 Hurricane Katrina that struck Mississippi and Louisiana. The damage might have been less severe if the delta landscape had not been so excessively modified by construction and draining wetland. As in Chapter 14, the author reminds us that as large and powerful as the water world is, the human presence is affecting it, and not always in a desirable fashion.
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