Chapter 16 The Oceans, Coastal Processes and Landforms Geosystems 5e An Introduction to Physical Geography Robert W. Christopherson Charlie Thomsen
Homework Assignment #3 (book review) is due on March 16.
Overview Coastal regions are unique and dynamic environments. Most of Earth's coastlines are relatively new and are the setting for continuous change. The land, ocean, and atmosphere interact to produce waves, tides, erosional features, and depositional features along the continental margins. The interaction of vast oceanic and atmospheric masses is dramatic along a shoreline. At times, the ocean attacks the coast with its erosive power; at other times, the sea breeze, salty mist, and repetitive motion of the water are gentle and calming.
Key Learning Concepts: ---After this lecture you should be able to: Describe the chemical composition of seawater and the physical structure of the ocean. Identify the components of the coastal environment and list the physical inputs to the coastal system, including tides and mean sea level. Describe wave motion at sea and near shore and explain coastal straightening as a product of wave refraction. Identify characteristic coastal erosional and depositional landforms. Describe barrier islands and their hazards as they relate to human settlement. Assess living coastal environments: corals, wetlands, salt marshes, and mangroves. Construct an environmentally sensitive model for settlement and land use along the coast.
1. Describe the salinity of seawater: its composition, amount, and distribution. Water acts as a solvent, dissolving at least 57 of the elements found in nature. In fact, most natural elements and the compounds they form are found in the seas as dissolved solids, or solutes. Thus, seawater is a solution, and the concentration of dissolved solids is called salinity. Seven elements comprise more than 99% of the dissolved solids in seawater: chlorine (Cl), sodium (Na), magnesium (Mg), sulfur (S), calcium (Ca), potassium (K), and bromine (Br). Seawater also contains dissolved gases (such as carbon dioxide, nitrogen, and oxygen), solid and dissolved organic matter, and a multitude of trace elements. Salinity worldwide normally varies between 34% and 37%; variations are attributable to atmospheric conditions above the water and to the quantity of freshwater inflows. The term brine is applied to water that exceeds the average of 35% salinity, whereas brackish applies to water that is less than 35%. (See figure 16.2).
Figure 16. 2: Variation in Ocean Salinity and Latitude Figure 16.2: Variation in Ocean Salinity and Latitude. Salinity (green line) is principally a function of climatic conditions. Specifically, important is the moisture relation expressed by the difference between evaporation and precipitation (E-P).
2. Analyze the latitudinal distribution of salinity shown in Figure 16-2. Why is salinity less along the equator and greater in the subtropics? Generally, oceans are lower in salinity near landmasses, because of river discharges and runoff. Extreme examples include the Baltic Sea (north of Poland and Germany) and the Gulf of Bothnia (between Sweden and Finland), which average 10% or less salinity because of heavy freshwater runoff and low evaporation rates. On the other hand, the Sargasso Sea, within the North Atlantic subtropics, averages 38%, and the Persian Gulf is at 40% as a result of high evaporation rates in an almost-enclosed basin. Deep pockets near the floor of the Red Sea register a very salty 225%. In equatorial water, precipitation is high throughout the year, diluting salinity values to slightly lower than average (34.5%). In subtropical oceans–where evaporation rates are high due to the influence of hot, dry subtropical high-pressure cells–salinity is more concentrated, increasing to 36.5%.
3. What are the three general zones relative to physical structure within the ocean? Characterize each by temperature, salinity, dissolved oxygen, and dissolved carbon dioxide. The ocean's surface layer is warmed by the Sun and is wind-driven. Variations in water temperature and solutes are blended rapidly in a mixing zone that represents only 2% of the oceanic mass. Below this is the thermocline transition zone, a region of strong temperature gradient that lacks the motion of the surface. Friction dampens the effect of surface currents, with colder water temperatures at the lower margin tending to inhibit any convective movements. Starting at a depth of 1-1.5 km (0.62-0.93 mi) and going down to the bottom, temperature and salinity values are quite uniform. Temperatures in this deep cold zone are near 0°C (32°F); but, due to its salinity, seawater freezes at about –2°C (28.4°F). The coldest water is at the bottom except near the poles, where cold water may be near or at the surface. (See Figure 16-3).
Figure 16.3: The Ocean’s Physical Structure
4. What are the key terms used to describe the coastal environment? The coastal environment is called the littoral zone. (Littoral comes from the Latin word for shore.) The littoral zone spans both land and water. Landward, it extends to the highest water line that occurs on shore during a storm. Seaward, it extends to the point at which storm waves can no longer move sediments on the seafloor (usually at depths of approximately 60 m or 200 ft). The specific contact line between the sea and the land is the shoreline, and adjacent land is considered the coast. (See Figure 16-4).
Figure 16. 4: The Littoral Zone Figure 16.4: The Littoral Zone. The littoral zone includes the coast, beach, and nearshore environments.
5. Define mean sea level. How is this value determined 5. Define mean sea level. How is this value determined? Is it constant or variable around the world? Mean sea level is a calculated value based on average tidal levels recorded hourly at a given site over a period of years. Mean sea level varies spatially from place to place because of ocean currents and waves, tidal variations, air temperature and pressure differences, and ocean temperature variations.
6. What interacting forces generate the pattern of tides? Earth's orientation to the Sun and the Moon (astronomical relationships) produce the pattern of tides, the complex daily oscillations in sea level that are experienced to varying degrees around the world. Tides also are influenced by the size, depth, and topography of ocean basins, by latitude, and by shoreline configuration. Tides are produced by the gravitational pull exerted on Earth by both the Sun and the Moon. Although the Sun’s influence is only about half that of the Moon (46%) because of the Sun's greater distance from Earth, it is still a significant force. Figure 16-6 illustrates the relationship among the Moon, the Sun, and Earth and the generation of variable tidal bulges on opposite sides of the planet.
Figure 16. 6 The Cause of Tides Figure 16.6 The Cause of Tides. Gravitational relations of Sun, Moon, and Earth combine to produce the tides. Gravity and inertia are essential elements in understanding tides. Gravity is the force of attraction between two bodies. Inertia is the tendency of objects to stay still if motionless or to keep moving in the same direction if in motion. In the case of figure 16.6a for example, the corresponding tidal bulge on the Earth’s opposite side is primarily the result of farside water’s remaining in position (being left behind) because its inertia exceeds the gravitational pull of the Moon and Sun.
Do tides really move in and out along the shoreline? Tides appear to move in and out along the shoreline, but they do not actually do so. Instead, the Earth’s surface rotates into and out of the relatively “fixed” tidal bulges as Earth changes its position in relation to the Moon and Sun. Hence, every day, most coastal locations experience two high (rising) tides, known as flood tides, and two low (falling) tides, known as ebb tides. The difference between consecutive high and low tides is considered the tidal range.
7. What characteristic tides are expected during a new Moon or a full Moon? During the first-quarter and third-quarter phases of the Moon? Figure 16-6a shows the Moon and the Sun in conjunction (lined up with Earth–new Moon or full Moon), a position in which their gravitational forces add together. The combined gravitational effect is strongest in the conjunction alignment and results in the greatest tidal range between high and low tides, known as spring tides. Figure 16-6b shows the other alignment that gives rise to spring tides, when the Moon and Sun are at opposition. When the Moon and the Sun are neither in conjunction nor in opposition, but are more-or-less in the positions shown in c and d (first- and third-quarter phases), their gravitational influences are offset and counteract each other somewhat, producing a lesser tidal range known as neap tide.
8. Is tidal power being used anywhere to generate electricity 8. Is tidal power being used anywhere to generate electricity? Explain briefly how such a plant would utilize the tides to produce electricity. Are there any sites in North America? Where are they? The fact that sea level changes daily with the tides suggests an opportunity that these could be harnessed to produce electricity. The bay or estuary under consideration must have a narrow entrance suitable for the construction of a dam with gates and locks, and it must experience a tidal range of flood and ebb tides large enough to turn turbines, at least a 5 m range. Tidal power generation is possible at about 30 locations in the world, although at present only two of them are actually producing electricity: an experimental 1 megawatt station in Russia at Kislaya-Guba Bay, on the White Sea, since 1969; and a facility in the Rance River estuary on the Brittany coast of France since 1967. According to studies completed by the Canadian government, the present cost of tidal power at ideal sites is economically competitive with that of fossil fuels, although certain environmental concerns must be addressed. Among several favorable sites on the Bay of Fundy, one plant is in operation. The Annapolis Tidal Generating Station was built in 1984 to test electrical production using the tides. Nova Scotia Power Incorporated operates the 20 megawatts plant.
9. What is a wave? How are waves generated, and how do they travel across the ocean? Does the water travel with the wave? Discuss the process of wave formation and transmission. Undulations of ocean water called waves travel in wave trains, or groups of waves. Storms around the world generate large groups of wave trains. A stormy area at sea is the generating region for these large waves, which radiate outward from their formation center. As a result, the ocean is crisscrossed with intricate patterns of waves traveling in all directions. The waves seen along a coast may be the product of a storm center thousands of kilometers away. Water within such a wave is not really migrating but is transferring energy through the water in simple cyclic undulations, which form waves of transition. In a breaker, the orbital motion of transition gives way to waves of translation, in which both energy and water move forward toward shore as water cascades down from the wave crest (See Figure 16-8).
Figure 16.8: The orbiting tracks of water particles change from circular motions and swells in deep water (waves of transition) to more elliptical orbits near the bottom in shallow water (waves of translation).
10. Describe the refraction process that occurs when waves reach an irregular coastline. Why is the coastline straightened? Generally, wave action is a process that results in coastal straightening. As waves approach an irregular coast, they bend and focus around headlands, or protruding landforms generally composed of more resistant rocks (See Figure 16-9). Thus, headlands represent a specific point of wave attack along a coastline. Waves tend to disperse their energy in coves and bays on either side of the headlands. This wave refraction (wave bending) along a coastline redistributes wave energy so that different sections of the coastline are subjected to variations in erosion potential.
Figure 19. 9 Coastal Straightening Figure 19.9 Coastal Straightening. The process of coastal straightening is brought about by wave refraction (deflection from a straight path). Wave energy is concentrated as it converges on headlands.
11. Define the components of beach drift and the longshore current and longshore drift. Particles on the beach are moved along as beach drift, or littoral drift, shifting back and forth between water and land in the effective wind and wave direction. These dislodged materials are available for transport and eventual deposition in coves and inlets and can represent a significant volume. The longshore current transports beach drift. A longshore current generates only in the surf zone and works in combination with wave action to transport large amounts of sand, gravel, sediment and debris along the shore as longshore drift. See Figure 16.10.
Figure 16. 10: Longshore Current and Beach Drift Figure 16.10: Longshore Current and Beach Drift. Longshore currents are produced as waves approach the surf zone and shallower water. Longshore and beach drift results as substantial volumes of material are moved along the shore.
13. What is meant by an erosional coast? What are its features? The active margins of the Pacific along the North and South American continents are characteristic coastlines affected by erosional landform processes. Erosional coastlines tend to be rugged, of high relief, and tectonically active, as expected from their association with the leading edge of a drifting lithospheric plate. Sea cliffs are formed along a coastline by the undercutting action of the sea. As indentations are produced at water level, such a cliff becomes notched, leading to subsequent collapse and retreat of the cliff. Other erosional forms evolve along cliff-dominated coastlines, including sea caves, sea arches, and sea stacks. As erosion continues, arches may collapse, leaving isolated stacks out in the water. The coasts of southern England and Oregon are prime examples of such erosional landscapes. (Fig. 16-12).
14. What is meant by a depositional coast? What are the features? Depositional coasts generally are located near onshore plains of gentle relief, where sediments are available from many sources. Such is the case with the Atlantic and Gulf coastal plains of the United States, which lie along the relatively passive, trailing edge of the North American lithospheric plate. A spit consists of sand deposited in a long ridge extending out from a coast; it partially crosses and blocks the mouth of a bay. A classic example is Cape Cod, Mass. The spit becomes a bay barrier if it completely cuts the bay off from the ocean and forms an inland lagoon. Spits are made up of materials that have been eroded and transported by drift; for much sand to accumulate, offshore currents must be weak. A tombolo occurs when sand deposits connect the shoreline with an offshore island. (Fig 16-13).
15. How do people modify littoral drift? Figure 16-14, illustrates several of the common approaches: jetties to block material from harbor entrances, groins to slow drift action along the coast, and a breakwater to create a zone of still water near the coastline. However, interrupting the coastal drift that is the natural replenishment for beaches may lead to unwanted changes in sand distribution downcurrent. In addition, enormous energy and materials must be committed to counteract the enormous and relentless energy that nature invests along the coast.
16. Describe a beach—its form and composition. A beach is that place along a coast where sediment is in motion. Material from the land temporarily resides there while it is in active transit along the shore. The beach zone ranges, on average, from 5 m above high tide to 10 m below low tide, although specific definition varies greatly along individual shorelines. Beaches are dominated by quartz (SiO2) because it is the most abundant mineral on Earth, resists weathering, and therefore remains after other minerals are removed. A beach acts to stabilize a shoreline by absorbing wave energy, as is evident by the amount of material that is in almost constant motion.
17. On the basis of the information in the text, do you think barrier islands and beaches should be used for development? If so, under what conditions? If not, why not?
Answer: Offshore sand bars gradually migrate toward shore as the sea level rises. Because many barrier beaches evidence this landward migration today, they are an unwise choice for a home site or commercial building. Nonetheless, they are a common choice, even though they take the brunt of storm energy and actually act as protection for the mainland. The hazard represented by the settlement of barrier islands was made graphically clear when Hurricane Hugo (1989) assaulted South Carolina. Beachfront houses, barrier beach developments, and millions of tons of sand were swept away; up to 95% of the single-family homes in Garden City alone were destroyed.
Next Topic: Living Coastal Environments: Corals, wetlands, salt marshes, and mangroves. 20. How are corals able to construct reefs and islands? A coral is a simple marine animal with a cylindrical, saclike body; it is related to other marine invertebrates, such as anemones and jellyfish. Corals secrete calcium carbonate (CaCO3) from the lower half of their bodies, forming a hard external skeleton. Although both solitary and colonial corals exist, it is the colonial forms that produce enormous structures, varying from treelike and branching forms to round and flat shapes. Through many generations, live corals near the ocean's surface build on the foundation of older corals below, which in turn may rest upon a volcanic seamount or some other submarine feature built up from the ocean floor. An organically derived sedimentary formation of coral rock is called a reef and can assume one of several shapes, principally, a fringing reef, a barrier reef, or an atoll.
21. A trend in corals that is troubling scientists, some possible causes. Scientists are tracking an unprecedented bleaching and dying-off of corals worldwide. The Caribbean, Australia, Japan, Indonesia, Kenya, Florida, Texas, and Hawaii are experiencing this phenomenon. The bleaching is due to a loss of colorful algae from within and upon the coral itself. Normally colorful corals have turned stark white as the host coral expels nutrient-supplying algae. Exactly why the coral ejects its living partner is unknown. Possibilities include local pollution, disease, sedimentation, and changes in salinity. Another probable cause is the 1 to 2C° warming of sea-surface temperatures, as stimulated by greenhouse warming of the atmosphere. During the 1982-1983 areas of the Pacific Ocean were warmer than normal and widespread coral bleaching occurred. Coral bleaching worldwide is continuing as average ocean temperatures climb higher. (see Figure 16.17 for coral riff distribution).
Figure 16. 17. Worldwide Distribution of Living Coral Formations Figure 16.17. Worldwide Distribution of Living Coral Formations. Yellow patches are areas of prolific reef growth. The red dotted line marks the limits of coral activity.
22. Why are coastal wetlands poleward of 30° N and S latitude different from those that are equatorward? In terms of wetland distribution, salt marshes tend to form north of the 30th parallel, whereas mangrove swamps form equatorward of that point. This is dictated by the occurrence of freezing conditions, which control the survival of mangrove seedlings. Roughly the same latitudinal limits apply in the Southern Hemisphere. Salt marshes usually form in estuaries and behind barrier beaches and sand spits. An accumulation of mud produces a site for the growth of halophytic (salt-tolerant) plants. Plant growth then traps additional alluvial sediments and adds to the salt marsh area. Sediment accumulation on tropical coastlines provides the site for growth of mangrove trees, shrubs, and other small trees. The adventurous prop roots of the mangrove are constantly finding new anchorages and are visible above the water line but reach below the water surface, providing a habitat for a multitude of specialized life forms. Mangrove swamps often secure and fix enough material to form islands.
Movie: Waves, Beaches and Coasts This program shows the dynamic interaction of two geologic agents: rocky landmasses and the energy of the ocean. Aspects of waves — their types, parts, movement, and impact on the shore — are illustrated. The program also covers shoreline characteristics, currents, sea barriers, tides, and how the greenhouse effect could impact sea level and coastal lands. http://www.learner.org/resources/series78.html
Geosystems 5e An Introduction to Physical Geography End of Chapter 16 Geosystems 5e An Introduction to Physical Geography Robert W. Christopherson Charlie Thomsen