The Restless Ocean opens with a discussion of ocean surface currents and their importance. Deep-ocean circulation is also briefly examined, tides are analyzed, along with a detailed look at wave mechanics and wave erosion. Also investigated are the shoreline features and erosional problems that are caused by wave action and rising sea levels. The chapter concludes with a discussion of emergent and submergent coastal regions.
Learning Objectives
After reading, studying, and discussing this chapter, you should be able to:
•List the factors that influence surface ocean currents.
•Discuss the importance of surface ocean currents.
•Describe deep-ocean circulation.
•Discuss the factors that influence titles.
•Describe wave characteristics and types.
•Describe wave erosion and the features produced by wave erosion.
•Discuss shoreline erosional problems and solutions.
•Explain the differences between an emergent and submergent coast.
Chapter Summary
•Surface ocean currents are parts of huge, slowly moving, circular whirls, or gyres, that begin near the equator in each ocean. Wind is the driving force for the ocean‘s surface currents. Where wind is in contact with the ocean, it passes energy to the water through friction and causes the surface layer to move. The most significant factor other than wind that influences the movement of ocean waters is the Coriolis effect, the deflective force of Earth’s rotation which causes free-moving objects to he deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Because of the Coriolis effect, surface currents form clockwise gyres in the Northern Hemisphere and counterclockwise gyres in the Southern Hemisphere. Winds may also produce vertical water movements. Upwelling is the rising of cold water from deeper layers to replace warmer surface water. It is most characteristic along the eastern shores of the oceans.
•Ocean currents are important in navigation and travel and for the effect that they have on climates. The moderating effect of poleward-moving warm ocean currents during the winter in middle latitudes is well known.
•In contrast to surface currents, deep-ocean circulation is governed by gravity and driven by density differences. The two factors that are most significant in creating a dense mass of water are temperature and salinity.
•Tides, the daily rise and fall in the elevation of the ocean surface at a specific location, are caused by the gravitational attraction of the moon and, to a lesser extent, by the sun. Near the times of new and full moons, the sun and moon are aligned and their gravitational forces are added together to produce especially high and low tides. These are called the spring tides. Conversely, at about the times of the first and third quarters of the moon, when the gravitational forces of the moon and sun are at right angles, the daily tidal range is less. These are called neap tides. The three types of tides are 1) semidiurnal, with two high and two low tides each tidal day and relatively small differences in the high and low water heights, 2) diurnal, with a single high and low water height each tidal day, and 3) mixed, which have two high and two low waters each day and are characterized by a large inequality in high water heights, low water heights, or both.
•The three factors that influence the height, wavelength, and period of a wave are 1) wind speed, 2) length of time the wind has blown, and 3) fetch, the distance that the wind has traveled across the open water.
•The two types of wind-generated waves are 1) waves of oscillation, which are waves in the open sea in which the wave form advances as the water particles move in circular orbits, and 2) waves of translation, the turbulent advance of water formed near the shore as waves of oscillation collapse, or break, and form surf.
•Wave erosion is caused by wave impact, pressure, and abrasion (the sawing and grinding action of water armed with rock fragments). The bending of waves is called wave refraction. Due to refraction, wave impact is concentrated against the sides and ends of headlands.
•Most waves reach the shore at an angle. The uprush and backwash of water from each breaking wave moves the sediment in a zigzag pattern along the beach. This movement, called beach drift, can transport sand and pebbles hundreds or even thousands of meters each day. Oblique waves also produce longshore currents within the surf zone that flow parallel to the shore.
•Features produced by shoreline erosion include wave-cut cliffs (which originate from the cutting action of the surf against the base of coastal land), wave-cut platforms (relatively flat, bench-like surfaces left behind by receding cliffs), sea arches (formed when a headland is eroded and two caves from opposite sides unite), and sea stacks (formed when the roof of a sea arch collapses).
•Some of the features formed when sediment is moved by beach drift and longshore currents are spits (elongated ridges of sand that project from the land into the mouth of an adjacent bay), baymouth bars (sand bars that completely cross a bay) and tombolos (ridges of sand that connect an island to the mainland or to another island).
•Local factors that influence shoreline erosion are 1) the proximity of a coast to sediment-laden rivers, 2) the degree of tectonic activity, 3) the topography and composition of the land, 4) prevailing winds and weather patterns, and 5) the configuration of the coastline and nearshore areas.
•Three basic responses to shoreline erosion problems are 1) building structures such as groins (short walls built at a right angle to the shore to trap moving sand), breakwaters (structures built parallel to the shoreline to protect boats from the force of large breaking waves), and seawalls (barriers constructed to prevent waves from reaching the area behind the wall) to hold the shoreline in place, 2) beach nourishment, which involves the addition of sand to replenish eroding beaches, and 3) relocation of buildings away from the beach.
•Because of basic geological differences, the nature of shoreline erosion problems along America's Pacific and Atlantic Coasts is very different. Much of the development along the Atlantic and Gulf Coasts has occurred on barrier islands which receive the full force of major storms. A majority of the Pacific Coast is characterized by narrow beaches hacked by steep cliffs and mountain ranges. A major problem facing the Pacific shoreline is a narrowing of beaches because the natural flow of materials to the coast has been interrupted by dams built for irrigation and flood control.
•One frequently used classification of coasts is based upon changes that have occurred with respect to sea level. Emergent coasts, often with wave-cut cliffs and wave-cut platforms above sea level, develop either because an area experiences uplift or as a result of a drop in sea level. Conversely, submergent coasts, with their drowned river mouths, called estuaries, are created when sea level rises or the land adjacent to the sea subsides.