The following statements summarize and describe many of the key terms and concepts presented
in the chapter.
- The ocean's surface currents follow the general pattern of the world's major
wind belts. Surface currents are parts of huge, slowly moving loops of water called
gyres that are centered in the subtropics of each ocean basin. The
positions of the continents and the Coriolis effect also
influence the movement of ocean water within gyres. Because of the Coriolis effect,
subtropical gyres move clockwise in the Northern Hemisphere and
counterclockwise in the Southern Hemisphere. Generally, four main
currents comprise each subtropical gyre.
- Ocean currents are important in navigation and for the effect they have on
climates. Poleward-moving warm ocean currents moderate winter
temperatures in the middle latitudes. Cold currents exert their greatest influence
during summer in middle latitudes and year-round in the tropics. In addition to cooler
temperatures, cold currents are associated with greater fog frequency and
drought.
- Upwelling, the rising of colder water from deeper layers, is a
wind-induced movement that brings cold, nutrient-rich water to the surface. Coastal
upwelling is most characteristic along the west coasts of continents.
- 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, so the movement of deep ocean water is often termed
thermohaline circulation. Most water involved in thermohaline circulation
begins in high latitudes at the surface when the salinity of the cold water increases due
to sea ice formation. This dense water sinks, initiating deep-ocean currents.
- The shore is the area extending between the lowest tide level and the
highest elevation on land that is affected by storm waves. The coast extends inland from the
shore as far as ocean-related features can be found. The shore is divided into the
foreshore and backshore. Seaward of the foreshore are the
nearshore and offshore zones.
- A beach is an accumulation of sediment found along the landward margin
of the ocean or a lake. Among its parts are one or more berms and the
beach face. Beaches are composed of whatever material is locally abundant and
should be thought of as material in transit along the shore.
- Waves are moving energy and most ocean waves are initiated by the
wind. 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 open water. Once waves leave a storm area, they are termed
swells, which are symmetrical, longer-wavelength waves.
- As waves travel, water particles transmit energy by circular orbital
motion, which extends to a depth equal to one half the wavelength. When a wave
travels into shallow water, it experiences physical changes that can cause the wave to
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. Owing to refraction, wave
impact is concentrated against the sides and ends of headlands.
- Most waves reach the shore at an angle. The uprush (swash) and backwash of water from
each breaking wave moves the sediment in a zigzag pattern along the beach. This movement is
called beach drift. Oblique waves also produce longshore
currents within the surf zone that flow parallel to the shore and transport more
sediment than beach drift.
- Erosional features include wave-cut cliffs (which
originate from the cutting action of the surf against the base of coastal land),
wave-cut platforms (relatively flat, benchlike surfaces left behind by
receding cliffs), and marine terraces (uplifted wave-cut platforms). Erosional
features also include sea arches (formed when a headland is eroded and two sea
caves from opposite sides unite), and sea stacks (formed when the roof of a
sea arch collapses).
- Some of the depositional features that form 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
(sandbars that completely cross a bay), and tombolos (ridges of sand that
connect an island to the mainland or to another island). Along the Atlantic and Gulf Coastal
Plains, the coastal region is characterized by offshore barrier islands, which
are low ridges of sand that parallel the coast.
- 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.
- Hard stabilization involves building hard, massive structures in an
attempt to protect a coast from erosion or prevent the movement of sand along the beach.
Hard stabilization includes groins (short walls constructed at a right angle
to the shore to trap moving sand), breakwaters (structures built parallel to
the shore to protect it from the force of large breaking waves), and seawalls
(armoring the coast to prevent waves from reaching the area behind the wall).
Alternatives to hard stabilization include beach nourishment,
which involves the addition of sand to replenish eroding beaches, and
relocation of damaged or threatened buildings.
- Because of basic geological differences, the nature of shoreline erosion
problems along America's Pacific and Atlantic/Gulf 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. Much of the Pacific Coast is characterized by narrow
beaches backed by steep cliffs and mountain ranges. A major problem facing the Pacific
shoreline is the narrowing of beaches caused by irrigation and flood control dams that
interrupt the natural flow of sand to the coast.
- One commonly used classification of coasts is based upon changes that have occurred
with respect to sea level. Emergent coasts often exhibit wave-cut cliffs and marine terraces and develop either because an area experiences uplift or as a result of a drop in sea level. Conversely, submergent coasts commonly display drowned river mouths called estuaries and are created when sea level rises or the land adjacent to the sea subsides.
- 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, the Sun. The Moon and the Sun each produce a pair of tidal bulges on Earth. These tidal bulges remain in fixed positions relative to the generating bodies as Earth rotates through them, resulting in alternating high and low tides. Spring Tides occur near the times of new and full moons when the Sun and Moon are aligned and their bulges are added together to produce especially high and low tides (a large daily tidal range). Conversely, neap tides occur at about the times of the first and third quarters of the Moon when the bulges of the Moon and Sun are at right angles producing a smaller daily tidal range.
- Three main tidal patterns exist worldwide. A diurnal tidal pattern exhibits one high and low tide daily. A semi-diurnal tidal pattern exhibits two high and low tides daily of about the same height; and a mixed tidal pattern usually has two high and low tides daily of different heights.
- Tidal currents are horizontal movements of water that accompany the rise and fall of the tides. Tidal flats are the areas that are affected by the advancing and retreating tidal currents. When tidal currents slow after emerging from narrow inlets, they deposit sediment that may eventually create tidal deltas.
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