Environmental Engineering Reference
In-Depth Information
The deep ocean floor does not consist exclusively of the abyssal plain. Some areas
have considerable relief. Among the more important of such features are the
following:
•
Seamounts
are isolated mountain-shaped elevations more than 3000 feet
high.
•
Mid-oceanic ridges
are submarine mountains, extending more than 37,000
miles through the oceans, generally 10,000 feet above the abyssal plain.
•
Trenches
are deep, steep-sided troughs in an abyssal plain.
•
Guyots
are flat-topped volcanic mountains that rise from the ocean bottom
and usually are covered by 3000 to 6000 feet of water.
OCEAN TIDES, CURRENTS, AND WAVES
The Earth's coastline, including sea cliffs, shores, and beaches, is primarily
sculpted by water—tides, currents, and waves. The ceaseless, restless motion of
the sea is an extremely effective geologic agent. Seawater in motion erodes cliffs,
transports debris along shores, and dumps it on beaches. Most coasts retreat or
advance over time.
T
ides
Tides are the periodic rise and fall of the sea, once every 12 hours and 26 minutes.
The gravitational pull of the moon and, to a lesser extent, the sun causes water on
Earth to bulge toward them. It is interesting to note that a similar bulge occurs on
the opposite side of the Earth due to inertial forces (further explanation is beyond the
scope of this text). The effect of the tides is not too noticeable in the open sea, as the
difference between high and low tide amounts to only about 2 feet. The tidal range
may be considerably greater near shore, however. It may range from less than 2 feet
to as much as 50 feet. The tidal range will vary according to the phase of the moon
and the distance of the moon from the Earth. The type of shoreline and the physical
configuration of the ocean floor will also affect the tidal range.
C
urrenTs
Ocean currents are localized movements of masses of seawater. They are the result
of drift of the upper 50 to 100 m of the ocean due to drag by wind. Thus, surface
ocean currents generally follow the same patterns as atmospheric circulation, with
the exception that atmospheric currents continue over the land surface while ocean
currents are deflected by the land. Along with wind action, currents may also be
caused by tides, variations in the salinity of the water, rotation of the Earth, and con-
centrations of turbid or muddy water. In addition to this surface circulation, seawater
also circulates vertically due to differences in temperature and salinity that affect
water density.
