[Illustration: _Six ships checking the Gulf Stream’s course through
the Atlantic Ocean over a 2-week period found the variations shown
above._]
[Illustration: _The infrared film photograph shows the edge of the
Gulf Stream. The visible line between the Gulf Stream, which is on
the right, and Labrador water is made by Sargassum weed concentrated
at the interface._]
The ocean is constantly in motion—not just in the waves and tides that
characterize its surface but in great currents that swirl between
continents, moving (among other things) great quantities of heat from
one part of the world to another. Beneath these surface currents are
others, deeply hidden, that flow as often as not in an entirely
different direction from the surface course.
These enormous “rivers”—quite unconstant, sometimes shifting, often
branching and eddying in a manner that defies explanation and
prediction—occasionally create disastrous results. One example is El
Niño, the periodic catastrophe that plagues the west coast of South
America. This coast normally is caressed by the cold, rich Humboldt
Current. Usually the Humboldt hugs the shore and extends 200 to 300
miles out to sea. It is rich in life. It fosters the largest commercial
fishery in the world and is the home of one of the mightiest game fish
on record, the black marlin. The droppings of marine birds that feed
from its waters are responsible for the fertilizer (guano) exports that
undergird the Chilean, Peruvian, and Ecuadorian economies.
Every few years, however, the Humboldt disappears. It moves out from
shore or simply sinks, and a flow of warm, exhausted surface water known
as El Niño takes its place. Simultaneously, torrential rains assault the
coast. Fishes and birds die by the millions. Commercial fisheries are
closed. The beaches reek with death. El Niño is a stark demonstration of
man’s dependence on the sea and why he must learn more about it.
There are other motions in the restless sea. The water masses are
constantly “turning over” in a cycle that may take hundreds of years,
yet is essential to bring oxygen down to the creatures of the deeps, and
nutrients (fertilizers) up from the sea floor to the surface. Here the
floating phytoplankton (the plants of the sea) build through
photosynthesis the organic material that will start the nutrient cycle
all over again. Enormous tonnages of these tiny sea plants, rather than
being rooted in the soil, are separated from solid earth by up to
several vertical miles of saltwater. Sometimes, too, there is a more
rapid surge of deep water to the surface, a process known as upwelling.
Internal waves, far below the surface, develop between water masses that
have different densities and between which there is relative motion.
These waves are much like the wind-driven waves on the surface, though
much bigger: Internal waves may have heights of 300 feet or more and be
6 miles or more in length!
[Illustration: _A dividing cell of the diatom_ Corethron hystrix.
_Diatoms, one-celled photosynthetic plants, are the primary
producers of organic matter in fresh waters._]
[Illustration: _Ocean currents feed sand from nearby beaches into
this “sandfall”, which is about 30 feet high, in a submarine canyon
off Baja California._]
Among other motions of the sea there are landslides, or turbidity
currents, which are great boiling mixes of mud, rock, sand, and water
rushing down submarine mountainsides at speeds of a mile a minute. They
destroy everything in their paths and spread clouds of debris over the
abyssal plains like a sandstorm, producing fanlike deposits radiating
far out from the base of the slope. And there are tsunamis, or seismic
sea waves—popularly misnamed “tidal waves”—that transmit energy from
undersea earthquakes or volcanic eruptions. At sea, these waves are only
a few inches high, but they may travel great distances at 500 miles an
hour. As they approach the shoaling waters of a coast, they are slowed
to about 30 miles an hour and build up great surface waves capable of
destroying harbor and coastal installations.