OCEANOGRAPHY 791
These plates are in constant motion relative to each other. As
one would expect, the borders between adjoining plates are
regions of intense seismic activity.
The visible manifestation of this activity varies according
to its strength, and ranges from seismically inactive moun-
tain belts to active volcanic chains. Along areas where plates
are diverging (“ridge axes”), there is a constant formation
of new surface area, as volcanic material from the earth’s
interior rises to fill the gap caused by the plate divergence.
Quite logically then, and in light of the fact that the earth
is undergoing little or no expansion, there must be zones
where plates are converging. These areas are called subduc-
tion zones and are characterized by the sliding of one plate
beneath the edge of the adjoining plate. The theory of sea-
floor spreading states that the ocean basins were formed, and
are continuing to change at a rate of order 2 centimeters per
year (EOS, 1988) as a result of the divergence of plates along
axes called oceanic ridges. As an example of this phenom-
enon, as recently as 165 million years ago, the continents
bordering what is now the Atlantic Ocean were very much
more closely separated, possibly constituting one very large
land mass. The plates on which these continents rest then
began diverging along an axis known as the Mid-Atlantic
ridge, creating the ocean basin that presently exists. This
ridge runs along the approximate centerline of the Atlantic
Ocean, from Iceland south to approximately 1800 kilome-
ters north of Antarctica.
For the purpose of categorization, we can divide the
world’s oceans into three bodies: the Atlantic Ocean (includ-
ing the Arctic, Baltic, and Mediterranean Seas), bordered by
the Americas, Africa, Europe, and the Arctic land mass; the
Pacific Ocean, bordered by the Americas, Asia, Australia,
and Antarctica; and the Indian Ocean, bordered by Africa,
Asia, Australia, and Antarctica.
Table 1 indicates the area and average depth of the three
ocean basins (including adjacent seas). Note that the Pacific
Ocean encompasses the largest area and has the largest aver-
age depth of the three. We should caution that this separation
of the oceans into three, distinct bodies of water is somewhat
misleading, since the adjacent seas (e.g., the Mediterranean
and Arctic) are often quite different from the major ocean
basins in physical, chemical and biological characteristics.
As partial evidence of this point, we also list in Table 1 the
area and depth of the three ocean basins, excluding the adja-
cent seas. Note that in comparison with the Pacific and Indian
Oceans, a much larger percentage of the Atlantic Ocean’s
assigned area consists of smaller seas. The neglect of these
seas results in a more uniform average depth among the three
major oceans, although the Pacific remains the largest and
deepest on average.
In general, if one were to move seaward from the bound-
ary between continent and ocean, one would first encounter
a continental shelf, characterized by relatively shallow water
with depth gradually increasing in the seaward direction to a
maximum of the order to 200 meters. The continental slope
would then be encountered, representing the transition struc-
ture from shallow to deep ocean, and characterized by an
abrupt increase in water depth to the order of 3000 meters.One typically finds that the water depth will continue to
increase away from the continent (although at a slower rate)
to maximum depths of between 5000 and 6000 meters until
reaching the spreading centers, or mid-ocean ridges dis-
cussed earlier. The water depth generally decreases toward
the ridges to the order of 2000 meters.
This pattern of increasing depth away from the con-
tinents, followed by decreasing depth as the ridge axis is
approached appears at first glance to be a contradiction to the
theory of ocean basin formation explained earlier. If we are to
accept the notion of a mid-ocean ridge, or zone of divergence,
at which new ocean crust is continually being uplifted and
forced out on either side of the ridge axis, we would expect
to find shallow water depths along these axes, and symmet-
rically increasing depths as one approaches the continental
land masses on either side. The reason for this apparent con-
tradiction is the process of sedimentation, the deposition of
land-derived sediments along the ocean floor. As one would
expect, ocean regions most closely bordering the continents
experience the highest rates of sedimentation, both through
wind-driven atmospheric input as well as the more dominant
mechanism of water-borne inputs at the coast. Rates of sedi-
ment accumulation in these regions vary considerably, but
have typical magnitudes of several hundred meters (verti-
cally) per million years on most continental shelves. The rate
of sedimentation decreases by an order of magnitude on the
continental slope, to tens of meters per million years. Finally,
there is in general very little or negligible deposition in the
deep ocean. Figure 1 illustrates a typical cross-section of the
ocean floor as it exists today; the product of the combined
actions of seafloor spreading and sedimentation.SEA WATER PROPERTIESAs a forcing mechanism for both deep water and coastal
motions, a tracer for the identification of water movements,
and a critical parameter in the acoustic characteristics of
ocean waters, density, r, is perhaps the most important sea-
water property to ocean engineers and scientists. In fact, many
oceanographic calculations require a knowledge of the water
density to an accuracy of 6 significant digits! In practice, theTABLE 1
Area and average depth of major ocean basinsOcean Basin Area (10^6 km^2 ) Average Depth
(meters)Including Adjacent Seas
Indian 74.917 3897
Atlantic 106.463 3332
Pacific 179.679 4028
Excluding Adjacent Seas
Indian 73.443 3963
Atlantic 82.441 3926
Pacific 165.246 4282C015_001_r03.indd 791C015_001_r03.indd 791 11/18/2005 11:10:00 AM11/18/2005 11:10:00 AM