Chapter 3 - The Physical Setting
3.3 Sea-Floor Features
Earth's rocky surface is divided into two types: oceanic,
with a thin dense crust about 10 km thick, and continental, with a thick
light crust about 40 km thick. The deep, lighter continental crust floats
higher on the denser mantle than does the oceanic crust, and the mean height
of the crust relative to sea level has two distinct values: continents have
a mean elevation of 1100 m, the ocean has a mean depth of -3400 m (Figure 3.5).
||Figure 3.5 Left Histogram
of elevations of land and depth of the sea floor as percentage of area
of the Earth, in 50m intervals
showing the clear distinction between continents and sea floor. Right Cumulative
frequency curve of height, the hypsographic curve. The curves are calculated
from the ETOPO 30´ data set.
The volume of the water in the ocean
exceeds the volume of the ocean basins, and some water spills over on to
the low lying areas of the continents.
These shallow seas are the continental shelves. Some, such as the South China
Sea, are more than 1100 km wide. Most are relatively shallow, with typical
depths of 50-100 m. A few of the more important shelves are: the East China
Sea, the Bering Sea, the North Sea, the Grand Banks, the Patagonian Shelf,
the Arafura Sea and Gulf of Carpentaria, and the Siberian Shelf. The shallow
help dissipate tides, they are often areas of high biological productivity,
and they are usually included in the exclusive economic zone of adjacent countries.
crust is broken into large plates that move relative to each other. New
crust is created at the mid-ocean ridges, and old crust is lost at trenches.
relative motion of crust, due to plate tectonics, produces the distinctive
features of the sea-floor sketched in Figure 3.6, include mid-ocean ridges,
trenches, island arcs, and basins.
||Figure 3.6 Schematic section through the ocean showing principal features of the
sea floor. Note that the slope of the sea floor is greatly exaggerated in the figure.
The names of the sub-sea features have been
defined by the International Hydrographic Bureau (1953), and the following
definitions are taken from
Sverdrup, Johnson, and Fleming (1942), Shephard (1963), and Dietrich
et al., (1980).
Basins are deep depressions of the sea
floor of more or
less circular or oval form.
Canyons are relatively narrow, deep furrows
steep slopes, cutting across the continental shelf and slope, with bottoms
Continental (or island) shelves are zones
to a continent (or around an island) and extending from the low-water line
to the depth,
usually about 120 m, where there is a marked or rather steep descent toward
depths. (figure 3.7).
||Figure 3.7 An example of a continental shelf,
the shelf offshore of Monterey California showing the Monterey and other canyons.
Canyons are common on shelves, often extending across the shelf and
down the continental slope to deep water. Figures copyright Monterey
Bay Aquarium Research Institute (MBARI).
(or island) slopes are the declivities seaward from the shelf edge into greater
Plains are flat, gently sloping or nearly level
regions of the sea-floor, e. g. an abyssal plain.
Ridges are long, narrow elevations
of the sea-floor with
sides and rough
Seamounts are isolated or comparatively
elevations rising 1000 m or more from the sea floor and with small summit
area (figure 3.8).
| Figure 3.8 An example of a seamount,
the Wilde guyot. A guyot is a seamount with a flat top created by
wave action when the seamount extended above sea level. As the seamount
is carried by plate motion, it gradually sinks deeper below sea level.
The depth was contoured from echo sounder data collected along the
ship track (thin straight lines) supplemented with side-scan sonar
data. Depths are in units of 100 m. From William Sager, Texas A&M University.
Sills are the low parts of the ridges separating
ocean basins from one another or from the adjacent sea-floor.
long, narrow, and deep depressions
of the sea-floor, with relatively steep sides (Figure 3.9).
||Figure 3.9 An
example of a trench, the Aleutian Trench; an island arc, the Aleutian
Islands; and a
continental shelf, the Bering Sea. The island arc is composed of
volcanos produced when
oceanic crust carried deep into a trench melts and rises to the surface.
Top: Map of the Aleutian
region of the North Pacific. Bottom: Cross-section
through the region.
Sub-sea features have important influences on the ocean circulation. Ridges
separate deep waters of the ocean into distinct basins separated by
sills. Water deeper
than the sill between two basins cannot move from one to the other.
Tens of thousands of isolated peaks, seamounts, are scattered throughout
the ocean basins. They interrupt ocean currents, and produce turbulence leading
to vertical mixing of water in the ocean.