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 seas help dissipate tides, they are often areas of high biological productivity, and they are usually included in the exclusive economic zone of adjacent countries.

The 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. The 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 with steep slopes, cutting across the continental shelf and slope, with bottoms sloping continuously downward.

Continental (or island) shelves are zones adjacent 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 great 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).

Continental (or island) slopes are the declivities seaward from the shelf edge into greater depth.

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 steep sides and rough topography.

Seamounts are isolated or comparatively isolated 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.

Trenches are 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.

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