Australia: The Land Where Time Began

A biography of the Australian continent 

Seafloor Features

The detailed features of the margins of the continents, the major lateral boundaries of the ocean, are the shoreline and the ocean floor are important as a result of their effect on circulation. The main divisions of the ocean that are recognised are, from shore to ocean, the shore, the continental shelf, the continental slope or rise, the deep sea bottom, of which the deepest part is the abyssal plain. Plate tectonics and submarine volcanism have produced some of the major ocean floor features such as mid-ocean ridges, trenches, island arcs and seamounts.

The ocean floor is very smooth in some places, the authors2 suggesting that it may be even smoother than on open plains on the land. Large regions of very smooth topography have been produced by sedimentation, composed mostly of organic matter from the upper layers of the ocean that falls as a constant rain. In the western North Atlantic areas of the abyssal plain have been measured to be within 2 m over a distance of 100 km. In the northeast Indian Ocean/Bay of Bengal the ocean is very smooth, sloping from 2,000 m depth to more than 5,000 m over a distance of 3,000 km, the sedimentation resulting from the sediment brought to the ocean by the Ganges River and the Brahmaputra River, both of which drain the Himalayas. Deep currents can move sediments around, and the formation of dunes and canyons are common, deep currents being sometimes being found as a result of erosional features in deep sediments.

In the distribution of water masses and the location of currents the topography of the ocean floor often is an important factor, such as relates to the Walvis Ridge in the South Atlantic Ocean, the height of the ridge preventing the eastern part of the Atlantic Basin filling directly with the bottom water from the Weddell Sea, Antarctica. The bottom water flows to the north along the western boundary of the South Atlantic until reaching a deep passage in the MAR and flowing south and fills the basin to the east of the ridge. Sills, shallowest parts of channels that define marginal seas, strongly influence the mid-level currents and the water mass distribution of the sea. Coastal shape, along with the topography of the bottom, directly the upwelling along the coast. The topography of the bottom often influence the alongshore currents, and the horizontal scales of the bottom topography can determine the instabilities in the system. The breaking of surface gravity waves is determined by the topography of the coast and directly influences the local tidal expressions.

The boundaries, that include the bottom, are the sites where most of the mixing of the oceans takes place. It is suggested by intensive experiments on the mixing, including its genesis, as well as microstructure observations carried out in many regions, that a major mechanism of dissipation of the ocean's energy is the flow of deep internal tides over steep bottom slopes in the. The largest slopes have been found by analysis of bathymetry to tend to be the flanks of the mid-ocean ridges that are spreading the fastest. The mid-ocean ridges of the Atlantic Ocean, Southern Ocean, and Indian Ocean have been suggested by the latest analysis of the bathymetric data of bathymetric slopes, as well as information concerning the stratification of the deep ocean, may be the sites of most vigorous dissipation of ocean energy (Becker & Sandwell, 2008).

Spatial Scales

According to the authors2 it is usual when illustrating vertical cross-section of the ocean floor to greatly exaggerate the vertical scale, as if the cross-section was viewed to actual scale it would not show the vertical details or the alternatively it would need to be too long for convenience. They suggest an example of the scale that could be used as 1 cm:100 km for the horizontal scale and 1 cm:100 m for the vertical scale, resulting in a magnification of 1000 times for the vertical dimension compared to the horizontal dimension (1000:1), though this method exaggerates the ocean floor slope, or of isopleths (contours of constant water properties. On a display using the exaggerated vertical dimension the slope of a line of constant temperature (isotherm) would be represented as a slope of 1in 10, whereas the actual slope would be 1 in 10,000.

Sources & Further reading

  1. Emery, William J., Pickard, George L., Tally, Lynne D., & Swift, James H., 2011, Descriptive Physical Oceanography, an Introduction, Academic Press.


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Author: M. H. Monroe
Last Updated 16/04/2012



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