Australia: The Land Where Time Began

A biography of the Australian continent 

Continental Shelf, Slope and Rise

Continental shelf

Extending seaward from the shore the continental shelf has a gradient of 1 in 500, to the shelf break, the outer limit, that has been set to where the gradient becomes an average of 1 in 20, usually with an average depth of 130 m, down to the floor of the deep ocean where the predominant sediment is sand, and less commonly sand or mud. The average width of the shelf is 65 km, though it is much narrower in places, though it is as much as 10 times this width in other places such as the northeastern Bering Sea or the Arctic Shelf off Siberia. In a vertical cross-section of the ocean floor outward from the shore the shelf break is usually easily evident. There are many reasons why the fishing grounds of the world are situated on continental slope, among which are the proximity to estuaries, the dept the sunlight penetrates compared with the depth of the bottom, and on some shelves, the upwelling of nutrient-rich water, especially those off the western coasts of continents.

The continental slope

From the shelf to the ocean floor the average depth is about 4,000 m vertically, though it extends to more than 9,000 m vertically in a horizontal distance that is relatively short. Continental slopes are generally much steeper than the slopes between highlands and lowlands on the land. Though the slope has some rock outcrops it is mostly mud. Submarine canyons, that occur throughout the world, are usually included in the continental slope. The canyons are valleys in the slope that have either V-shaped or vertical sides, and are usually present off coasts that have rivers. Some that are usually found in hard granitic rock were carved by rivers prior to inundation by rising sea levels, examples being the Mediterranean and the Baja, California. Turbidity currents are responsible for some, usually in softer sedimentary rocks. The continental rise is the term used for lower section of the slope where it grades into the deep seafloor.

Turbidity currents, that are comprised of a mixture of water and sediment, are episodic events that occur on continental slopes as a result of unstable sediment and not by internal forces within the water. They occur when the sedimentary material accumulates on the slopes to the point where they become unstable and ultimately the force of gravity causes them to flow down the slope, large amounts of sediment, as well as some bottom material, rush down the slope reaching speeds of up to 100 km/hr with enough force to snap undersea cables. Some factors that determine when turbidity currents occur include the slope of the valley and the nature of the material in the valley. Many of the submarine canyons on slopes are formed by turbidity currents, and in some cases, such as the Congo River, that carry a dense load of material in suspension, form turbidity currents continuously flow down thir canyons.

Deep Ocean

The bathymetric gradient decreases down the continental rise from the bottom of the continental slope to the floor of the deep sea, that is last lone from the shore, that is most extensive zone. An area of 74 % of the deep sea floor the depth ranges from 3000-6000 m, and 1 % of the  area of the deep sea area is deeper. The variety of topography of the deep-sea floor is considered by some, such as the authors1, to be its most characteristic feature, though prior to the studies by deep ocean soundings the floor of the deep ocean was believed to be uniformly smooth. The actual condition of the ocean floor only became known when detailed soundings were used to determine the nature of the bottom prior to the laying of cables, after which it became apparent that it was predominantly rugged, though it is now known that the actual nature of the ocean floor is similar to that of the land, with mountains, valleys and plains. Once satellite altimetry became available for studying the topography of the oceans the distribution of ocean bottom features could be mapped which allowed a global view of  the features on the floor (Smith & Sandwell, 1997) to be associated with plate tectonic processes and the sources of the sediment could be associated with rivers that emptied into the oceans.

 

Sources & Further reading

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

Links

Website for source 1

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last Updated 16/04/2012

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading