Plateaus – Plateaus are “flat or nearly flat elevations of considerable areal extent, dropping off abruptly on one or more sides” (IHO, 2008).  Areas of flat ocean floor that are raised above the level of surrounding seafloor, delimited by a steep slope, are known as submarine plateaus.  The plate tectonic theory explains how the rifting apart of ancient super-continents, Gondwanaland and Laurasia, over the past 100 million years has resulted in the present day configuration of ocean basins and continents.  The disintegration of Gondwanaland not only created the six large continents, but also a number of smaller continental fragments that are scattered around the globe.  Some of these so-called micro continents rise above the ocean surface forming large islands like New Zealand; others remain entirely submerged, forming the largest, broad undersea plateaus.

A total of 184 plateaus were mapped in this study, covering an area of 18,486,600 km2, or 5.11% of the oceans.  The largest plateau is located in the South Pacific Ocean, extending from New Zealand to northeast Australia, including Challenger Plateau and Lord Howe Rise (Harris, 2011) and covers a total area of 1,505,370 km2.  Other plateaus of notable size are the Campbell Plateau (1,229,370 km2) and the Kerguelen Plateau (1,226,230 km2).  Plateaus are generally important features in the South Pacific and Indian Oceans, where they cover areas of over 7% and 8% of those ocean regions, respectively.  There were no plateaus mapped in the Mediterranean and Black Seas in this study.

Other plateaus are produced from large volcanic eruptions or from tectonic uplift of ocean crust.  Plateaus are normally draped by sediments, but since the tops of plateaus are cut-off from receiving any sediment from the continents because of their raised profile, the only food source to the benthos is derived from the rain of pelagic organic matter from the overlying water column.

An example of a large oceanic plateau is the Lord Howe Rise located in the Tasman Sea between Australia and New Zealand (Harris, 2011).  The plateau is a fragment of continental crust that separated from Australia around 40 million years ago.  It has subsided to a depth of around 1,500 m and because the overlying ocean is oligotrophic, pelagic sediments are accumulating on its flat top at a sluggish rate of only a few centimetres per thousand years. It is characterised mostly (95%) by sediment infauna with isolated volcanic cones providing hard substrate for filter-feeding epifauna.

In the northeast Atlantic, Sayago-Gil et al. (2012) describe multibeam

bathymetry and benthic samples collected from Hatton Bank, between 600 and 2,000 m water depth. Benthic communities are sparse where the seabed comprises mobile sediments (contourite drift deposits) in deeper water, whereas cold-water corals are common on rocky outcrops closer to the top of the bank.

Nodder et al. (2012) describe the benthic communities of the Chatham Rise, a 160,000 km2  submarine continental ridge that extends eastward from New Zealand into the Southwest Pacific Ocean in water depths of 50 to 2,000 m. Isolated groups of volcanic seamounts are present on the flanks of the rise. Most of the Chatham Rise consists of sediment substrates populated by mobile fauna, with conspicuous examples including scampi (Metanephrops challengeri ), squat lobsters (Munida gracilis ), several crab species, quill worms (Hyalinoecia tubicola ), urchins (Parametia peloria ), and other spatangids and asteroids.

Figure 4.3 Unpublished photographs from Geoscience Australia taken on substrates of the Lord Howe Rise, deep sea plateau in around 1,500 m water depth, showing hard substrate filter-feeders: (A) stalked Crinoid; (B) Crinoid; (C) Cnidaria; soft-substrate burrows of (D) Quill worm (?); and (E, F) unknown deposit feeders.

Statistics of plateaus (after Harris et al., 2014).  The percentage areas refer to fraction of ocean regions that is submarine plateau.

OceanArea km2Plateau area%Number of PlateausAverage plateau area km2
Arctic Ocean1,193,7409.191299,480
Indian Ocean5,036,8707.0637136,130
Mediterranean and Black Sea00.0000
North Atlantic Ocean1,628,3603.643645,230
North Pacific Ocean1,856,7902.263356,270
South Atlantic Ocean1,220,2303.029135,580
South Pacific Ocean7,054,8008.0950141,100
Southern Ocean495,8302.441241,320
All Oceans18,486,6105.11184100,470

Harris, P.T., 2011. Benthic environments of the Lord Howe Rise submarine plateau: Introduction to the special volume. Deep Sea Research Part II 58, 883-888.

Harris, P.T., MacMillan-Lawler, M., Rupp, J., Baker, E.K., 2014. Geomorphology of the oceans. Marine Geology 352, 4-24.

IHO, 2008. Standardization of Undersea Feature Names: Guidelines Proposal form Terminology, 4th ed. International Hydrographic Organisation and Intergovernmental Oceanographic Commission, Monaco, p. 32.

Nodder, S.D., Bowden, D.A., Pallentin, A., Mackay, K., 2012. Ch. 56: Seafloor Habitats and Benthos of a Continental Ridge: Chatham Rise, New Zealand, in: Harris, P.T., Baker, E.K. (Eds.), Seafloor geomorphology as benthic habitat: GeoHAB Atlas of seafloor geomorphic features and benthic habitats. Elsevier, Amsterdam, pp. 763-776.

Sayago-Gil, M., Durán-Muñoz, P., Murillo, F.J., Díaz-del-Río, V., Serrano, A., Fernández-Salas, L.M., 2012. Ch. 55: A Study of Geomorphological Features of the Seabed and the Relationship to Deep-Sea Communities on the Western Slope of Hatton Bank (NE Atlantic Ocean), in: Harris, P.T., Baker, E.K. (Eds.), Seafloor geomorphology as benthic habitat: GeoHAB Atlas of seafloor geomorphic features and benthic habitats. Elsevier, Amsterdam, pp. 751-761.