Over that time span, the ocean floor cools, contracts, and
sinks, rapid first and slower later (see “Subsidence of Oceanic Crust”), and this subsidence controls the depth of the
largest part of the seafloor.
Abyssal plains grade into the foot regions of continental
margins towards the land masses and into areas of increasing
topographical roughness towards mid-ocean ridges
(Figure 1a, b). In order to delimit this physiographic domain,
the International Hydrographic Organization (IHO) agreed
upon a definition that includes water depth and the variation
in relief over a certain radius (e.g., the variation in relief
has to be <300 m over a radius of 25 cells of 30 arc sec).
Following this definition, Harris et al. (2014) calculated
that globally, abyssal plains make up 27.9 % of the seafloor
or 19.8 % of the entire surface of the Earth.
The flatness of abyssal plains is exemplified in
Figure 1b. It shows a bathymetric profile of the western
North Atlantic Ocean from offshore Newfoundland to
Abyssal Plains, Figure 1 (a) Bathymetric map of the western North Atlantic, showing a broad continental shelf and slope, a prominent
abyssal plain (the Hatteras Abyssal Plain), and the Mid-Atlantic Ridge. The map was created with the GeoMapApp (www.geomapapp.org);
the underlying bathymetric data were published as Global Multi-Resolution Topography (GMRT) by Ryan et al. (2009). (b) shows a depth
profile from the continental shelf offshore Newfoundland to the Mid-Atlantic Ridge (A–D). In this profile, the continental platform and the
continental slope are clearly depicted. Towards the Mid-Atlantic Ridge, the roughness of the seafloor increases with decreasing thickness
of the sediment cover. Stippled lines indicate seafloor bedrock age according to Mu ¨ ller et al. (2008).
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ABYSSAL PLAINS