Ridges – Ridges are defined as “an isolated (or group of) elongated narrow elevation(s) of varying complexity having steep sides, often separating basin features” (IHO 2008). In this study “ridges” were confined to features greater than 1,000 m in relief (i.e. they overlay the abyssal mountains classification layer). Ridges partially overlap with the mid-ocean ridges, which were mapped as a separate feature, but in many areas mid-ocean ridges do not attain sufficient vertical relief (>1,000 m) to qualify as “ridges” in the present study.
In order to distinguish between seamounts and ridges, it is important to note the IHO (2008) definition of seamounts includes the specification that seamounts are “conical in form”. Thus features having a width/length ratio of < 0.5 are defined here as ridges; ridges are generally larger (mean size of 2,570 km2 versus 810 km2 for seamounts) and less steep-sided than seamounts. Escarpments (areas of seafloor dipping at an angle of over 5o) characterize 46.1% of ridge flanks compared with 63.4% of seamounts and guyots globally.
Overall, ridges are most common in the North and South Pacific Oceans, covering 3.50% and 3.00% of the two ocean regions, respectively. The Arctic Ocean and the Mediterranean and Black Sea have the fewest number of ridges and least amount of ridge area (< 1%). The largest single ridge feature mapped in this study is an un-named ridge near the Aleutian Islands that covers an area of 63,400 km2. It is interesting that, although geomorphic ridges are often associated with mid-ocean spreading (seafloor rifting), they cover the greatest areas in regions of plate convergence and subduction (eg. in the western Pacific and in Drake Passage).