Rift valleys – Rift valleys are confined to the central axis of mid-ocean spreading ridges; they are elongate, local depressions flanked generally on both sides by spreading ridges (Macdonald, 2001). Rift valleys cover the largest fraction of abyssal zone in the Arctic Ocean, where they characterise 0.622% of that area. The greatest total area of rift valleys occurs in the Indian Ocean where they cover 165,220 km2.
Harris et al. (2014) mapped 658 separate rift valley segments, found mainly along mid-ocean ridges, covering an area of 710,060 km2. Segmentation of the rift valley, due to transform faults and other factors (Macdonald, 2001), is manifest as a greater number of smaller-sized segments in the Indian, North Pacific and South Pacific Oceans, compared with the Arctic, North Atlantic and South Atlantic Oceans, where rift valleys are fewer in number (less segmented) and much greater in size than the global average (Table). Earth’s largest mid-ocean rift valleys occur in the Atlantic Ocean and they are up to 21,390 km2 in area, compared with the global mean size of only 1,080 km2 (see Table), although, as noted above, the Indian Ocean contains the largest absolute area of rift valleys.
Harris et al. (2014) used the EarthByte (Müller et al., 1997) database on mid-ocean ridge spreading rates to assign a mean spreading rate to each of the 658 rift valley segments and to then estimate a mean rift valley segment size and spreading rate for each major ocean region. The results (see Figure) demonstrate that larger rift valley segments are generally associated with slow-spreading rates and smaller rift valley segments are associated with fast spreading. The relationship appears to generally hold true but is complicated by other factors that include crustal thickness, the development of fracture zones and patterns of upwelling magma (Macdonald, 2001). The data indicate that Order 3 rift valley segments (as per Macdonald, 2001) are the most abundant, although the 1 km bathymetric grid size used is too coarse to resolve Order 4 rift valley segments.
For a discussion of the biota of rift valleys, see “Spreading Ridges”.