Southeast Indian Ridge

Amsterdam−St. Paul hotspot During the past few million years hotspot activity has produced a plateau straddling on the SEIR. This Amsterdam–St. Paul Plateau while formed in the last 10 million years, started this formation beneath the Australian Plate so the plateau is now built on the components of two tectonic plates (see Kumar et al. for diagram of this complex process). For several reasons, including that the composition at and near Amsterdam and Saint Paul Island is distinct from other Kerguelen hotspot material, this has suggested to many that the Amsterdam–St. Paul hotspot (ASP) is separate from the Kerguelen hotspot. The ASP Plateau covers an area of and rises above the surrounding seafloor. North-east of the ASP Plateau a string of submarine volcanoes, the Chain of the Dead Poets, Kerguelen hotspot The Kerguelen plume is likely to have played a role in the breakup of eastern Gondwana about 136 and be relevant to the formation of the SEIR later but this relevance is still not fully understood. The Kerguelen hotspot, located more than from the SEIR currently, also influences the MORB composition of the SEIR near the ASP Plateau. Many recent authors do not try to portray a continuous connection between Broken Ridge which has Kerguele plume basalts formed 37 Ma ago and the Kerguelen plateau basalts formed between 37 Ma and now for several reasons, including that the ocean floor south of the ASP Plateau and north of the Kerguele Plateau is poorly studied. Australian−Antarctic Discordance Trending east-west between Australia and Antarctica, the SEIR traverses the Australian-Antarctic Discordance (AAD), a morphologically complex region overlying an area of mantle down-welling. Located midway between the ASP-Kerguelen and the Balleny-Tasmantid hotspots, the AAD overlies a region where cooler mantle temperatures have produced a thin oceanic crust and a rough topography with deep valleys. The AAD is found between 120° and 128° E and covers about of the SEIR which at this point is deep mid-ocean ridge at between in the centre of the Australian–Antarctic depression. Between the AAD and the Amsterdam and St. Paul islands, spreading rate is constant at while axial depth increases by more than . This has been interpreted as an eastward decrease in mantle temperature of perhaps caused by a magma flow from the Kerguelen–ASP hotspots to the AAD 'cold spot' at 120–128°E. Located at 126°E, the AAD would thus mark the -long transition between Indian Ocean and Pacific MORBs (mid-ocean ridge basalts), a boundary that has been migrating westward during the past tens of million years. Between 102°E and the AAD, where the spreading rate is constant, the left-stepping transform faults suggest the presence of oblique extensional forces while the presence of a long, elevated ridge near the 96°E right-stepping transform suggests a compressional force is also active. Together these features indicate the two tectonic plate made a recent counter-clockwise change in relative motion. == Tectonic history ==

Australia and Antarctica were neighbours before the break-up of Gondwana in the Cretaceous and several conjugate structures exist on either side of the SEIR. In south-western Australia the Albany-Fraser Orogen formed during the Mesoproterozoic collision between the Australian Yilgarn and Antarctic Mawson cratons. The continental basement of the submarine Naturaliste Plateau is also associated with this orogeny. The Darling Fault on the Australian west coast has a possible continuation beneath the Antarctic Denman Glacier. Archaean and Paleoproterzoic rocks in the Kalinjala Mylonite Zone of the Eyre Peninsula, Australia, match those found in Terre Adelie in Eastern Wilkes Land, Antarctica. Faults in Tasmania–Victoria and Northern Victoria Land have been identified as Cambrian remains of the west-dipping subduction zone along the eastern margin of Gondwana. Australia and Antarctica broke-up around but spreading in the SEIR first began during the Eocene () when the Kerguelen hotspot separated Broken Ridge from the rest of the Kerguelen Plateau. The SEIR has been migrating northeast since and is now located from the Kerguelen hotspot. The ASP hotspot was originally located beneath Australia and a chain of seamounts connecting it to the southern end of the Ninety East Ridge, i.e. the ASP hotspot track, indicate it probably contributed to the formation of the Ninety East Ridge before the SEIR opened. The opening of the Southern Ocean began west of Australia around from where it propagated eastward at about . This rifting was not the direct product of hotspot interaction as it occurred over a cooler than normal mantle. Initially spreading was extremely slow, a half rate of during the period after which it accelerated to . == Oceanography ==

The SEIR divides the channel between Australia and Antarctica into the South Indian Basin to the south and the South Australian and Tasman basins to the north. The AAD forms a saddle across the channel while also offering the deepest connection between the Australian and South Indian basins. There is a voluminous contourite drift along the southern flank of SEIR. Volcanic in origin, it is most likely derived from the slopes of the Kerguelen Plateau and Crozet Islands. This redistribution of sediments has occurred during the last 40000 years. Elevated contributions during the Last Glacial Maximum are thought to be caused by the Antarctic Circumpolar Current and Circumpolar Deep Water and their interaction with the Circumpolar Bottom Water. == See also ==