Ninety East Ridge

The Ninety East Ridge (also rendered as Ninetyeast Ridge, 90E Ridge or 90°E Ridge) is a linear, age-progressive seamount chain in the Indian Ocean and is named for its near-parallel strike along the 90th meridian at the center of the Eastern Hemisphere. It is approximately 5,000 kilometres (3,100 mi) in length and can be traced topographically from the Bay of Bengal southward towards the Southeast Indian Ridge (SEIR), though the feature continues to the north where it is hidden beneath the sediments of the Bengal Fan. The ridge extends between latitudes 33°S and 17°N and has an average width of 200 km.

The ridge divides the Indian Ocean into the West and East Indian Ocean. The northeastern side is named the Wharton Basin and ceases at the western end of the Diamantina Fracture Zone which passes to the east and almost to the Australian continent.

The ridge is primarily composed of Ocean Island Tholeiites (OIT), a subset of basalt which increase in age from approximately 43.2 ± 0.5 Ma in the south to 81.8 ± 2.6 Ma in the north though a more recent analysis using modern Ar–Ar techniques is currently pending publication. This age progression has led geologists to theorize that a hotspot in the mantle beneath the Indo-Australian Plate created the ridge as the plate has moved northward in the late Mesozoic and Cenozoic. This theory is supported by a detailed analysis of the chemistry of the Kerguelen Plateau and Rajmahal Traps, which together, geologists believe, represent the flood basalts erupted at the initiation of volcanism at the Kerguelen hotspot which was then sheared in two as the Indian subcontinent moved northward. However, the existence of so-called deep mantle hotspots is currently a topic of debate in the geologic community, with a few geochemists favoring an alternative hypothesis which postulates a much shallower origin for hotspot volcanism.

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