The Ninety East Ridge (also rendered as Ninetyeast Ridge, 90E Ridge or 90°E Ridge) is a

mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverge ...

on the

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by th ...

floor named for its near-parallel strike along the 90th meridian at the center of the

Eastern Hemisphere The Eastern Hemisphere is the half of the planet Earth which is east of the prime meridian (which crosses Greenwich, London, United Kingdom) and west of the antimeridian (which crosses the Pacific Ocean and relatively little land from pole to pol ...

. It is approximately in length and can be traced topographically from the

Bay of Bengal The Bay of Bengal is the northeastern part of the Indian Ocean, bounded on the west and northwest by India, on the north by Bangladesh, and on the east by Myanmar and the Andaman and Nicobar Islands of India. Its southern limit is a line between ...

southward towards the

Southeast Indian Ridge The Southeast Indian Ridge (SEIR) is a mid-ocean ridge in the southern Indian Ocean. A divergent tectonic plate boundary stretching almost between the Rodrigues Triple Junction () in the Indian Ocean and the Macquarie Triple Junction () in the ...

(SEIR), though the feature continues to the north where it is hidden beneath the sediments of the

Bengal Fan The Bengal Fan, also known as the Ganges Fan, is the largest submarine fan on Earth. Geography It is located in Eurasia, being about long, wide with a maximum thickness of . The fan resulted from the uplift and erosion of the Himalayas and th ...

. The ridge extends between latitudes 33°S and 17°N and has an average width of 200 km.

Description

The ridge divides the Indian Ocean into the West and East Indian Ocean. The northeastern side is named the

Wharton Basin Wharton Basin is the marine area of the north east quarter of the Indian Ocean. It is named after William Wharton (1843-1905), Hydrographer of the Navy. Alternative names are Cocos Basin (after the Cocos Islands) and West Australian Basin. It ...

and ceases at the western end of the

Diamantina Fracture Zone The Diamantina Fracture Zone (DFZ) is an area of the south-eastern Indian Ocean seafloor, consisting of a range of ridges and trenches. It lies to the south of the mideastern Indian Ocean features of the Wharton Basin and Perth Basin, and t ...

which passes to the east and almost to the Australian continent.Stow, D. A. V. (2006) ''Oceans : an illustrated reference'' Chicago : University of Chicago Press, - page 127 for map of Indian Ocean and ridges The ridge is primarily composed of Ocean Island Tholeiites (OIT), a subset of

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

which increase in age from approximately 43.2 ± 0.5 Ma in the south to 81.8 ± 2.6 Ma in the north. A more recent analysis using modern Ar–Ar techniques gives an age progression from 77 Ma at 5°N to 43 Ma at 31°S. This age progression has led geologists to theorize that a hotspot in the mantle beneath the

Indo-Australian Plate The Indo-Australian Plate is a major tectonic plate that includes the continent of Australia and the surrounding ocean and extends northwest to include the Indian subcontinent and the adjacent waters. It was formed by the fusion of the Indian an ...

created the ridge as the plate has moved northward in the late

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceo ...

and

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...

. This theory is supported by a detailed analysis of the chemistry of the

Kerguelen Plateau The Kerguelen Plateau (, ), also known as the Kerguelen–Heard Plateau, is an oceanic plateau and a large igneous province (LIP) located on the Antarctic Plate, in the southern Indian Ocean. It is about to the southwest of Australia and is ...

and

Rajmahal Traps The Rajmahal Traps is a volcanic igneous province in Eastern India, covering the parts of Jharkhand, West Bengal and Meghalaya. The Rajmahal Hills of Jharkhand is the type area of this province. Multiple layers of solidified lava made the Rajma ...

, which together, geologists believe, represent the

flood basalt A flood basalt (or plateau basalt) is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basalt lava. Many flood basalts have been attributed to the onset of a hotspot reach ...

s erupted at the initiation of volcanism at the

Kerguelen hotspot The Kerguelen hotspot is a volcanic hotspot at the Kerguelen Plateau in the Southern Indian Ocean. The Kerguelen hotspot has produced basaltic lava for about 130 million years and has also produced the Kerguelen Islands, Naturaliste Plateau, Hear ...

which was then sheared in two as the Indian subcontinent moved northward.

Surveying

The ridge has been surveyed several times in the past, including several times by the

Deep Sea Drilling Program The Deep Sea Drilling Project (DSDP) was an ocean drilling project operated from 1968 to 1983. The program was a success, as evidenced by the data and publications that have resulted from it. The data are now hosted by Texas A&M University, alth ...

(DSDP). In 2007, the

RV Roger Revelle R/V ''Roger Revelle'' is a operated by Scripps Institution of Oceanography under charter agreement with Office of Naval Research as part of the University-National Oceanographic Laboratory System (UNOLS) fleet. The ship is named after Roger Rand ...

collected bathymetric, magnetic and seismic data together with dredge samples from nine sites along the ridge as part of an

Integrated Ocean Drilling Program The Integrated Ocean Drilling Program (IODP) was an international marine research program. The program used heavy drilling equipment mounted aboard ships to monitor and sample sub-seafloor environments. With this research, the IODP documented e ...

(IODP) site survey intended to examine the hotspot hypothesis for the ridge.

Origins

It had been assumed that India and Australia were on a single tectonic plate for at least the last 32 million years. However, considering the high level of large earthquakes in the Ninety East Ridge area and the evidence of deformation in the central Indian Ocean, it is more appropriate to consider the deformed region in the central Indian Ocean as a broad plate boundary zone separating the Indian Plate and the Australian Plate.

Paleontology

During the late

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), E ...

around 60 million years ago, parts of the Ninety East Ridge were temporarily exposed for 2-3 million years as

volcanic islands Geologically, a high island or volcanic island is an island of volcanic origin. The term can be used to distinguish such islands from low islands, which are formed from sedimentation or the uplifting of coral reefs (which have often formed ...

probably 1,000 km from the nearest land. Preserved pollen and plant cuticle fragments have been found in boreholes drilled on the ridge. The flora has been noted to be most similar to Australian and Antarctic floras, rather than to Indian floras, including

Podocarpaceae Podocarpaceae is a large family of mainly Southern Hemisphere conifers, known in English as podocarps, comprising about 156 species of evergreen trees and shrubs.James E. Eckenwalder. 2009. ''Conifers of the World''. Portland, Oregon: Timber Pre ...

conifers, as well as 15 species of angiosperms, including members of

Arecaceae The Arecaceae is a family of perennial flowering plants in the monocot order Arecales. Their growth form can be climbers, shrubs, tree-like and stemless plants, all commonly known as palms. Those having a tree-like form are called palm trees ...

Chloranthaceae Chloranthaceae is a family of flowering plants (angiosperms), the only family in the order Chloranthales. It is not closely related to any other family of flowering plants, and is among the early-diverging lineages in the angiosperms. They are ...

''sensu lato,''

Lauraceae Lauraceae, or the laurels, is a plant family that includes the true laurel and its closest relatives. This family comprises about 2850 known species in about 45 genera worldwide (Christenhusz & Byng 2016 ). They are dicotyledons, and occur ma ...

, ''

Gunnera ''Gunnera'' is the sole genus of herbaceous flowering plants in the family Gunneraceae, which contains 63 species. Some species in this genus, namely those in the subgenus ''Panke'', have extremely large leaves. Species in the genus are various ...

Gillbeea ''Gillbeea'' is a genus of three species of Australasian rainforest trees from the family Cunoniaceae. Botanical science has formally published , the names and descriptions of one species endemic to New Guinea and two species endemic to the Wet ...

,'' and possibly Callitrichaceae.

References

External links

90 East Ridge Formation (94 Ma - present)

* Video including 90 East Ridge Formation (94 Ma (million years ago) - present)
YouTube. ''Plate tectonics, Paleogeography, & Ice Ages (dual hemispheres)''
by

Christopher Scotese Christopher R. Scotese (born 4 May 1953) is an American geologist and paleogeographer. He received his PhD from the University of Chicago in 1985. He is the creator of the Paleomap Project, which aims to map Earth over the last billion years, ...

, 2019. (

Field Museum of Natural History The Field Museum of Natural History (FMNH), also known as The Field Museum, is a natural history museum in Chicago, Illinois, and is one of the largest such museums in the world. The museum is popular for the size and quality of its educational ...

Northwestern University Northwestern University is a private research university in Evanston, Illinois. Founded in 1851, Northwestern is the oldest chartered university in Illinois and is ranked among the most prestigious academic institutions in the world. Charte ...

). * Video including 90 East Ridge Formation (94 Ma (million years ago) - present)
YouTube. ''Plate Tectonics, 540Ma - Modern World''
by

, 2016. (

). {{Coord, 3, S, 90, E, type:mountain, display=title Plate tectonics Underwater ridges of the Indian Ocean Seamount chains