Ninety East Ridge

The Ninety East Ridge (also rendered as Ninetyeast Ridge, 90E Ridge or 90°E Ridge) is a linear structure on the Indian Ocean floor 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.[1]

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.[2]

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[3] 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.[3] 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.[4]

The ridge has been surveyed several times in the past, including several times by the Deep Sea Drilling Program (DSDP). In 2007, the RV Roger Revelle collected bathymetric, magnetic and seismic data together with dredge samples from nine sites along the ridge as part of an Integrated Ocean Drilling Program (IODP) site survey intended to examine the hotspot hypothesis for the ridge.[5]

It had been assumed that the India and Australia are 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.[6][7]

The Ninety East Ridge at the centre of the picture and the Chagos-Laccadive Ridge on the upper left side

See also


  1. ^ Ramana, M.V.; T. Ramprasad; M. Desa; V. Subrahmanyam (2000). "Integrated geophysical studies over the 85°E ridge - Evaluation and interpretation". Visakha Science Journal. 4 (1): 45–56.
  2. ^ Stow, D. A. V. (2006) Oceans : an illustrated reference Chicago : University of Chicago Press, ISBN 0-226-77664-6 - page 127 for map of Indian Ocean and ridges
  3. ^ a b Weis, D.; et al. (1993). The Influence of Mantle Plumes in Generation of Indian Oceanic Crust. Geophysical Monograph. Geophysical Monograph Series. 70. pp. 57–89. Bibcode:1992GMS....70...57W. doi:10.1029/gm070p0057. ISBN 9781118668030.
  4. ^ King, S. D.; Anderson, D. L. (1995). "An Alternative Mechanism of Flood Basalt Formation". Earth and Planetary Science Letters. 136 (3–4): 269–279. Bibcode:1995E&PSL.136..269K. doi:10.1016/0012-821X(95)00205-Q.
  5. ^ "Seismic Project Information KNOX06RR Ninetyeast Ridge IODP Survey". The University of Texas at Austin.
  6. ^ Stein, S.; Okal, W. A. (1974). "Seismicity and Tectonics of the Ninetyeast Ridge Area: Evidence for Internal Deformation of the Indian Plate" (PDF). Journal of Geophysical Research. 83 (B5): 2233. Bibcode:1978JGR....83.2233S. doi:10.1029/jb083ib05p02233. Retrieved 23 May 2015.
  7. ^ Van Orman, J.; Cochran, J. R.; Weissel, J. K.; Jestin, F. (1995). "Distribution of shortening between the Indian and Australian plates in the central Indian Ocean". Earth and Planetary Science Letters. 133 (1–2): 35–46. Bibcode:1995E&PSL.133...35V. CiteSeerX doi:10.1016/0012-821x(95)00061-g.

Further reading

Coordinates: 3°S 90°E / 3°S 90°E

90th meridian east

The meridian 90° east of Greenwich is a line of longitude that extends from the North Pole across the Arctic Ocean, Asia, the Indian Ocean, the Southern Ocean, and Antarctica to the South Pole.

It is the border between two tropical cyclone basins: the Australian region, and the Southwest Indian Ocean basin.The Ninety East Ridge is named after the meridian.

The 90th meridian east forms a great circle with the 90th meridian west.

This meridian is halfway between the Prime meridian and the 180th meridian and the center of the Eastern Hemisphere is on this meridian.

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 Islands of India and Myanmar and the Nicobar Islands of India. Its southern limit is a line between Sri Lanka and the north western most point of Sumatra (Indonesia). It is the largest water region called a bay in the world. There are countries dependent on the Bay of Bengal in South Asia and Southeast Asia.

The Bay of Bengal occupies an area of 2,172,000 square kilometres (839,000 sq mi). A number of large rivers flow into the Bay of Bengal: the Ganges–Hooghly, the Padma, the Brahmaputra–Jamuna, the Barak–Surma–Meghna, the Irrawaddy, the Godavari, the Mahanadi, the Brahmani, the Baitarani, the Krishna and the Kaveri. Among the important ports are Chennai-Ennore, Chittagong, Colombo, Kolkata-Haldia, Mongla, Paradip, Port Blair, Tuticorin, Visakhapatnam and Dhamra. Among the smaller ports are Gopalpur Port, Kakinada and Payra.

Bengal Fan

The Bengal Fan, also known as the Ganges Fan, is the largest submarine fan on Earth. The fan is about 3,000 km (1,900 mi) long, 1,430 km (890 mi) wide with a maximum thickness of 16.5 km (10.3 mi). The fan resulted from the uplift and erosion of the Himalayas and the Tibetan Plateau produced by the collision between the Indian Plate and the Eurasian Plate. Most of the sediment is supplied by the Ganges and Brahmaputra rivers which supply the Lower Meghna delta in Bangladesh and the Hoogly delta in West Bengal (India). Several other large rivers in Bangladesh and India provide smaller contributions. Turbidity currents have transported the sediment through a series of submarine canyons, some of which are more than 1,500 miles (2,414 km) in length, to be deposited in the Bay of Bengal up to 30 degrees latitude from where it began. To date, the oldest sediments recovered from the Bengal fan are from Early Miocene age. Their mineralogical and geochemical characteristics allow to identify their Himalayan origin and demonstrate that the Himalaya was already a major mountain range 20 million years ago.The fan completely covers the floor of the Bay of Bengal. It is bordered to the west by the continental slope of eastern India, to the north by the continental slope of Bangladesh and to east by the northern part of Sunda Trench off Myanmar and the Andaman Islands, the accretionary wedge associated with subduction of the Indo-Australian Plate beneath the Sunda Plate and continues along the west side of the Ninety East Ridge. The Nicobar Fan, another lobe of the fan, lies east of the Ninety East Ridge.The fan is now being explored as a possible source of fossil fuels for the surrounding developing nations.

The fan was first identified by bathymetric survey in the sixties by Bruce C. Heezen and Marie Tharp which identified the abyssal cone and canyon structures. It was delineated and named by Joseph Curray and David Moore following a geological and geophysical survey in 1968.A deep sea canyon called Swatch of no Ground is located south of Sundarbans National Park and the island of Dublar Char. This area hosts important habitats for cetaceans including endangered species such as various dolphins, Irrawaddy dolphins, and Bryde's whales (see Sundarbans National Park).

Broken Ridge

Broken Ridge or Broken Plateau is an oceanic plateau in the south-eastern Indian Ocean. Broken Ridge once formed a large igneous province (LIP) together with the Kerguelen Plateau. When Australia and Antarctica started to separate, Broken Ridge and the Kerguelen Plateau got separated by the Southeast Indian Ridge. Alkalic basalt from Broken Ridge has been dated to 95 Ma.Broken Ridge stretches 1,200 km (750 mi) from the southern end of Ninety East Ridge towards the south-western corner of Australia. It is up to 400 km (250 mi) wide and reaches 1,000 m (3,300 ft) below sea level. It is separated from the Diamantina Fracture Zone on its southern side by a 3,000 m (9,800 ft) escarpment, while on the northern side the ridge slopes gently towards the abyssal Wharton Basin. The sediment cover on the ridge reaches 800 m (2,600 ft) and the Moho is found at about 20 km (12 mi). It is separated from the Naturaliste Plateau by the Dirck Hartog Ridge.

The Kerguelen LIP covered 2.3×10^6 km2 (0.89×10^6 sq mi) making it the second largest LIP on Earth (after the Ontong Java Plateau in the Pacific). Both these enormous LIPs reaches 2–4 km (1.2–2.5 mi) above the surrounding ocean floor and have a crustal thickness of 20–40 km (12–25 mi) (compared to oceanic crust typically around 7 km (4.3 mi) thick.)

The Broken Ridge and Kerguelen Plateau are now separated by 1,800 km (1,100 mi). When they broke-up, the southern flank of Broken Ridge was uplifted some 2,000 m (6,600 ft) and reached above sea level.The Kerguelen LIP has a long and complicated history, however, and is probably the least "typical" of all LIPs.

Rocks from both the Broken Ridge and the Kerguelen Plateau contain a continental component or "fingerprint". In the Early Cretaceous, the Kerguelen hotspot was split into several diapirs of various sizes, composition, and ascent rates. These separate diapirs created the Bunbury Basalt, the Southern Kerguelen Plateau, the Rajmahal Traps/Indian lamprophyres, Antarctic lamprophyres, and the Central Kerguelen Plateau/Broken Ridge. In the late Cretaceous, activity in the mantle slowed and the Kerguelen hotspot was reduced to a single plume which created the Ninety East Ridge.

120-95 Ma when the Southern and Central Kerguelen Plateau formed together with the Broken Ridge, the Kerguelen hotspot produced 1 km3 (0.24 cu mi)/year, but 95-25 Ma the output decreased to 0.1 km3 (0.024 cu mi).

Central Indian Ridge

The Central Indian Ridge (CIR) is a north-south-trending mid-ocean ridge in the western Indian Ocean.

Coryphaenoides woodmasoni

Coryphaenoides woodmasoni is a fish species belonging to the family Macrouridae and the order Gadiformes. The species was described by Alfred William Alcock in 1873 and named in honour of James Wood-Mason. There are no subspecies listed in the Catalogue of Life.

Diamantina Deep

The Diamantina Deep is located in the Diamantina Trench southwest of Perth, Western Australia. The Diamantina Trench is in the eastern part of the larger Diamantina Fracture Zone, which stretches 1,900 kilometres (1,200 mi) from the Ninety East Ridge to the Naturaliste Plateau, off the lower part of Southwest Australia. It is one of the deepest points (surpassed by the Sunda Trench) in the Indian Ocean at 7,079 m (23,225 ft). It is located about 1,125 kilometres (699 mi) west-southwest of Perth at 35°S 104°E / -35; 104.

A survey in 1961 by the Australian oceanographic survey ship HMAS Diamantina (K377) confirmed the bathymetry and conducted a scientific survey. The trench (and the Fracture Zone) was named after her.

Diamantina Fracture Zone

The Diamantina Fracture Zone is an area of the south-eastern Indian Ocean seafloor. It has 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 to the south west of the Naturaliste Plateau.

Eastern Hemisphere

The Eastern Hemisphere is a geographical term for the half of Earth which is east of the prime meridian (which crosses Greenwich, London, UK) and west of the antimeridian (which crosses the Pacific Ocean and relatively little land from pole to pole). It is also used to refer to Afro-Eurasia (Africa and Eurasia) and Australia, in contrast with the Western Hemisphere, which includes mainly North and South America. The Eastern Hemisphere may also be called the "Oriental Hemisphere". In addition, it may be used in a cultural or geopolitical sense as a synonym for the "Old World".

Eighty Five East Ridge

The Eighty Five East Ridge or 85°E Ridge is a near-linear, aseismic, age-progressive ridge in the northeastern Indian Ocean. It is named for its near-parallel strike along the 85th meridian. It is one of two major aseismic ridges in the Bay of Bengal, the other being the Ninety East Ridge. The feature extends from the Mahanadi Basin in the north, off the northeastern coast of India, shifts westwards by about 250 km around 5°N, southeast of Sri Lanka and continues south to the Afanasy Nikitin Seamount in the Central Indian Basin.No wells have been drilled on the ridge. Samples of the ridge from the Afanasy Nikitin Seamount were ultramafic dunite. Seismic studies have shown that the morphology of the ridge including its depth of occurrence varies along the ridge track and that in general the ridge has been buried beneath sediments deposited since the Oligocene. The ridge is associated with complicated gravity and magnetic signatures. The northern part of the ridge is buried beneath thick sediments of the Bengal Fan and shows a negative gravity anomaly. Ridge structures in the south occasionally rise above the sea floor and are associated with a positive gravity anomaly. The magnetic signatures associated with the ridge are complex, with alternate stripes of strong positive and negative anomalies. Magnetic modelling of the ridge suggests that it was emplaced over a period of rapid geomagnetic reversals whereas the underlying oceanic crust was formed in a normal magnetic field of the Cretaceous normal superchron or "Cretaceous quiet period". The correlation of the magnetization pattern of the ridge and the geomagnetic polarity timescale suggests that the volcanism that created the ridge started ~80 Ma (magnetic chron time 33r) in the Mahanadi Basin and the process continued southwards, ending at ~55 Ma near the Afanasy Nikitin Seamount. There are several proposals to explain the origin and development of the ridge, one of which is that the volcanism was caused by a short-lived hotspot.


Gondwana ( ) or Gondwanaland was a supercontinent that existed from the Neoproterozoic (about 550 million years ago) until the Jurassic (about 180 million years ago).

It was formed by the accretion of several cratons. Eventually, Gondwana became the largest piece of continental crust of the Paleozoic Era, covering an area of about 100,000,000 km2 (39,000,000 sq mi). During the Carboniferous Period, it merged with Euramerica to form a larger supercontinent called Pangaea. Gondwana (and Pangaea) gradually broke up during the Mesozoic Era. The remnants of Gondwana make up about two thirds of today's continental area, including South America, Africa, Antarctica, Australia, Indian Subcontinent and Arabia.

The formation of Gondwana began c. 800 to 650 Ma with the East African Orogeny, the collision of India and Madagascar with East Africa, and was completed c. 600 to 530 Ma with the overlapping Brasiliano and Kuunga orogenies, the collision of South America with Africa and the addition of Australia and Antarctica, respectively.

Hotspot (geology)

In geology, the places known as hotspots or hot spots are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the surrounding mantle. Their position on the Earth's surface is independent of tectonic plate boundaries. There are two hypotheses that attempt to explain their origins. One suggests that hotspots are due to mantle plumes that rise as thermal diapirs from the core–mantle boundary. The other hypothesis is that lithospheric extension permits the passive rising of melt from shallow depths. This hypothesis considers the term "hotspot" to be a misnomer, asserting that the mantle source beneath them is, in fact, not anomalously hot at all. Well-known examples include the Hawaii, Iceland and Yellowstone hotspots.

Indian Ocean

The Indian Ocean is the third-largest of the world's oceanic divisions, covering 70,560,000 km2 (27,240,000 sq mi) (19.8% of the water on the Earth's surface). It is bounded by Asia on the north, on the west by Africa, on the east by Australia, and on the south by the Southern Ocean or, depending on definition, by Antarctica.

Kelly Jemison

Kelly Jemison is an American academic geologist specializing in Antarctic diatoms. She studied at Florida State University. She has participated in the ANDRILL (ANtarctic geological DRILLing) project. In 2011, she was awarded the Antarctica Service Medal.

Kerguelen Plateau

The Kerguelen Plateau ( , ) is an oceanic plateau and a large igneous province (LIP) located on the Antarctic Plate, in the southern Indian Ocean. It is also a microcontinent and submerged continent. It is about 3,000 km (1,900 mi) to the southwest of Australia and is nearly three times the size of Japan. The plateau extends for more than 2,200 km (1,400 mi) in a northwest–southeast direction and lies in deep water.

The plateau was produced by the Kerguelen hotspot, starting with or following the breakup of Gondwana about 130 million years ago. A small portion of the plateau breaks sea level, forming the Kerguelen Islands (a French territory) plus the Heard and McDonald Islands (an Australian territory). Intermittent volcanism continues on the Heard and McDonald Islands.

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, Heard Island, the McDonald Islands, and the Ninety East Ridge.

Macrosmia phalacra

Macrosmia phalacra is a species of rattail known from the Ninety East Ridge in the Indian Ocean. This fish is found at depths of from 1,650 to 1,660 metres (5,410 to 5,450 ft).

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 6,000 km (3,700 mi) between the Rodrigues Triple Junction (25°S 70°E) in the Indian Ocean and the Macquarie Triple Junction (63°S 165°E) in the Pacific Ocean, the SEIR forms the plate boundary between the Australian and Antarctic plates since the Oligocene (anomaly 13).The SEIR is the spreading centre closest to the Kerguelen and Amsterdam–Saint-Paul hotspot.

The SEIR has an intermediate full spreading rate of 65 mm/yr, and, because Antarctica is virtually stationary, this results in a northward ridge migration of half that rate.

Spreading rates along the SEIR varies from 69 mm/yr near 88°E to 75 mm/yr near 120°E.

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 lies east of the Ninety East Ridge and west of Western Australia.

It is of interest in relation to Indian Ocean floor movement and adjacent fracture zones and the relationship between the Indian and Australian plates and is one of a number of features of the Indian Ocean that has been studied extensively. However, its floor has not been charted since the 1960s and is not well known.


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