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 div ...
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 ...
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 ...
. 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 bet ...
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 t ...
(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.
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, an ...
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 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 surface of a rocky planet or moon. More than 90 ...
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
A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that.
Mantle may refer to:
*Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear
**Mantle (vesture), an Eastern Orthodox ve ...
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 Cretace ...
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 configu ...
. 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, 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 reac ...
s erupted at the initiation of volcanism at the
Kerguelen hotspot 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 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.
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 that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pala ...
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 P ...
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 ''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 m ...
, ''
Gunnera,
Gillbeea,'' and possibly
Callitrichaceae.
See also
*
Eighty Five East Ridge
References
Further reading
*
*
*
*
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
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, ...
, 2016. (
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 ...
).
{{Coord, 3, S, 90, E, type:mountain, display=title
Plate tectonics
Underwater ridges of the Indian Ocean
Seamount chains