The Eighty Five East Ridge or 85°E Ridge is a near-linear, aseismic, age-progressive ridge in the northeastern

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 t ...

. 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 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 betwee ...

, the other being the

Ninety East Ridge The Ninety East Ridge (also rendered as Ninetyeast Ridge, 90E Ridge or 90°E Ridge) is a mid-ocean ridge on the Indian Ocean floor named for its near-parallel strike along the 90th meridian at the center of the Eastern Hemisphere. It is approxima ...

. The feature extends from the Mahanadi Basin in the north, off the northeastern coast of

India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...

, 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 Afanasy Nikitin (russian: Афана́сий Ники́тин; died 1472) was a Russian merchant from Tver and one of the first Europeans (after Niccolò de' Conti) to travel to and document his visit to India. He described his trip in a narrat ...

Seamount were

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...

dunite Dunite (), also known as olivinite (not to be confused with the mineral olivenite), is an intrusive igneous rock of ultramafic composition and with phaneritic (coarse-grained) texture. The mineral assemblage is greater than 90% olivine, with ...

. 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 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 the ...

and shows a negative

gravity anomaly The gravity anomaly at a location on the Earth's surface is the difference between the observed value of gravity and the value predicted by a theoretical model. If the Earth were an ideal oblate spheroid of uniform density, then the gravity meas ...

. 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 reversal A geomagnetic reversal is a change in a planet's magnetic field such that the positions of magnetic north and magnetic south are interchanged (not to be confused with geographic north and geographic south). The Earth's field has alternated ...

s whereas the underlying

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic ...

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.

References

Further reading