The Agulhas Passage is an

abyssal The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. The word ''abyss'' comes from the Greek word (), meaning "bottomless". At depths of , this zone remains in perpetual darkness. It covers 83% of the total area o ...

channel located south of

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...

between the

Agulhas Bank The Agulhas Bank (, from Portuguese for Cape Agulhas, ''Cabo das Agulhas'', "Cape of Needles") is a broad, shallow part of the southern African continental shelf which extends up to south of Cape Agulhas before falling steeply to the abyssal pla ...

and

Agulhas Plateau The Agulhas Plateau is an oceanic plateau located in the south-western Indian Ocean about south of South Africa. It is a remainder of a large igneous province (LIP), the Southeast African LIP, that formed (Ma) at or near the triple junction w ...

. About wide, it connects the Natal Valley and Transkei Basin in the north to the Agulhas Basin in the south and is the only near-shore connection between the south-western

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or approximately 20% of the water area of Earth#Surface, Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia (continent), ...

and

South Atlantic Ocean The Atlantic Ocean is the second largest of the world's five borders of the oceans, oceanic divisions, with an area of about . It covers approximately 17% of Earth#Surface, Earth's surface and about 24% of its water surface area. During the ...

Geology

The formation of the Agulhas Passage was initiated by the Gondwana break-up some (Ma) and the rifting of the Somali and Mozambique Basins, an event associated with the formation of the ocean floor in the

Weddell Sea The Weddell Sea is part of the Southern Ocean and contains the Weddell Gyre. Its land boundaries are defined by the bay formed from the coasts of Coats Land and the Antarctic Peninsula. The easternmost point is Cape Norvegia at Princess Martha C ...

147 Ma. Between the Agulhas Bank and the Agulhas Passage the boundary between the Earth's crust and the mantle (the

Moho Moho may refer to: Birds * ''Moho'' (genus), an extinct genus of birds in the family Mohoidae * The Hawaiian name of the Hawaiian rail, an extinct species * The Māori name of the North Island takahē, an extinct species * A local name for the o ...

) ascend from over which is normal for a

continent-ocean boundary The continent-ocean boundary (COB) or continent-ocean transition (COT) or continent-ocean transition zone (COTZ) is the boundary between continental crust and oceanic crust on a passive margin or the zone of transition between these two crustal type ...

. The crustal thickness under the Agulhas Passage varies from which is equally normal for

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 ultramaf ...

. It is possible, however, that volcanic flows from the

large igneous province A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive ( sills, dikes) and extrusive (lava flows, tephra deposits), arising when magma travels through the crust towards the surface. The format ...

(LIP) added crustal material to the Agulhas Passage (which is 160-120 Ma) during the LIP formation (100-80 Ma) and that the crust under the passage was originally thinner. The sedimentary layers are very thin in the western Agulhas Passage, in places only . In the central parts of the passage, in contrast, they can reach and form sediment drifts. The southern part is affected by thinning and erosion.

Oceanography

The Agulhas Current, the

western boundary current Boundary currents are ocean currents with dynamics determined by the presence of a coastline, and fall into two distinct categories: western boundary currents and eastern boundary currents. Eastern boundary currents Eastern boundary currents are ...

of the Indian Ocean, flows south through the Natal Valley, Transkei Basin, and Agulhas Passage into the south-east Atlantic Ocean where it

retroflect Retroflection is the movement of an ocean current that doubles back on itself. Usage history More commonly used to describe the way the mammalian intestine or uterus might turn back on itself, retroflection was first used in an oceanographic sense ...

s back into the Indian Ocean as the

Agulhas Return Current The Agulhas Return Current (ARC) is an ocean current in the South Indian Ocean. The ARC contributes to the water exchange between oceans by forming a link between the South Atlantic Current and the South Indian Ocean Current. It can reach veloc ...

. In the Indian Ocean, the Agulhas Current is dissipated into a

gyre In oceanography, a gyre () is any large system of ocean surface currents moving in a circular fashion driven by wind movements. Gyres are caused by the Coriolis effect; planetary vorticity, horizontal friction and vertical friction determine the ...

which finally brings parts of the water mass back to the Natal Valley. Since the development of the Antarctic Circumpolar Current in the

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

Antarctic Bottom Water The Antarctic bottom water (AABW) is a type of water mass in the Southern Ocean surrounding Antarctica with temperatures ranging from −0.8 to 2 °C (35 °F) and absolute salinities from 34.6 to 35.0 g/kg. As the densest water mass of ...

(AABW) flows both north through the Agulhas Passage and south of the Agulhas Plateau into the Transkei Basin and then into the Mozambique Basin.

North Atlantic Deep Water North Atlantic Deep Water (NADW) is a deep water mass formed in the North Atlantic Ocean. Thermohaline circulation (properly described as meridional overturning circulation) of the world's oceans involves the flow of warm surface waters from the ...

(NADW) flows north through the Agulhas Passage and then west of the Mozambique Ridge and through the Transkei Basin east of the ridge and then into the Indian Ocean. South of South Africa NADW is found close to a depth of where it transports 2.9

Sverdrup In oceanography, the sverdrup (symbol: Sv) is a non- SI metric unit of volumetric flow rate, with equal to . It is equivalent to the SI derived unit cubic hectometer per second (symbol: hm3/s or hm3⋅s−1): is equal to . It is used almost ...

s (million m³/s) at a speed of , resulting in an Agulhas Passage depleted of sediments at this level.

Antarctic Intermediate Water Antarctic Intermediate Water (AAIW) is a cold, relatively low salinity water mass found mostly at intermediate depths in the Southern Ocean. The AAIW is formed at the ocean surface in the Antarctic Convergence zone or more commonly called the An ...

flows from Antarctica into the Indian Ocean from where it reaches Africa from the east. It flows south through the Agulhas Passage and then follows the Agulhas Current and retroflects back into the Indian Ocean.

References

Notes

Sources

* * * * Landforms of the Indian Ocean {{Coord, 36, S, 25, E, display=title