Vema Seamount is a seamount in the South Atlantic Ocean. Discovered in 1959 by a ship with the same name, it lies 1,600 kilometres (1,000 mi) from Tristan da Cunha and 1,000 kilometres (620 mi) northwest of Cape Town. The seamount has a flat top at a mean depth of 73 metres (40 fathoms) which was eroded into the seamount at a time when sea levels were lower; the shallowest point lies at 26 metres (14 fathoms) depth. The seamount was formed between 15 and 11 million years ago, possibly by a hotspot. The seamount rises high enough that its summit is at shallow depth, allowing sunlight to reach it and thus permitting the growth of kelp and algae. A number of sea animals and fish are encountered on the seamount; active fisheries existed at Vema Seamount and caused the disappearance of some animal species.
History Vema Seamount was discovered by the research ship RV Vema of the Lamont–Doherty Earth Observatory in 1959. Vema was one of the first seamounts to be the subject of scientific study, and the first seamount investigated by scuba divers without special equipment. It lies in international waters and its summit is so shallow that it is a navigation hazard to ships.
Geomorphology and geography Vema Seamount lies in the South Atlantic Ocean, 1,600 kilometres (1,000 mi) from Tristan da Cunha. The cities of Cape Town and Lüderitz lie east-southeast and northeast of Vema, respectively; the distance from Cape Town is about 1,000 kilometres (620 mi). An unrelated seamount with the same name lies in the North Atlantic, close to the Mid-Atlantic Ridge. The seamount has a conical shape with a flat top; the shallowest point rises to an elevation of 26 metres (14 fathoms) below sea level – later determined to be 21.5 metres (71 ft) deep – and is called Collins Point. At least one source gives a minimum depth of 11 metres (36 ft) for the seamount, while bathymetric surveys in 2015 have found a minimum depth of 21.5 metres (71 ft). The flat top is a summit plateau with a width of 8.0 kilometres (5 mi) and according to a 2019 publication 11 by 8.5 kilometres (6.8 mi × 5.3 mi) at a mean depth of 73 metres (40 fathoms) and has been named Emerson Plateau; it has a vaguely triangular shape pointing west, and Collins Point lies close to the western margin of the Emerson Plateau. Other points on the Plateau also rise to depths of less than 55 metres (30 fathoms). The summit plateau mostly consists of hard rock, like the upper slopes, with rocky outcrops separated by sandy plains. The plateau appears to be a wave-cut platform of Pleistocene age, when sea levels were lower, and is swept by strong ocean currents. The seamount rises from a depth of 4,600 metres (2,500 fathoms), where it occupies a breadth of 56 kilometres (35 mi) and forms an isolated conical feature. The seafloor from which Vema rises is part of the abyssal plain of the Cape Basin. From there, the slopes of Vema first rise steeply and feature subsidiary summits; above 130 metres (70 fathoms) depth, the slopes flatten.
Geology Volcanic rocks such as tuff as well as calcareous aggregates are found on the plateau. Collins Point is composed of phonolite, which contains aegirine, alkali feldspar, augite, and nepheline. Olivine-containing basalt has also been found. A minimum age of 11.0 ± 0.3 million years has been obtained from samples taken at Collins Point by potassium-argon dating, with another age estimate being 15 million years. Older ages have been obtained deeper on the seamount; a sample from 3,000 metres (9,800 ft) depth gave an age of 18 million years. Light-coloured rocks on the summit platform may constitute a former carbonate platform. Vema is an intraplate volcano. It is considered to be the present-day location of a hotspot, the Vema hotspot, although the hotspot itself may have moved farther south or west (by about 200 kilometres (120 mi)) since it created the Vema Seamount and may not be active anymore. Earlier volcanism caused by the Vema hotspot may have manifested itself in southern Namibia in the form of alkaline volcanics that define a HIMU suite. Seismic tomography has shown what may be a mantle plume underneath Vema, while another theory considers the Vema hotspot is a consequence of the Tristan hotspot shedding a secondary diapir. The hotspot origin of the Vema Seamount is not universally agreed upon.
Water conditions Water temperatures at Vema range between 18–21 °C (64–70 °F), and decrease with increasing depth. The cold Benguela Current does not reach the seamount, which is instead influenced by the South Atlantic oceanic gyre. The movement and strength of ocean eddies are altered when they interact with Vema Seamount, with Agulhas eddies often splitting apart at the seamount. During the ice ages, sea level drop may have exposed part of the summit platform.
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