Alexa Bank is a seamount in Samoa, northwest of Rotuma. The seamount reaches a depth of 18–21 metres (59–69 ft) below sea level and has the appearance of an atoll with a flat top and steep slopes. Some active coral growth takes place at its top, but if it ever was an active atoll it has now drowned. It was probably formed by the Samoa hotspot 24 million years ago, although older volcanism about 40 million years ago has also been identified.
Alexa Bank rises to depths of 18–21 metres (59–69 ft), has an eastward elongated flat top and is 150 kilometres (93 mi) long and 20–65 kilometres (12–40 mi) wide, widening towards the west; such dimensions resemble these of Savaii and Upolu in Samoa. The flat top has some features of an atoll including a raised rim, and it may indeed once have been an atoll. The rim is surrounded by a narrow terrace and in turn encloses a much wider terrace, that surrounds the flat top at an average depth of 42 metres (138 ft). The northern and southern flanks are formed by steep scarps, with the eastern slope featuring a terrace at 750 metres (2,460 ft) depth and the western slope extending as a 150 kilometres (93 mi) long volcanic ridge.
The upper 1,600–800 metres (5,200–2,600 ft) of the Bank are formed by carbonate sediments, while the eastern slope terrace may be part of the original volcanic edifice that was carried to this depth by thermal subsidence of the oceanic crust; seismic data may support the existence of a volcano within the carbonate sediments that form the bulk of Alexa Bank as well. Such thicknesses of the sediments are comparable to these of Bikini Atoll and Eniwetok.
The seafloor around Alexa Bank has a rough relief, with additional seamounts known as the Louisa Bank, Morton Bank, Penguin Bank and Turpie Bank nearby; the last two are often considered to be part of Alexa Bank. The 4,000 metres (13,000 ft) deep Alexa Trough lies west and south of Alexa Bank and is part of the Vitiaz Trench, connecting it with the Tonga Trench. The bank itself is part of the "Melanesian Border Plateau", a group of seamounts and islands along the Vitiaz Trench that are mostly of volcanic origin.
Alexa Bank appears to be the westernmost extension of the Samoa hotspot trail, which has covered a length of 1,700 kilometres (1,100 mi) at least. The hotspot trail is in part coincident with a large group of islands and seamounts which were probably formed by the same hotspot but also featured substantial postshield volcanism, probably owing to tectonic phenomena triggered by the Tonga Trench close by. The seafloor is part of the Pacific Plate.
Samples dredged from the Alexa Bank include basaltic rocks and have alkali basalt to tholeiitic composition. Isotope data display affinities to these from other Samoan volcanoes. Other rocks dredged from the seamount are argillite, breccia, shale and siltstone but they may not have originated on Alexa Bank proper.
Rocks from Alexa Bank have been dated to be 37–36.9 million years old and may be a product of Eocene volcanism. More recent age estimates are 24 million years ago, while fossil data have been inferred to justify a Cretaceous age.
Alexa Bank has been considered to be a product of the Samoa hotspot, but its age may be too high to be a product of this hotspot. The newer age measurements however are consistent with the association with the Samoa hotspot. Some tectonic activity may have continued until later times, leading to the development of segmentation and tilting.
The bank was probably an atoll during the Pleistocene and then drowned. It is possible that unusually rapid geological subsidence of Alexa Bank outpaced the ability of its reefs to keep up. Originally, the flat surface of Alexa was explained by wave erosion during sea level lowstands.
Dead corals and foraminifera occur on Alexa Bank. Some corals are still alive and form pinnacles on the surface of the bank. Algal rhodoliths and stromatoliths have been found on Alexa as well. The Japanese snake blenny has been found at Alexa Bank. Radiolarians in Eocene-Quaternary sediments south of Alexa Bank probably originate there.
The Pacific Ocean evolved in the Mesozoic from the Panthalassic Ocean, which had formed when Rodinia rifted apart around 750 Ma. The first ocean floor which is part of the current Pacific Plate began 160 Ma to the west of the central Pacific and subsequently developed into the largest oceanic plate on Earth.The tectonic plates continue to move today. The slowest spreading ridge is the Gakkel Ridge on the Arctic Ocean floor, which spreads at less than 2.5 cm/year (1 in/year), while the fastest, the East Pacific Rise near Easter Island, has a spreading rate of over 15 cm/year (6 in/year).Outline of oceanography
The following outline is provided as an overview of and introduction to Oceanography.Samoa hotspot
The Samoa hotspot is a volcanic hotspot located in the south Pacific Ocean.
The hotspot model describes a hot upwelling plume of magma through the Earth's crust as an explanation of how volcanic islands are formed. The hotspot idea came from J. Tuzo Wilson in 1963 based on the Hawaii volcanic island chain.
In theory, the Samoa hotspot is based on the Pacific Tectonic Plate traveling over a fixed hotspot located deep underneath the Samoan Islands.
The Samoa hotspot includes the Samoan Islands (American Samoa and Samoa), and extends to the islands of Uvea or Wallis Island (Wallis and Futuna) and Niulakita (Tuvalu), as well as the submerged Pasco banks and Alexa Bank.As the Pacific Plate moves slowly over the hotspot, thermal activity builds up and is released in magma plume spewing through the Earth's crust, forming each island in a chain. The Samoa islands generally lie in a straight line, east to west, in the same direction of the tectonic plate 'drifting' over the hotspot.
A characteristic of a “classic” hotspot, like the Hawaii hotspot, results in islands located further from the hotspot being progressively older with newer and younger islands closest to the fixed hotspot, like the Loihi Seamount, the only submarine volcano which has been studied in detail by scientists. The scientific research from Loihi has resulted in a 'Hawaii' model for hotspots primarily limited to the information gathered from the Hawaii islands.However, the Samoa hotspot is currently an enigma for scientists. In the Samoa Islands, the easternmost island of Ta'u and the westernmost island of Savai'i have both erupted in the past 150 years. The most recent eruption on Sava'i occurred with Mount Matavanu (1905–1911) and on Ta'u in 1866.Vailulu'u
Vailulu'u is a volcanic seamount discovered in 1975. It rises from the sea floor to a depth of 593 m and is located between Ta'u and Rose islands at the eastern end of the Samoa hotspot chain. The basaltic seamount is considered to mark the current location of the Samoa hotspot. The summit of Vailulu'u contains a 2 km wide, 400 m deep oval-shaped caldera. Two principal rift zones extend east and west from the summit, parallel to the trend of the Samoan hotspot. A third less prominent rift extends southeast of the summit.
Eruptions at Vailulu'u were recorded in 1973. An earthquake swarm in 1995 may have been related to an eruption from the seamount. Turbid water above the summit shows evidence of ongoing hydrothermal plume activity. Vailulu'u may breach the surface of the ocean and officially become an island if a high rate of eruptions continue.