Alexa Bank is a

seamount A seamount is a large geologic landform that rises from the ocean floor that does not reach to the water's surface (sea level), and thus is not an island, islet or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise a ...

Samoa Samoa, officially the Independent State of Samoa; sm, Sāmoa, and until 1997 known as Western Samoa, is a Polynesian island country consisting of two main islands ( Savai'i and Upolu); two smaller, inhabited islands ( Manono and Apolima); ...

, northwest of

Rotuma Rotuma is a Fijian dependency, consisting of Rotuma Island and nearby islets. The island group is home to a large and unique Polynesian indigenous ethnic group which constitutes a recognisable minority within the population of Fiji, known as " ...

. The seamount reaches a depth of below sea level and has the appearance of an

atoll An atoll () is a ring-shaped island, including a coral rim that encircles a lagoon partially or completely. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical oceans and seas where corals can gro ...

with a flat top and steep slopes. Some active

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and se ...

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.

Geology and geomorphology

Local

Alexa Bank rises to depths of , has an eastward elongated flat top and is long and wide, widening towards the west; such dimensions resemble these of Savaii and

Upolu Upolu is an island in Samoa, formed by a massive basaltic shield volcano which rises from the seafloor of the western Pacific Ocean. The island is long and in area, making it the second largest of the Samoan Islands by area. With approxi ...

in Samoa. The flat top has some features of an

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 . The northern and southern flanks are formed by steep scarps, with the eastern slope featuring a terrace at depth and the western slope extending as a long volcanic ridge. The upper of the Bank are formed by

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...

sediments, while the eastern slope terrace may be part of the original volcanic edifice that was carried to this depth by

thermal subsidence In geology and geophysics, thermal subsidence is a mechanism of subsidence in which conductive cooling of the mantle thickens the lithosphere and causes it to decrease in elevation. This is because of thermal contraction: as mantle material cools a ...

of the oceanic crust;

seismic Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

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 Bikini Atoll ( or ; Marshallese: , , meaning "coconut place"), sometimes known as Eschscholtz Atoll between the 1800s and 1946 is a coral reef in the Marshall Islands consisting of 23 islands surrounding a central lagoon. After the Seco ...

and

Eniwetok Enewetak Atoll (; also spelled Eniwetok Atoll or sometimes Eniewetok; mh, Ānewetak, , or , ; known to the Japanese as Brown Atoll or Brown Island; ja, ブラウン環礁) is a large coral atoll of 40 islands in the Pacific Ocean and with i ...

Regional

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 deep Alexa Trough lies west and south of Alexa Bank and is part of the

Vitiaz Trench Vityaz may refer to: * Bogatyr, a folk or epic hero in Slavic culture Aircraft and spacecraft *Vityaz, the call sign of spacecraft Soyuz TM-14 * Russky Vityaz, an aircraft built by Igor Sikorsky in 1913 Military * , a ''Bogatyr'' class screw cor ...

, connecting it with the Tonga Trench. The bank itself is part of the "Melanesian Border Plateau", a group of

s and islands along the

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 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 The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and I ...

Composition

Samples dredged from the Alexa Bank include

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 ...

ic rocks and have

alkali basalt Alkali basalt or alkali olivine basalt is a dark-colored, porphyritic volcanic rock usually found in oceanic and continental areas associated with volcanic activity, such as oceanic islands, continental rifts and volcanic fields. Alkali basalt ...

tholeiitic The tholeiitic magma series is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma ...

composition.

Isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...

data display affinities to these from other Samoan volcanoes. Other rocks dredged from the seamount are

argillite :''"Argillite" may also refer to Argillite, Kentucky.'' Argillite () is a fine-grained sedimentary rock composed predominantly of indurated clay particles. Argillaceous rocks are basically lithified muds and oozes. They contain variable amounts ...

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...

and

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, ...

but they may not have originated on Alexa Bank proper.

Geologic history

Rocks from Alexa Bank have been dated to be 37–36.9 million years old and may be a product of

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', ...

volcanism. More recent age estimates are 24 million years ago, while fossil data have been inferred to justify a

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

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

during the

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

and then drowned. It is possible that unusually rapid geological

subsidence Subsidence is a general term for downward vertical movement of the Earth's surface, which can be caused by both natural processes and human activities. Subsidence involves little or no horizontal movement, which distinguishes it from slope mov ...

of Alexa Bank outpaced the ability of its

reef A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic processes—deposition of sand, wave erosion planing down rock ...

s to keep up. Originally, the flat surface of Alexa was explained by wave erosion during sea level lowstands.

Biology

Dead

s and

foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...

occur on Alexa Bank. Some corals are still alive and form pinnacles on the surface of the bank. Algal

rhodolith Rhodoliths (from Greek for ''red rocks'') are colorful, unattached calcareous nodules, composed of crustose, benthic marine red algae that resemble coral. Rhodolith beds create biogenic habitat for diverse benthic communities. The rhodolithic gr ...

s and

stromatolith Stromatolites () or stromatoliths () are layered sedimentary formations (microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). Th ...

s have been found on Alexa as well. The Japanese snake blenny has been found at Alexa Bank.

Radiolarian The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ectoplasm. The elab ...

s in

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million year ...

sediments south of Alexa Bank probably originate there.

References

Sources

* * * {{refend Eocene volcanoes Seamounts of the Pacific Ocean Volcanoes of the Pacific Ocean Undersea banks of the Pacific Ocean Western Samoan Volcanic Province