Alexa Bank is a

seamount A seamount is a large submarine landform that rises from the ocean floor without reaching the water surface (sea level), and thus is not an island, islet, or cliff-rock. Seamounts are typically formed from extinct volcanoes that rise abruptly a ...

Samoa Samoa, officially the Independent State of Samoa and known until 1997 as Western Samoa, is an island country in Polynesia, part of Oceania, in the South Pacific Ocean. It consists of two main islands (Savai'i and Upolu), two smaller, inhabited ...

, northwest of

Rotuma Rotuma () is a self-governing heptarchy, generally designated a Local government in Fiji, dependency of Fiji. Rotuma commonly refers to the Rotuma Island, the only permanently inhabited and by far the largest of all the islands in the Rotuma Gro ...

. The seamount reaches a depth of below

sea level Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...

and has the appearance of an

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

with a flat top and steep slopes. Some active

coral Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important 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 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 ...

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 approximate ...

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, (), 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 group ...

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 expansion: as mantle material cools ...

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 (or generally, quakes) and the generation and propagation of elastic ...

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 language, Marshallese: , , ), known as Eschscholtz Atoll between the 19th century and 1946, is a coral reef in the Marshall Islands consisting of 23 islands surrounding a central lagoon. The atoll is at the no ...

and

Eniwetok Enewetak Atoll (; also spelled Eniwetok Atoll or sometimes Eniewetok; , , or , ; known to the Japanese as Brown Atoll or Brown Island; ) is a large coral atoll of 40 islands in the Pacific Ocean and with its 296 people (as of 2021) forms a legi ...

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, connecting it with the

Tonga Trench The Tonga Trench is an oceanic trench located in the southwestern Pacific Ocean. It is the deepest trench in the Southern hemisphere and the second deepest on Earth after the Mariana Trench. The fastest plate-tectonic velocity on Earth is occurri ...

. The bank itself is part of the "Melanesian Border Plateau", a group of

s and islands along the Vitiaz Trench that are mostly of volcanic origin. Alexa Bank appears to be the westernmost extension of the

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

close by. The seafloor is part of the Pacific Plate.

Composition

Samples dredged from the Alexa Bank include

basalt Basalt (; ) is an aphanite, 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 planetary surface, surface of a terrestrial ...

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 i ...

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 ...

composition.

Isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

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

argillite Argillite () is a fine-grained sedimentary rock composed predominantly of Friability, indurated clay particles. Argillaceous rocks are basically lithified muds and Pelagic sediment, oozes. They contain variable amounts of silt-sized particles. T ...

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

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. Although its permeabil ...

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

volcanism. More recent age estimates are 24 million years ago, while

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

data have been inferred to justify a

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

age. Alexa Bank has been considered to be a product of the

, 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 ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

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 component, abiotic (non-living) processes such as deposition (geol ...

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 unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...

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

Radiolarian The Radiolaria, also called Radiozoa, are unicellular eukaryotes 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 ecto ...

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), as well as the current and most recent of the twelve periods of the ...

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