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Lamont seamount chain is a chain of submarine mountains in the eastern Pacific Ocean which are named "Sasha", "MIB", "MOK", "DTD" and "NEW". They are located close to the East Pacific Rise and reach a minimum depth of 1,629 metres (5,344 ft).
  Lamont seamount chain Lamont seamount chain (Pacific Ocean) | |
| Location | |
|---|---|
| Coordinates | 10°00′N 104°30′W[1] |
These seamounts are submarine volcanoes of Pleistocene to Holocene age that are usually capped off by summit calderas and craters. They have erupted lava flows of tholeiitic composition; the last eruption may have occurred less than 8,000 years ago.
The Lamont seamount chain is a group of five seamounts in the Pacific Ocean;[2] from southeast to northwest they are known as "Sasha",[3] "MIB", "MOK", "DTD" and "NEW"[1] and there is an additional unnamed seamount southwest from "NEW".[4] The seamounts were discovered in 1983 and named later.[5] Brisingid echinoderms have been observed on the seamounts.[6]
The seamounts reach heights of 1–1.4 kilometres (0.62–0.87 mi) and have summit craters/calderas,[2] that on "DTD", "MOK" and "NEW" form nested, complex calderas.[7] Horseshoe-shaped volcanic ridges accompany the calderas and lava effusion appears to have preferentially occurred on the margins of the calderas, along with mass wasting.[8] The Lamont seamount chain forms a 50 kilometres (31 mi) long chain and the individual seamounts surrounded by lava cones 140–100 metres (460–330 ft) high.[2] The shallowest part of the chain is on one of the western seamounts and lies at 1,629 metres (5,344 ft) depth[9] while Sasha reaches to 1,890 metres (6,200 ft), "MIB" to 1,630 metres (5,350 ft) depth, "MOK" to 1,640 metres (5,380 ft) and "NEW" to 1,670 metres (5,480 ft);[10] in general the seamounts shallow westward and their outline changes from conical to more elongated.[5] East of the Lamont seamounts is a nascent volcano.[11]
While the Clipperton Fracture Zone is located just north, the East Pacific Rise lies east of the seamounts;[12] Sasha Seamount lies just 8 kilometres (5.0 mi) from the Rise.[3] It appears to contain a magma chamber at that latitude and seems to have produced voluminous volcanism that generated a topographic elevation that is now associated with the Lamont seamounts.[2] The volcanism is quite young and accompanied by hydrothermal activity; seafloor spreading here proceeds at a rate of 11 centimetres per year (4.3 in/year)[3] and a volcanic eruption in 2003–2006.[13]
The seamounts have produced hyaloclastites,[14] sheet- and lobe-like lava flows as well as pillow lavas[12] and talus. The total volume of each seamount ranges between 140–20 cubic kilometres (33.6–4.8 cu mi)[2] and the volume increases away from the East Pacific Rise, which along with other patterns indicates a progressive development of the seamount as they move away from the East Pacific Rise.[3] They appear to have been formed under the influence both of fracture zones and of a melt anomaly that also formed a 200–400 metres (660–1,310 ft) topographic anomaly west of the Lamont seamount chain[15] along with elongated volcanic structures;[14] a bathymetric swell is also noted around the seamounts[16] as well as a 150 metres (490 ft) deep depression that may be an isostatic moat.[17] Two magma chambers have been identified between Sasha and the EPR.[18] The volcanism may be ultimately the consequence of hotspot activity.[19]
Composition
The Lamont seamount chain has erupted tholeiitic magmas[20] which contain olivine and plagioclase phenocrysts[21] as well as spinel but little clinopyroxene.[20] All samples taken from the seamounts contain sulfide globules, including cubanite and pyrrhotite.[22] In some samples, carbonates, celadonite, iron hydroxides, manganese crusts and mica are found.[23] They are relatively primitive magmas that were not stored over long timespans in the crust[24] and undergo simple fractional crystallization processes;[25] their sources appear to be distinct from the magma sources of the East Pacific Rise.[26]
The Lamont seamounts probably formed within the last 400,000 years.[15] While the ages of the seamounts are not known, they are by necessity younger than the underlying 740,000 - 100,000 years old seafloor; the lack of sediment cover and thick ferromanganese deposits as well as the youthful appearance of lava flows also speaks for a young age[2] although there is no evidence of hydrothermal activity, past or present.[16] The reflectance of the seafloorm has been used to infer increasing ages from 33,000 years at Sasha to 230,000 at "MOK".[10] Radiocarbon dating of foraminifera encased in lava flows of "NEW" seamount has yielded an age of 18,540 ± 216 radiocarbon years ago, indicating recent activity;[8] additionally, a sample from Sasha seamount may be less than 8,000 years old on the basis of a radium-thorium isotope disequilibrium.[27]
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