Lamont seamount chain

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]

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]