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Crough Seamount

Seamount in the Pacific Ocean, within the exclusive economic zone of Pitcairn

Crough Seamount

25°00′S 121°12′W / 25°S 121.2°W / -25; -121.2^[1]Crough Seamount (named after the geologist Thomas Crough^[2]) is a seamount in the Pacific Ocean, within the exclusive economic zoneofPitcairn.^[3] It rises to a depth of 650 metres (2,130 ft) and is paired with a taller but overall smaller seamount to the east. This seamount has a flat top and probably formed an island in the past. It is about 7-8 million years old, although a large earthquake recorded at its position in 1955 may indicate a recent eruption.

The seamount appears to be part of a long geological lineament with the neighbouring Henderson and Ducie islands, as well as the southern Tuamotus and Line Islands. Such a lineament may have been generated by a hotspot; the nearby Easter hotspot is a candidate hotspot.

Geology and geomorphology[edit]

Regional[edit]

The region lies between and around the islands of Pitcairn and Easter Island.^[4] There, the East Pacific Rise is interrupted by a trapezoid microplate known as the Easter Microplate^[5] about 400 kilometres (250 mi) wide. Seafloor spreading occurs at a rate of about 16 centimetres per year (6.3 in/year).^[4]

There is a topographic swell that connects the two islands and continues eastward towards Sala y Gomez. The origin of this swell and the various volcanoes and seamounts associated with it has been variously explained as either being due to a mantle plume which forms volcanoes that are then carried away through plate motion or by a "hot line" where a number of simultaneously active volcanic centres develop.^[4] This geological lineament may extend all the way to Tonga.^[6]

Crough seamount was probably formed by the Easter hotspot that also generated Easter Island^[7] albeit with the participation of a nearby fracture zone^[8] that modified the trend of the hotspot path.^[9] In this case the Easter Island-Sala y Gomez ridge and the Crough Seamount would be conjugate volcanic ridges paired across the East Pacific Rise.^[10] although it is possible that two separate hotspots were active on the eastern and western side of the East Pacific Rise.^[11][12] Another theory postulates that Crough was formed by its own hotspot, the Crough hotspot.^[13]

Together with Ducie and Henderson Crough forms a 1,300 kilometres (810 mi) long westward trending lineament^[14] with each volcano becoming older the farther west it lies,^[15] and which may be a prolongation of the southern Tuamotus^[16] which were generated by the same hotspot.^[10] Even farther west the hotspot track may include Oeno, Minerve Reef, Marutea, Acton, Rangiroa, the Line Islands and the Mid-Pacific Mountains, although a continuation through the Line Islands is problematic if it is assumed that the Easter hotspot generated this track.^[13][17] A different theory has Crough seamount as its own hotspot, that formed the seamounts and islands together with another hotspot ("Larson"),^[18] and lesser contributions of the Society and Marquesas hotspot.^[19] East of Crough, a series of even younger volcanic ridges continues until the East Pacific Rise^[20] where the hotspot may be located.^[13] The Crough hotspot may be a conjugate of the Easter hotspot,^[21] and sourced from the middle mantle.^[18]

Local[edit]

Crough is an east-west trending seamount^[5] which rises over 2 kilometres (1.2 mi) from the seafloor to a depth of less than 722 metres (2,369 ft)^[22] at 650 metres (2,130 ft).^[23] It has a flat top and the presence of coral sands indicates that Crough once emerged above sea level before subsiding to its present depth,^[22] having formerly hosted corals^[24] and pteropods. Wave erosion that took place when Crough emerged above sea level truncated the seamount, turning it into a flat guyot.^[25] Pillow lavas crop out between 1,400–950 metres (4,590–3,120 ft).^[26] Crough Seamount has a volume of 660 cubic kilometres (160 cu mi), comparable to that of other submarine volcanoes such as Macdonald seamount, Mehetia and Moua Pihaa.^[23]

A second seamount lies nearby and partly overlaps with Crough,^[1] it is named Thomas Seamount^[27] in honour of a geophysicist.^[28] This seamount is even shallower than Crough as it reaches a depth of 600 metres (2,000 ft) but has a smaller volume of 600 cubic kilometres (140 cu mi).^[23]

Composition[edit]

Dredging has yielded both vesicular and porphyritic basalt. Phenocrysts identified include clinopyroxene, olivine and plagioclase. Carbonates and hyaloclastites have also been found, and some samples were covered with manganese crusts^[29] and palagonite.^[30] Hydrothermal iron crusts have also been found.^[26]

Eruption history[edit]

Argon-argon dating has yielded ages of 8.4 to 7.6 million years ago for samples dredged from Crough,^[31] while other geological indicators suggest an age of between 7 and 10 million years ago.^[32] Other estimates of its age are 4^[15]-3 million years.^[33]

In 1955, a strong earthquake was recorded on the northern flank of Crough Seamount;^[34] the characteristics of the earthquake resemble these of volcanic processes and it is thus possible that Crough Seamount is still active. Such activity may constitute a post-shield stage of volcanism.^[33] The earthquake has also been interpreted as a normal fault earthquake^[2] which sometimes occur in young oceanic crust, but the 1955 Crough event was considerably stronger than other earthquakes of this type.^[35]

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