Crough Seamount (named after the

geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althou ...

Thomas Crough) 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 abru ...

in the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

, within the exclusive economic zone of

Pitcairn The Pitcairn Islands (; Pitkern: '), officially the Pitcairn, Henderson, Ducie and Oeno Islands, is a group of four volcanic islands in the southern Pacific Ocean that form the sole British Overseas Territory in the Pacific Ocean. The four isl ...

. It rises to a depth of 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 Henderson may refer to: People *Henderson (surname), description of the surname, and a list of people with the surname *Clan Henderson, a Scottish clan Places Argentina *Henderson, Buenos Aires Australia *Henderson, Western Australia Canada * H ...

and Ducie islands, as well as the southern

Tuamotus The Tuamotu Archipelago or the Tuamotu Islands (french: Îles Tuamotu, officially ) are a French Polynesian chain of just under 80 islands and atolls in the southern Pacific Ocean. They constitute the largest chain of atolls in the world, extendin ...

and

Line Islands The Line Islands, Teraina Islands or Equatorial Islands (in Gilbertese, ''Aono Raina'') are a chain of 11 atolls (with partly or fully enclosed lagoons) and coral islands (with a surrounding reef) in the central Pacific Ocean, south of the Hawa ...

. Such a lineament may have been generated by a hotspot; the nearby

Easter hotspot The Easter hotspot is a volcanic hotspot located in the southeastern Pacific Ocean. The hotspot created the Sala y Gómez Ridge which includes Easter Island, Salas y Gómez Island and the Pukao Seamount which is at the ridge's young western ed ...

is a candidate hotspot.

Geology and geomorphology

Regional

The region lies between and around the islands of

and

Easter Island Easter Island ( rap, Rapa Nui; es, Isla de Pascua) is an island and special territory of Chile in the southeastern Pacific Ocean, at the southeasternmost point of the Polynesian Triangle in Oceania. The island is most famous for its nearl ...

. There, the

East Pacific Rise The East Pacific Rise is a mid-ocean rise (termed an oceanic rise and not a mid-ocean ridge due to its higher rate of spreading that results in less elevation increase and more regular terrain), a divergent tectonic plate boundary located along ...

is interrupted by a trapezoid microplate known as the Easter Microplate about wide.

Seafloor spreading Seafloor spreading or Seafloor spread is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. History of study Earlier theories by Alfred Wegener an ...

occurs at a rate of about . 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 A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hot ...

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. This geological lineament may extend all the way to

Tonga Tonga (, ; ), officially the Kingdom of Tonga ( to, Puleʻanga Fakatuʻi ʻo Tonga), is a Polynesian country and archipelago. The country has 171 islands – of which 45 are inhabited. Its total surface area is about , scattered over in ...

. Crough seamount was probably formed by the

that also generated Easter Island albeit with the participation of a nearby

fracture zone A fracture zone is a linear feature on the ocean floor—often hundreds, even thousands of kilometers long—resulting from the action of offset mid-ocean ridge axis segments. They are a consequence of plate tectonics. Lithospheric plates on eit ...

that modified the trend of the hotspot path. 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. although it is possible that two separate hotspots were active on the eastern and western side of the East Pacific Rise. Another theory postulates that Crough was formed by its own hotspot, the Crough hotspot. Together with Ducie and

Crough forms a long westward trending

lineament ''See also Line (geometry)'' A lineament is a linear feature in a landscape which is an expression of an underlying geological structure such as a fault. Typically a lineament will appear as a fault-aligned valley, a series of fault or fold-aligne ...

with each volcano becoming older the farther west it lies, and which may be a prolongation of the southern

Tuamotu The Tuamotu Archipelago or the Tuamotu Islands (french: Îles Tuamotu, officially ) are a French Polynesian chain of just under 80 islands and atolls in the southern Pacific Ocean. They constitute the largest chain of atolls in the world, extendin ...

s which were generated by the same hotspot. Even farther west the hotspot track may include Oeno, Minerve Reef, Marutea,

Acton Acton may refer to: Places Antarctica * Mount Acton Australia * Acton, Australian Capital Territory, a suburb of Canberra * Acton, Tasmania, a suburb of Burnie * Acton Park, Tasmania, a suburb of Hobart, Tasmania, formerly known as Acton Canada ...

Rangiroa Rangiroa ( Tuamotuan for 'vast sky') or Te Kokōta (Cook Islands Māori for 'the Hyades star cluster') is the largest atoll in the Tuamotus and one of the largest in the world (smaller than Kwajalein in the Marshall Islands and Huvadhu in the M ...

and the

, although a continuation through the Line Islands is problematic if it is assumed that the Easter hotspot generated this track but more plausible if Crough seamount is supposed to be its own hotspot. East of Crough, a series of even younger volcanic ridges continues until the East Pacific Rise where the hotspot may be located. The Crough hotspot may be a conjugate of the

Local

Crough is an east-west trending seamount which rises over from the seafloor to a depth of less than at . 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, having formerly hosted

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 sec ...

s and pteropods. Wave erosion that took place when Crough emerged above sea level truncated the seamount, turning it into a flat

guyot In marine geology, a guyot (pronounced ), also known as a tablemount, is an isolated underwater volcanic mountain ( seamount) with a flat top more than below the surface of the sea. The diameters of these flat summits can exceed .Pillow lava Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or ''subaqueous extrusion''. Pillow lavas in volcanic rock are characterized by thick sequences of disconti ...

s crop out between . Crough Seamount has a volume of , comparable to that of other submarine volcanoes such as

Macdonald seamount Macdonald seamount (named after Gordon A. Macdonald) is a seamount in Polynesia, southeast of the Austral Islands and in the neighbourhood of a system of seamounts that include the Ngatemato seamounts and the Taukina seamounts. It rises fro ...

Mehetia Meheti'a or Me'eti'a is a volcanic island in the Windward Islands, in the east of the Society Islands in French Polynesia. This island is a very young active stratovolcano east of the Taiarapu Peninsula of Tahiti. It belongs to the Teahiti'a-M ...

and

Moua Pihaa Moua Pihaa is a submarine volcano in French Polynesia. It is located southwest of Mehetia in the Society Islands, and is the largest of a group of seamounts stretching between Mehetia and Tahiti Tahiti (; Tahitian ; ; previously also know ...

. A second seamount lies nearby and partly overlaps with Crough, it is named Thomas Seamount in honour of a

geophysicist Geophysics () is a subject of natural science concerned with the physical processes and physical properties of the Earth and its surrounding space environment, and the use of quantitative methods for their analysis. The term ''geophysics'' som ...

. This seamount is even shallower than Crough as it reaches a depth of but has a smaller volume of .

Composition

Dredging has yielded both vesicular and porphyritic

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 ...

Phenocryst 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...

s identified include

clinopyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

and

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...

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 g ...

s and

hyaloclastite Hyaloclastite is a volcanoclastic accumulation or breccia consisting of glass (from the Greek ''hyalus'') fragments (clasts) formed by quench fragmentation of lava flow surfaces during submarine or subglacial extrusion. It occurs as thin margin ...

s have also been found, and some samples were covered with

manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...

crusts and

palagonite Palagonite is an alteration product from the interaction of water with volcanic glass of chemical composition similar to basalt. Palagonite can also result from the interaction between water and basalt melt. The water flashes to steam on contact w ...

Hydrothermal Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...

iron crusts have also been found.

Eruption history

Argon-argon dating has yielded ages of 8.4 to 7.6 million years ago for samples dredged from Crough, while other geological indicators suggest an age of between 7 and 10 million years ago. Other estimates of its age are 4-3 million years. In 1955, a strong

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...

was recorded on the northern flank of Crough Seamount; 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. The earthquake has also been interpreted as a

normal fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

earthquake which sometimes occur in young oceanic crust, but the 1955 Crough event was considerably stronger than other earthquakes of this type.

References

Sources

* * * * * * * * * {{refend Seamounts of the Pacific Ocean Miocene volcanoes Pitcairn Islands