Limalok
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| Limalok | |
|---|---|
_Guyot.png) | |
  Limalok Location in the Marshall Islands | |
| Summit depth | 1,255 metres (4,117 ft) |
| Summit area | 636-square-kilometre (246 sq mi) |
| Location | |
| Group | Ratak Chain |
| Coordinates | 5°36′N 172°18′E / 5.6°N 172.3°E[1] |
| Country | Marshall Islands |
| Geology | |
| Type | Guyot |
| Age of rock | Cretaceous |
Limalok (formerly known as Harrie or Harriet) is a Cretaceous[a]-Paleocene[b] guyot/tablemount in the southeastern Marshall Islands, one of a number of seamounts (a type of underwater volcanic mountain) in the Pacific Ocean. It was probably formed by a volcanic hotspot in present-day French Polynesia. Limalok lies southeast of Mili Atoll and Knox Atoll, which rise above sea level, and is joined to each of them through a volcanic ridge. It is located at a depth of 1,255 metres (4,117 ft) and has a summit platform with an area of 636 square kilometres (246 sq mi).
Limalok is formed by basaltic rocks and was probably a shield volcano at first; the Macdonald, Rarotonga, Rurutu and Society hotspots may have been involved in its formation. After volcanic activity ceased, the volcano was eroded and thereby flattened, and a carbonate platform formed on it during the Paleocene and Eocene. These carbonates were chiefly produced by red algae, forming an atoll or atoll-like structure with reefs.
The platform sank below sea level 48 ± 2 million years ago during the Eocene, perhaps because it moved through the equatorial area, which was too hot or nutrient-rich to support the growth of a coral reef. Thermal subsidence lowered the drowned seamount to its present depth. After a hiatus lasting into the Miocene,[c] sedimentation commenced on the seamount leading to the deposition of manganese crusts and pelagic sediments; phosphate accumulated in some sediments over time.
Limalok was formerly known as Harrie Guyot[3] and is also known as Harriet Guyot;[4] Limalok refers to a traditional chieftess of Mile Atoll.[5] Limalok is one of the seamounts targeted during the Ocean Drilling Program,[6] which was a research program that aimed at elucidating the geological history of the sea by obtaining drill cores from the oceans.[6][7] The proportion of material recovered during the drilling[8] was low, making it difficult to reconstruct the geologic history of Limalok.[9]
Geography and geology
Local setting
Limalok lies at the southernmost[10] end of the Ratak Chain[11] in the southeastern Marshall Islands[12] in the western Pacific Ocean.[6] Mili Atoll is located 53.7 kilometres (33.4 mi) from Limalok,[3] with Knox Atoll in between the two.[13]
The relatively small[14] seamount rises from a depth of 4,500 metres (14,800 ft)[15] to a minimum depth of 1,255 metres (4,117 ft) below sea level.[16] The top of Limalok is 47.5 kilometres (29.5 mi) long[3] and broadens southeastward from less than 5 kilometres (3.1 mi) to more than 24 kilometres (15 mi),[13] forming a 636-square-kilometre (246 sq mi) summit platform.[17] The carbonate platform of Limalok crops out at the edges of the summit plateau.[10] Wide terraces[10] and numerous fault blocks surround the summit plateau;[18] some of the latter may have formed after the carbonate platform ceased growing.[19]
Mili Atoll and Limalok emerge from a common pedestal[9] and are connected by a ridge at 1.5 kilometres (0.93 mi) depth.[15] The seafloor is 152[20]–158 million years old,[21] but it is possible that Limalok rises from Cretaceous flood basalts[d] rather than the seafloor itself.[23] Volcanic sediments in the Eastern Mariana Basin may come from this seamount.[24]
Regional setting
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The Pacific Ocean seafloor, especially the parts that are of Mesozoic age, contains most of the world's guyots (also known as tablemounts[25]). These are submarine mountains[26] which are characterized by steep slopes, a flat top and usually the presence of corals and carbonate platforms.[1] These structures originally formed as volcanoes in the Mesozoic Ocean. Fringing reefs may have developed on the volcanoes, which then were replaced by barrier reefs as the volcanoes subsided and turned into atolls. Continued subsidence balanced by upward growth of the reefs led to the formation of thick carbonate platforms.[27] Volcanic activity can occur even after the formation of the atoll or atoll-like[e] landforms, and during episodes where the platforms were lifted above sea level, erosional features such as channels and blue holes[f] developed.[30] The crust underneath these seamounts tends to subside as it cools and thus the islands and seamounts sink.[31]
The formation of many seamounts[32] including Limalok[33] has been explained with the hotspot theory, in which a "hot spot" rising from the mantle leads to the formation of chains of volcanoes which get progressively older along the length of the chain, with an active volcano at only one end of the system, as the plate moves over the hotspot.[34] Seamounts and islands in the Marshall Islands do not appear to have originated from simple age-progressive hotspot volcanism as the age progressions in the individual island and seamount chains are often inconsistent with this explanation.[35] One solution to this dilemma may be that more than one hotspot passed through the Marshall Islands,[36] and it is also possible that hotspot volcanism was affected by extensional deformation of the lithosphere.[37] For Limalok, geochemical evidence shows affinities to the Rarotonga hotspot[38] which is unlike the geochemical trends in the other volcanoes of the Ratak Chain.[39] Reconstructions of the area's geological history suggest that the first hotspot to pass by Limalok was the Macdonald hotspot 95–85 million years ago, followed by the Rurutu hotspot and the Society hotspot 75–65 million years ago.[40] The Rarotonga and especially the Rurutu hotspots are considered to be the most likely candidates for the hotspot that formed Limalok.[41] However, some paleogeographical inconsistencies indicate that lithospheric fractures secondary to hotspot activity were also involved.[42]
From plate motion reconstructions, it has been established that the Marshall Islands were located in the era now occupied by present-day French Polynesia during the time of active volcanism. Both regions display numerous island chains, anomalously shallow ocean floors and the presence of volcanoes.[43] About eight hotspots have formed a large number of islands and seamounts in that region, with disparate geochemistries;[44] the geological province has been called "South Pacific Isotopic and Thermal Anomaly" or DUPAL anomaly.[45]
Composition
Limalok has erupted basaltic rocks,[13] which have been classified as alkali basalts,[46] basanite[39] and nephelinite.[47] Minerals contained in the rocks are apatite,[48] augite,[49] biotite, clinopyroxene, olivine, nepheline and plagioclase,[48] and there are ultramafic xenoliths.[50] Shallow crystal fractionation processes appear to have been involved in the genesis of the magmas erupted by Limalok.[51]
Alteration of the original material has formed calcite, chlorite, clay, iddingsite, montmorillonite, zeolite, and a mineral that could be celadonite.[41][48] Volcanogenic sandstones[52] and traces of hydrothermal alteration also exist on Limalok.[48]
Carbonate, clay,[13] manganese phosphate crust materials[g][54] and mudstones have been found in boreholes[28] or have been dredged from the seamount.[54] The carbonates take various forms, such as grainstone, packstone,[28] limestone,[55] rudstone and wackestone.[28] Porosity is usually low owing to cementation of the deposits,[55] a process in which grains in rock are solidified and pores filled by the deposition of minerals such as calcium carbonate.[56] The carbonate rocks show widespread evidence of diagenetic alteration,[57] meaning the carbonates have been chemically or physically modified after they were buried.[56] For example, aragonite, pyrite[58] and organic material were formed by alteration of living beings within the clays and limestones.[59]