Lo-En or Hess is an

Albian The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/ Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± 0 ...

–

Campanian The Campanian is the fifth of six ages of the Late Cretaceous Epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campani ...

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 .Marshall Islands The Marshall Islands ( mh, Ṃajeḷ), officially the Republic of the Marshall Islands ( mh, Aolepān Aorōkin Ṃajeḷ),'' () is an independent island country and microstate near the Equator in the Pacific Ocean, slightly west of the Intern ...

. One among a number of seamounts 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 contin ...

, it was probably formed by a hotspot in what is present-day French Polynesia. Lo-En lies southeast of

Eniwetok Enewetak Atoll (; also spelled Eniwetok Atoll or sometimes Eniewetok; mh, Ānewetak, , or , ; known to the Japanese as Brown Atoll or Brown Island; ja, ブラウン環礁) is a large coral atoll of 40 islands in the Pacific Ocean and with i ...

which rises above sea level, and Lo-En is almost connected to it through a ridge. The seamount is formed by

basalt Basalt (; ) is an 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 surface of a rocky planet or moon. More than 90 ...

ic rocks that probably formed a shield volcano first. It is believed that a number of hotspots such as the Macdonald hotspot, the

Rarotonga hotspot The Rarotonga hotspot is a volcanic hotspot in the southern Pacific Ocean. The hotspot was responsible for the formation of Rarotonga and some volcanics of Aitutaki. In addition to these volcanoes in the Cook Islands, the composition of volcan ...

and the Rurutu hotspot may have been involved in the formation of Lo-En. After volcanic activity, by the

Turonian The Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous Epoch, or a stage in the Upper Cretaceous Series. It spans the time between 93.9 ± 0.8 Ma and 89.8 ± 1 Ma (million years ago). The Turonian is preceded b ...

the seamount was submerged although it is possible that a

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 ...

platform formed during the Albian. After a hiatus,

sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the ...

commenced on the seamount in Oligocene time and led to the deposition of manganese crusts and

pelagic The pelagic zone consists of the water column of the open ocean, and can be further divided into regions by depth (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or w ...

sediments including

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

, some of which were later modified by

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...

Name and research history

The name "Lo-En" is a reference to the

hibiscus ''Hibiscus'' is a genus of flowering plants in the mallow family, Malvaceae. The genus is quite large, comprising several hundred species that are native to warm temperate, subtropical and tropical regions throughout the world. Member species ...

tree. The seamount was formerly known as Hess Guyot. In 1992 the seamount was targeted for drilling in the

Ocean Drilling Program The Ocean Drilling Program (ODP) was a multinational effort to explore and study the composition and structure of the Earth's oceanic basins. ODP, which began in 1985, was the successor to the Deep Sea Drilling Project initiated in 1968 by th ...

Geography and geology

Local setting

Lo-En lies within the northern

Marshall Islands The Marshall Islands ( mh, Ṃajeḷ), officially the Republic of the Marshall Islands ( mh, Aolepān Aorōkin Ṃajeḷ),'' () is an independent island country and microstate near the Equator in the Pacific Ocean, slightly west of the Intern ...

, less than south-southeast from Eniwetok. It is part of a cluster of seamounts and islands that surrounds

, but also appears to be part of a chain running parallel to the

Ralik Chain The Ralik Chain ( Marshallese: , ) is a chain of islands within the island nation of the Marshall Islands. Ralik means "sunset". It is west of the Ratak Chain. In 1999 the total population of the Ralik islands was 19,915. Christopher Loeak, who b ...

and the

Ratak Chain The Ratak Chain ( Marshallese: , ) is a chain of islands within the island nation of the Marshall Islands. Ratak means "sunrise". It lies to the east of the country's other island chain, the Ralik Chain. In 1999 the total population of the Rata ...

, two seamount-and-island chains in the Marshall Islands. The seamount rises above the seafloor to a depth of below sea level. Lo-En is elongated in north–south direction and is a typical

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 .landslides. The magnetization pattern of the seamount has been investigated; it is classified as "

normal Normal(s) or The Normal(s) may refer to: Film and television * ''Normal'' (2003 film), starring Jessica Lange and Tom Wilkinson * ''Normal'' (2007 film), starring Carrie-Anne Moss, Kevin Zegers, Callum Keith Rennie, and Andrew Airlie * ''Norma ...

" but with particular magnetization patterns that are different from the topography. No

cap or

s have been found on the platform of Lo-En, unlike in several other guyots of the region; a drill core at Lo-En found

sediments directly on the volcanic basement. However, remains of

reef A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic processes— deposition of sand, wave erosion planing down rock o ...

s and of

lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') ...

al sediments have been detected on

seismic Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

profiles although the existence of a

barrier reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups. Co ...

has been questioned, and Lo-En has a thick pelagic sediment cap. It is possible that shallow water limestones exist at the margins of the platform, which were not drilled. Lo-En shares its volcanic edifice with Eniwetok; a northern spur from Lo-En almost reaches Eniwetok while another spur emerges in south-southeast direction and is about long. Another seamount lies farther west from Lo-En. The seafloor underneath Lo-En is 113–156.9 ± 5 million years old and lies at a depth of more than .

Regional setting

The

seafloor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth an ...

, especially the parts that are of

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

age, contains most of the world's guyots. These are flat-topped submarine mountains which are characterized by steep slopes, a flat top and usually the presence of

corals 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 secr ...

and carbonate platforms. It is not clear whether the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

guyots were

atoll An atoll () is a ring-shaped island, including a coral rim that encircles a lagoon partially or completely. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical oceans and seas where corals can gr ...

s in the present-day sense but many of these seamounts were, which today still exist. All these structures originally formed as volcanoes in the Mesozoic Ocean. First

fringing reef A fringing reef is one of the three main types of coral reef. It is distinguished from the other main types, barrier reefs and atolls, in that it has either an entirely shallow backreef zone (lagoon) or none at all. If a fringing reef grows direc ...

s may have developed on the volcanoes, which then became barrier reefs as the volcano subsided and turned into an atoll. Continued subsidence balanced by upward growth of the reefs led to the formation of thick carbonate platforms. Sometimes volcanic activity occurred even after the formation of the atoll or atoll-like landforms, and during episodes where the carbonate platforms were lifted above sea level erosional features such as channels and

blue hole A blue hole is a large marine cavern or sinkhole, which is open to the surface and has developed in a bank or island composed of a carbonate bedrock ( limestone or coral reef). Their existence was discovered in the late 20th century by fishe ...

s developed. The crust underneath these seamounts tends to subside as it cools and thus the islands and seamounts sink. Pacific Basin Island Geography Hotspots

The formation of many such seamounts including Limalok has been explained with the hotspot theory, which discusses the formation of chains of volcanoes which get progressively older along the length of the chain, with an active volcano only at one end of the system. Seamounts and islands in the Marshall Islands do not appear to have originated from such simple age-progressive hotspot volcanism as the age progressions in the individual island and seamount chains are often inconsistent with a hotspot origin. One solution to this dilemma may be that more than one hotspot passed through the Marshall Islands, as well as the possibility that hotspot volcanism is affected by contemporaneous

lithospheric A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years or ...

extension. Candidate hotspots responsible for the formation of Lo-En are the Macdonald hotspot which passed close to Lo-En between 115 and 105 million years ago and the

and Rurutu hotspot, both of which were at Lo-En between 90 and 74 million years ago. Of these, the first two also have the strongest geochemical similarity to Lo-En. In the case of Lo-En, volcanism on the Ogasawara Fracture Zone may also have contributed to its formation considering that the seamount is much older than surrounding seamounts. Based on plate motion reconstructions, the region of the Marshall Islands was located in the region of 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. About 8 hotspots have formed a large number of islands and seamounts in that region, with disparate geochemistries, and that geological province has been called the "South Pacific Isotopic and Thermal Anomaly" or DUPAL anomaly.

Composition

Lo-En has erupted

alkali basalt Alkali basalt or alkali olivine basalt is a dark-colored, porphyritic volcanic rock usually found in oceanic and continental areas associated with volcanic activity, such as oceanic islands, continental rifts and volcanic fields. Alkali basalt ...

and

hawaiite Hawaiite is an olivine basalt with a composition between alkali basalt and mugearite. It was first used as a name for some lavas found on the island of Hawaii. It occurs during the later stages of volcanic activity on oceanic islands such as Ha ...

. Minerals contained in the rocks include apatite, biotite,

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 I ...

ilmenite Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printing ...

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...

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 pro ...

. Strong alteration has occurred and has given rise to calcite, carbonate,

chabazite Chabazite ()The New Oxford Dictionary of English (1998) - p. 300 "chabazite /'kabəzʌɪt/ noun "A colourless, pink or yellow zeolite mineral, typically occurring as rhombohedral crystals.". is a tectosilicate mineral of the zeolite group, clos ...

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

, hematite,

smectite A smectite (from ancient Greek ''σμηκτός'' smektos 'lubricated'; ''σμηκτρίς'' smektris 'walker's earth', 'fuller's earth'; rubbing earth; earth that has the property of cleaning) is a mineral mixtures of various swelling sheet sil ...

and

zeolite Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ･y where is either a metal ion or H+. These p ...

, as well as

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 ...

. Other rocks found on Lo-En are

chalk Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Ch ...

, limestone,

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

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

Geologic history

The geologic history of Lo-En seamount is relatively poorly known;

paleomagnetic Paleomagnetism (or palaeomagnetismsee ), is the study of magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.'' Certain magnetic minerals in roc ...

data have been used to infer an age of 45–85 million years ago for the seamount. It was located at about 25–30 degrees southern latitude when it formed, south of the hotspots that are presently active in the

Cook Islands ) , image_map = Cook Islands on the globe (small islands magnified) (Polynesia centered).svg , capital = Avarua , coordinates = , largest_city = Avarua , official_languages = , lan ...

–

Austral Islands The Austral Islands (french: Îles Australes, officially ''Archipel des Australes;'' ty, Tuha'a Pae) are the southernmost group of islands in French Polynesia, an overseas country of the French Republic in the South Pacific. Geographically ...

. Eniwetok is about 36 million years younger than Lo-En.

Volcanism and first biotic phenomena

A first phase of volcanic activity occurred 114 million years ago, and was followed by a second episode during the

, separated by as much as 30 million years. The first episode has been dated at 111.6–114.0 million years ago and was contemporaneous with volcanic activity at other western Pacific seamounts. The second episode may be responsible for the cones that emerge from or are embedded in sediments that cover Lo-En. Eniwetok was active 76.9 million years ago and flexural effects from its growth may have influenced the second volcanic phase at Lo-En. Potentially, the first episode was caused by the Macdonald hotspot and the second by the Rarotonga and Rurutu hotspots. The volcanic rocks encountered in drill cores consist of

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

and lava flows. Their alkalic composition implies that they may be post-shield volcanics. Claystones indicate that weathered terrain occurred on Lo-En, with subaerial alteration of volcanic rocks.

Post-volcanic

Lo-En was submerged in the

by about 112.8 ± 1.2 million years ago, long before surrounding seamounts and islands formed. The absence of a barrier reef would have facilitated the erosion of Lo-En's summit. It is possible that flexural loading by Eniwetok pushed Lo-En downward and thus prevented the formation of reefs on Lo-En during the Cretaceous when the seamount was uplifted by a hotspot. It took about 2–7 million years from the end of volcanic activity until sedimentation began. During the

, limestones were emplaced on Lo-En. These appear to reflect the formation of reefs and a shallow carbonate bank, material from which has been dredged on the southern flank. This platform drowned during the late Albian, similar to many other carbonate platforms of similar age, and there is no evidence of mid-Cretaceous platforms. Starting in the Albian– Cenomanian, pelagic limestone was deposited directly on the volcanic rocks during

Santonian The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series. It spans the time between 86.3 ± 0.7 mya (million years ago) and 83.6 ± 0.7 mya. ...

– Coniacian times; in light of fossil data it is likely that the seamount was not overly deep at that time. Sediments of Cretaceous–

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago (mya). It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name is a combination of the Ancient Greek ''pal ...

age reach thicknesses of less than . Starting in the Oligocene, pelagic oozes were deposited on Lo-En. The previously emplaced limestones also underwent phosphate reactions as they were exposed on the seafloor, and some of the sediments underwent reworking.

Foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...

encountered on the seamount in the form of nannofossils are: * '' Arkangelskiella specillata'' * '' Aspidolithus parcus expansus'' * '' Eiffellithus eximius'' * '' Gephyrorhabdus coronadventis'' * '' Lithastrinus septenarius'' * '' Micula sp.'' * '' Nannoconus farinacciae'' * '' Reinhardtites sp.'' * '' Stoverius sp.''

References

Sources

* * * * * * * * * * * * * * * {{Marshall Islands topics Seamounts of the Pacific Ocean Extinct volcanoes Landforms of the Marshall Islands Mesozoic volcanoes Ralik Chain