page_title: Lo-En
section_title: Regional setting
hierarchical_section_title: Lo-En / Geography and geology / Regional setting
caption_attribution_description: English: Various Islands of the Pacific Ocean basin made through hotspot and subduction processes.
context_page_description: Lo-En or Hess is an Albian–Campanian guyot in the Marshall Islands. One among a number of seamounts in the Pacific Ocean, it was probably formed by a hotspot in what is present-day French Polynesia. Lo-En lies southeast of Eniwetok which rises above sea level, and Lo-En is almost connected to it through a ridge.
  The seamount is formed by basaltic rocks that probably formed a shield volcano first. It is believed that a number of hotspots such as the Macdonald hotspot, the Rarotonga hotspot and the Rurutu hotspot may have been involved in the formation of Lo-En. After volcanic activity, by the Turonian the seamount was submerged although it is possible that a carbonate platform formed during the Albian. After a hiatus, sedimentation commenced on the seamount in Oligocene time and led to the deposition of manganese crusts and pelagic sediments including limestone, some of which were later modified by phosphate.
context_section_description: The Pacific Ocean seafloor, especially the parts that are of Mesozoic 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 and carbonate platforms. It is not clear whether the Cretaceous guyots were atolls 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 reefs 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 holes developed. The crust underneath these seamounts tends to subside as it cools and thus the islands and seamounts sink.
  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 extension. 
  Candidate hotspots responsible for the formation of Lo-En are the Macdonald hotspot which passed close to Lo-En between 115–105 million years ago and the Rarotonga hotspot 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.
caption: Maps of the paths of some Pacific hotspots
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