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Monowai Seamount is a Volcano seamount to the north of New Zealand. It is formed by a large caldera and a volcanic cone just south-southeast from the caldera. The volcanic cone rises to depths of up to but its depth varies with ongoing volcanic activity, including and the growth of . The seamount and its volcanism were discovered after 1877, but only in 1980 was it named "Monowai" after a research ship of the same name.
The subduction of the Pacific plate beneath the Australia plate has given rise to volcanic and hydrothermal activity on the Kermadec Ridge that Monowai is part of. The volcano is located at the site where the Osbourn Trough and the Louisville seamount chain subduct in the Tonga Trench and this subduction process probably has influenced its volcanism. Monowai is one of the most active volcanoes in the Kermadec Islands, with many eruptions since 1977, and perhaps the most active submarine volcano in the world. Volcanic activity is characterised by the emission of gas and discolouration of water, along with seismic activity and a substantial growth rate of the volcano. The ongoing hydrothermal activity has also been observed and hydrothermal vents on Monowai feature a rich variety of fauna.
To its north-northeast lies a large and deep caldera that is elongated in the northwest-southeast direction; the Monowai Seamount as a whole has a similar elongation. The caldera appears to consist of two nested calderas, the outer of which is the largest in the northern Kermadec arc with a surface area of . although an origin as one single long-lived caldera is also possible. This caldera is the largest in the Kermadec volcanic arc.
The rim of the caldera lies at a depth of about . occur around the caldera, and a cone is also found within the inner caldera and appears to be about high resurgent dome; additional cones can be found on the caldera floor which is covered by sediments. On Mussel Ridge, a ridge located within the caldera and close to its southwestern margin, seafloor observations have found cemented volcanic ash, dispersed rocks, mud, , pillow tube lavas, and Scree. Another about high cone is situated between the Monowai cone and the caldera. Both Monowai Seamount and Monowai Caldera rise from a lava shield and the entire volcanic complex covers an area of about .
Hydrothermal activity including venting at temperatures of less than occurs at Mussel Ridge where several low-temperature vents can be found; additional venting occurs on the Monowai Seamount main cone and satellite vents on its flanks; deep hydrothermal plumes hint at additional vents at greater depths. In addition to "true" hydrothermal fluids, the slopes of Monowai are the sources of non-hydrothermal plumes that probably originate from landslides or when material is remobilized by eruptions.
Other volcanic centres close to Monowai Seamount are the Hinepuia centre about south-southwest and the "U" volcanic centre about farther north. Less than north lies Volcano-19, which has erupted basaltic andesite and is active.
The basement that Monowai Seamount is constructed on may be the same as the one underpinning the rest of the Kermadec Ridge – compacted Miocene to Oligocene volcanic sediments above an Eocene volcanic arc which in the area of Monowai Seamount form a graben structure. Several faults that occur west of the volcano relate to the developing graben and additional faults dissect the volcano, including that surround the caldera. The faults that dissect the Monowai Seamount have strikes and trends comparable to those of other volcanoes in the central Kermadec arc.
The Kermadec arc has been active for the past 1 million years. About 5–6 million years ago, the Lau-Havre backarc trough opened up and separated the Kermadec arc from the Colville-Lau arc, where volcanic activity ceased about 5-3.5 million years ago. The subduction of the Louisville seamount chain appears to have altered the behaviour of the backarc since north of the subducting area the Lau Basin is undergoing full-fledged seafloor spreading while the Havre Trough south of the subducting area only features short rift segments.
On Mussel Ridge, hydrothermal alteration of rocks has produced several minerals like alunite, amorphous silica, anhydrite, barite, chalcopyrite, cristobalite, magnetite, marcasite, natroalunite, natrojarosite, pyrite, pyrophyllite, smectite, and native sulfur; in some places, the volcanic rocks have been entirely replaced by alteration products. Hyaloclastite rocks have also been observed.
Present-day activity occurs at the top of Monowai Seamount, and manifests itself in the form of , discoloured water, emission of gases and , rumbling sounds and upwelling water. Underwater, this activity generates cones, , and as well as and lava dome growth, which has caused the summit of Monowai Seamount to shift southward. Several have been observed on Monowai Seamount, including a strong swarm in May 2002 that may be associated with a sector collapse, and sound waves from the volcano have been recorded as far aways as Ascension Island in the Atlantic Ocean. Monowai Seamount is a fast-growing edifice, with growth rates ranging between . Monowai Seamount's magma output rate reaches during some periods and exceeds that of many oceanic volcanoes such as Hawaii; this fast growth is accompanied by cyclical landslides and sector collapses that redistribute material down its slopes. These landslides, while much smaller than comparable landslides at other volcanoes, appear to occur at a very high frequency. Such submarine landslides can lead to ; there is however no evidence for tsunamis triggered by eruptions at Monowai Seamount.
Early observations of volcanic activity at Monowai Seamount occurred in 1977 and 1978. The last eruption sequence may have occurred in October 2014 or May 2016, when pumice rafts and water discolouration were observed, respectively. Both events were accompanied by seismic episodes which indicate that between April 2014 and January 2017 there were about two eruptions per month. Monowai Seamount may be the most active submarine volcano in the world.
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