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Asubaqueous volcano is avolcano formed from the eruption or flow ofmagma that occurs under water (as opposed to asubaerial volcanic eruption).[1] Subaqueous volcanic eruptions are significantly more abundant than subaerial eruptions and are estimated to be responsible for 85% of global volcanism by volume.[2]
Subaqueous volcanoes can be classified into three types:
Deep-sea submarine volcanoes form underwater mountains calledseamounts. Many thousands of volcanic seamounts are scattered across the floors of the oceans. Other subaqueous volcanoes can be in the form of gently slopingtuff cones, although they can have steeper slopes, such asWhite Horse Bluff in theWells Gray-Clearwater volcanic field of east-centralBritish Columbia,Canada.[3]
Subaqueous volcanoes can be compared to subaerial volcanoes which are formed and erupt on land. The major differences in their eruptions are due to the effects ofpressure,heat capacity,thermal conductivity of water, and the presence of steam and waterrheology. The thermal conductivity of water is about 20 times that of air, and steam has a thermal conductivity nearly 50 times that of water.[4]
The study of subaqueous volcanoes has changed substantially. Modern studies offer fresh and unaltered observances, and one can see and map surface features and the water depth is known in areas that allow observation. Ancient studies have hadstratigraphic exposure to sections[clarification needed][maybe some words missing?], are easier to work on, have more and better exposures and have an existing relationship to resources.[5]
Somegeologists restrict the term "subaqueouspyroclastic flow deposits" tovolcaniclastic units that show characteristics ofemplacement in a hot state deposited underwater. However, this cannot always be done because of the subsequent process ofalteration ordiagenesis such as can be found in activehot springs and the associatedhydrothermal alteration. Deposits from pyroclastic flows, the kind that interacts with water before transforming into water-supported mass flows, are called "subaqueous pyroclasticdebris flow deposits" by some geologists.[6][7]
On the other hand, processes that are associated witheruption,transportation anddeposition are notably different because of the presence of water, including the ability tovaporize when in contact with water, a high density and resultingconfining pressure, highviscosity relative to air and differences in the thermal conductivity/specific heat capacity of air relative to water.[4]
Some understanding of subaqueous volcanoes can be inferred from knowledge of volcanic processes based on ancientsuccessions. Subaqueous volcano deposits have been occurring in the south ofHonshu,[8] the largest island amongJapan's four principal islands. The four subaqueous volcanic deposits that have been documented offer significant evidence to study.
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Subaqueous volcanic deposits are associated with subaqueoussedimentary deposits, which range from near shore and offshore to abyssal mudstone deposits. Unfortunately, paleo-depth constraints[clarification needed] for sedimentary strata are poor and subject to conflicting interpretations. However, the depth of emplacement can be conjectured with minor control of water depth. In determining the characteristics of pyroclastic flows in subaerial versus subaqueous deposits, it is commonly believed that waterfluidized volcaniclastic flows becomenormally graded in terms of all components except for large, buoyantpumice blocks which settle to form large pumice layers. However, such layers are usually seen assubaerialignimbrite (pumice-rich pyroclastic flows) deposits. Because of this, they are not considered clear evidence for the interpretation of the fluidizing agent (hot gas or water) and can therefore only be interpreted in conjunction with other criteria.
Characteristics can be sorted[clarification needed] to infer subaqueous eruption or emplacement ofsilicic pyroclastic deposits. Larger pumice blocks rise for a more extended period of time (minutes to hours) in comparison to smaller pumice fragments because of gases trapped withinvesicles and the possibility of fine ash fragments becomingentrained into the rising plume of gas and heated water because of the low density and weight. Therefore, subaqueous silicic pyroclastic eruptions may be diminished in the course size fraction as well as the very fine ash size fraction based on the buoyancy of the material in the water medium. These characteristics are important in determining the style of subaqueous eruption and emplacement mechanism. The characteristics of texture, such as grain morphology and grain size abundances also provide information on the process of the eruption style, transport, or flow properties, whether turbulent orlaminar.
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Seafloor exploration has discovered that more volcanic eruptions occur at the bottom of the sea than on land. However, the effects of ambient water andhydrostatic pressure on silicic volcanic eruptions in subaqueous settings are not entirely understood due to the inability of deep marine eruptions to be directly observed and studied. Because of this, information about recent deep-water volcanic eruptions are still incomplete and limited.
Studies of subaqueous volcanoes in Japan have concluded that clear evidence for eruption and/or emplacement of pyroclastic flows continues to require examination of these deposits, although inferential evidence such as grain morphology, sorting and grading can be used to identify and document ancient subaqueous volcanic deposits.
TheUniversity of California, Santa Barbara intends to continue to conduct further research, which may provide further information on styles of subaqueous volcanic eruptions and/or flow characteristics of volcanic deposits.[9]
