The so-called “small dot” underwater volcanoes are very special. At the top of the subduction trenches, in older and colder oceanic lithosphere, far from the intense volcanic activity exposed at the level of the ridges, they nevertheless play a significant role in the global carbon cycle.
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On every continent, many volcanoes are active. However, the intensity of this terrestrial volcanism lags far behind that occurring at the bottom of the oceans. Often associated with ocean ridges, underwater volcanoes often go completely unnoticed. However, it plays an important role in major chemical cycles and, through the intense hydrothermal activity it generates, supports abundant life at the bottom of the oceans.
These hydrothermal systems rely on the circulation of fluids within the crust, activated by a source of magmatic heat, and are suspected of being the origin of life on Earth. The interest in this high-temperature hydrothermalism associated with the magmatic activity of oceanic ridges has obscured the existence of another type of hydrothermal system occurring at the level of more discreet volcanoes called “small-spots”.
Volcanoes we don’t expect
To find these small underwater volcanoes fifty meters high, you have to leave the mid-ocean and approach the subduction zones at the edge of the lithospheric plate. The discovery of these types of volcanoes is fairly recent, and for good reason: no one expected volcanic activity in areas of the oceanic crust that are generally older, denser, and colder. Although they are located near subduction zones, where one oceanic plate sinks under another plate, they are not directly linked to this phenomenon, unlike, for example, arc magmatism in the origin of Pacific Fire Belt volcanoes.
Small-point volcanism is associated with the upwelling of very low-level magma through fractures, affecting the oceanic crust overlying the subduction zone. In fact, the sinking of an oceanic plate in subduction leads to a bulge in the area approximately 600 kilometers long in front of the subduction zone. This swelling opens up faults that cross the old and brittle crust. Faults can act as conduits for magmatic fluid in small amounts at the top of the asthenosphere. During ascent, these lavas are contaminated by contact with the rocks and highly hydrated sediments of the oceanic crust, enriching the surface lavas with a specific composition, alkalinity, and CO concentration.2. By solidifying, these lavas yield rocks called peperites. Nuggets are found in other types of volcanic environments, but these rocks are almost always the result of lava and submerged sediments.
Methane is a significant producer
This particular magmatic activity indicates the presence of hydrothermal activity. A function demonstrated by a team of Japanese researchers from Waseda University, who published their results in the journal Communication Earth and Environment. The scientists focused on characterizing hydrothermal deposits around a small-spot volcano located in 5.7 kilometers of water at the top of the Japan Trench.
In fact, this is the deepest hydrothermal activity described to date. It is the source of deposits rich in iron and manganese oxides, which precipitated directly from the cold hydrothermal fluid (below 200°C) in contact with seawater.2 than hydrothermal fluids at ocean ridges.
For scientists, these gas levels are large enough to play a significant role in the global carbon cycle. A role that has not yet been considered.