Arc Volcanoes Are More Wet Than Previously Assumed, Which Has Scientific and Economic Ramifications

Many prior studies have determined a far lower amount of water in arc volcanoes that develop above subduction zones.

According to new research, the increasing volume of water has significant consequences for understanding how the Earth's lower crust evolves, how magma erupts through the crust, and how commercially important mineral ore deposits form.

Arc volcanoes are moister than originally imagined

(Photo : ALEXIS HUGUET/AFP via Getty Images)
This aerial photograph taken on April 1, 2022 shows a road passing through the lava flow from Nyiragongo volcano on the border between the Democratic Republic of Congo and Rwanda, south of Virunga National Park and north of the city of Goma

According to the research, the estimated water concentrations in primitive arc magmas from this investigation are more varied and much greater than the average of roughly four weight percent of water discovered in prior studies.

According to the research, the results demonstrated that primitive arc H2O follows substantial crystal fractionation in the lower arc crust, as per ScienceDaily.

The overall picture here is that water acts as a lubricant in plate tectonics.

According to lead author Benjamin Urann, who is now a Ph.D. student in the Massachusetts Institute of Technology (MIT) WHOI Joint Program in Oceanography/Applied Ocean Science and Engineering at the time of this research, the water content will affect all sorts of different parameters involved in how tectonic plates move.

Urann and his Ph.D. supervisor, Véronique Le Roux, who is also a co-author of the paper, established methodologies for measuring moisture contents in minerals using the Secondary Ion Mass Spectrometry instrument at WHOI, building on previous work that recommended that arc magmas could perhaps contain substantially more H20 than melt-inclusion measurements suggested.

Instead of investigating lava samples that have erupted to the Earth's surface, the researchers concluded that it would be more useful to analyze deep tectonic magmas that have not lost too much of their water content.

Rather than looking at exterior rocks that move far up through into the crust as molten rock and lose much more of their water content along the way, the researchers looked at magma lower crustal cumulates that calcified deep down in the crust at a high sufficient pressure to maintain their initial moisture contents signature.

Depending on the nature of the magma, the researchers determined that it contained between 10 and 20% water by weight.

Although this mass percent of water had been anticipated experimentally, it had never been demonstrated on natural samples, according to Le Roux.

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Volcanic arc complexes

A volcanic arc is a lengthy sequence of volcanoes that develops over a subduction zone.

Ocean-ocean subduction creates an island volcanic arc in an ocean basin, as per the journal Keys to the Interpretation of Geological History

Examples are the Aleutian Islands off Alaska's coast and the Lesser Antilles south of Puerto Rico.

Where there is much oceanic crust subducts beneath the continental crust, a continental volcanic arc develops. One example is the Cascade Volcanoes.

The volcanic arc is a living landmass in both situations.

In geology, the term complex refers to a distinct group of deformed rocks.

A volcanic arc complex is a distinct group of rocks thought to be the distorted relics of a long-gone volcanic arc landform.

There are several instances of volcanic arc systems in the Appalachian and Cordilleran Mountain belts, all of which began offshore and were assimilated towards the mountain belt or, like the Cascade volcanoes, developed within the mountainous region itself.

Volcanic arc complexes are quite straightforward to identify when they are unmetamorphosed or have a low metamorphic grade.

They often contain silicic and basaltic volcanic rock, as well as volcaniclastic rock, which is clastic sedimentary rock generated from volcanic erosion.

Certain forms of shale and greywacke are examples of volcaniclastic rocks (mud-rich sandstone). These rocks transform into greenschists in low-grade circumstances.

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