The water content of the magma determines where it is stored.

Not all volcanic eruptions are the same. The silica content of a magma determines its greater or lesser viscosity and, therefore, the explosive or effusive behavior of a volcano. The style, frequency, and magnitude of the eruptions largely depend on that aspect.

In certain types of volcanoes, the factors that control the depth at which magma is stored in reservoirs, or magma chambers, remain poorly understood. Daniel Rasmussen, a researcher at the Smithsonian Institution’s National Museum of Natural History in Washington, DC, and his colleagues have concluded that, in the case of volcanic arcs (chains of volcanoes that are often characterized by explosive eruptions), one of those factors could be the water content of the magma. The team has published its results in Science.

In contrast, at mid-ocean ridges, another type of volcanic region, the phenomenon that determines the depth of magma chambers is well known. In general, it is accepted that the depth of the reservoirs is conditioned by the “neutral buoyancy level”, the position where the weight of the melt and its buoyancy are balanced. The magma, initially hotter (and therefore less dense) than the surrounding rock, rises to the surface. Upon reaching the earth’s crust, the difference in density decreases until the magma stabilizes at a certain depth, and the magma chamber is formed there.

However, comparative analysis of a series of geophysical and geochemical studies reveals that not all magmas are stored at the neutral buoyancy level. The team led by Christian Huber, a researcher at the Department of Earth, Environmental and Planetary Sciences at Brown University, has established that the density of silica-rich magmas is usually lower than that of the surrounding crust, so difficult to apply the above hypothesis.

Why do some magmas stop rising after a certain depth? A possible explanation has to do with the water content of the magmas coming from the deep crust. Rasmussen’s team studied the case of volcanic arcs, which originate at the edges of converging tectonic plates and give rise to archipelagos aligned along the Pacific Ring of Fire, such as the Aleutian Islands. Its magmatic chambers are found at variable depths that can reach about 15 kilometers.

In order to identify the role of water, the researchers collected ash from six volcanoes to analyze the olivine crystals they contained. As they grow, these crystals trap glassy inclusions of magma. By measuring the chemical composition of these microscopic inclusions, the scientists calculated the water content of the magma. They then supplemented their data with that of fifty-six other volcanoes around the world and, for each of them, compared the amount of water with the estimated depth of the reservoir.

The results reveal that magmas that contained more water tended to stop (and thus be stored) deeper in the Earth’s crust. According to the researchers, this correlation between water content and storage depth could be related to a change in the viscosity of the magma. When the melt rises through the crust, the water it contains is transformed into steam through a degassing process. This causes the magma to cool and crystallize, so that it would become more viscous and its rise could slow down or even stop.

However, in the case of more evolved magmas, the situation could be different, as Olivier Bachmann, professor at the Federal Polytechnic School of Zurich, points out: “It is likely that the arrest occurs at the beginning, when the poorly differentiated (basaltic, andesitic ) ascends from the lower crust. But that process would become less important as the melts continue to differentiate in the upper crust to form the most highly evolved magmas on our planet, such as rhyolitic magmas, which give rise, for example, to obsidian flows.”

Elisa Dore

Reference: “Magmatic water content controls the pre-eruptive depth of arc magmas». Daniel J. Rasmussen et al. in Science, vol. 375, pp. 1169-1172, March 10, 2022.

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