The water content in the magma controls the depth at which the reservoirs are located and is essential to create models that recreate the phenomenon
Of the 1,500 active volcanoes in the world , between 40 and 50 are erupting or in a state of agitated activity . For this reason, hundreds of millions of people are threatened by its potential risks, so having reliable prediction models to know when these ‘giants’ can wake up is vital. However, despite scientific and technological advances, there is still a long way to go to have an accurate and totally reliable method. Now, a new study published in ‘ Science ‘ led by volcanologist Dan Rasmussen , of the Smithsonian Institution’s National Museum of Natural History, may bring experts one step closer to accurate forecasts.
Volcanologists do not yet fully understand the natural dynamics and processes of magma beneath a volcano before it makes its way to the surface.
Although it may sound contradictory, water is very important in the process: the volatile elements of the magma, mainly water, cause very explosive volcanic eruptions. This new research has found that for arc-type volcanoes – the most common that form an island arc – magma with higher water content tends to be stored deeper in the Earth’s crust. These findings rule out previous assumptions that magma storage depth is largely controlled by neutral buoyancy in surrounding rock, with magma rising through cracks in the Earth’s crust because the molten rock is more buoyant than molten rock. surrounding crust.
“This study connects the depth at which magma is stored with water, which is important because water initiates and largely fuels eruptions,” says Rasmussen, who explains that water drives eruptions in the same way that carbon dioxide can explode a bottle of soda that has been previously shaken. “With water dissolved in magma that is stored under a volcano, if there is a sudden drop in pressure, such as when the lid of a shaken soda bottle is suddenly opened, gas bubbles form and that causes the magma to collapse. rise up and shoot out of the volcano. More water content in the magma means more gas bubbles and potentially a more violent eruption.”
Looking for olivine among the ashes
Rasmussen began his research in 2015 while completing his doctorate at Columbia University’s Lamont-Doherty Earth Observatory. During his stay, his colleague, volcanologist Terry Plank, suggested that he delve into why magma storage depth varies from volcano to volcano and what controls that depth. Together with a team that included geophysicist Diana Roman of the Carnegie Institution for Science, Rasmussen collected volcanic material from eight volcanoes located on Alaska’s rugged and remote Aleutian Islands.
The researchers focused on a specific type of volcano: the so-called arc volcanoes, which occur at the intersection of two converging tectonic plates. Arc volcanoes, like those found in the Aleutians, are the most numerous type on Earth and comprise the entirety of the so-called ‘Ring of Fire’ that surrounds the Pacific Plate, making them the most obvious target. to improve predictive capabilities.
Their goal was to find bits of olivine in the ash. These tiny green crystals form underground, sometimes trapping small shards of magma inside. The eruptions throw them to the surface, remaining as witnesses of what has happened inside the volcano and being able to estimate the water content of the magma.
After estimating the water content of trapped pieces of magma collected from six of the eight Aleutian volcanoes, the team combined that data with other estimates of magmatic water content taken from the scientific literature for an additional 56 volcanoes around the world. The final list comprised 3,856 individual samples from 62 volcanoes.
More water at greater depth
Rasmussen and the research team focused on recent eruptions because magma reservoirs don’t appear to move much after an eruption, so any estimates of depth or water content made using recent eruption material are more likely to reflect accurately the current state of the volcano’s magma reservoir.
After years of fieldwork, geochemical analyses, and literature review, the team pitted the depth of the magma reservoirs of 28 volcanoes around the world against their respective estimated magmatic water contents. The results were surprisingly clear: the water content of a magma reservoir was strongly correlated with its storage depth. In other words, magmas that contained more water tended to be stored deeper in the Earth’s crust.
The study also shows that a magma’s water content is responsible for controlling its depth, rather than simply correlating with it. The team showed this causal relationship by detecting the presence of chemical tracers associated with the formation of water-bearing magmas in the Earth’s mantle. “If storage depth determined the water content in the magma, it could still create the correlation between water content and depth that we observe, but it would not produce the chemical tracers of initial magma water content that we found,” Rasmussen said.
The water in the magma determines the depth
As for how water content might determine magma storage depth, Rasmussen and his co-authors argue that it has to do with a process known as degassing in which water mixed with magma forms gas bubbles. When magma rising through the Earth’s crust begins to degas, it becomes more viscous, which the researchers suggest causes the magma’s rise to slow and stop.
Evidence that water content largely controls magma storage depth overturns the explanation most widely accepted today, which holds that magma rises through cracks in the Earth’s crust because molten rock is buoyant. more than the surrounding crust, and it settles to its storage depth because it reaches neutral buoyancy where the magma does not buoy more than its surroundings.
Rasmussen explains that the next step for this research is to see if these findings hold for volcanoes in other geologic settings, such as hot-spot volcanoes like the Hawaiian Islands or rift volcanoes like those in East Africa. Beyond this extension of the research, Rasmussen said an even bigger question arises: “If magma water content controls magma storage depth, what controls magma water content?”
