Water determines magma depth, a key to accurate models of volcanic
activity, eruption

Date:
March 10, 2022
Source:
Smithsonian
Summary:
Around the world, between 40 and 50 volcanoes are currently erupting
or in states of unrest, and hundreds of millions of people are at
risk of hazards posed by these potentially active volcanos. Yet,
despite the profound hazards posed to human life and property by
volcanic eruptions, humanity still cannot reliably and accurately
predict them, and even when forecasts are accurately made by
experts, they may not afford ample time for people to evacuate
and make emergency preparations.



FULL STORY ========================================================================== Around the world, between 40 and 50 volcanoes are currently erupting
or in states of unrest, and hundreds of millions of people are at risk
of hazards posed by these potentially active volcanos. Yet, despite the profound hazards posed to human life and property by volcanic eruptions, humanity still cannot reliably and accurately predict them, and even when forecasts are accurately made by experts, they may not afford ample time
for people to evacuate and make emergency preparations.


========================================================================== Accurate and reliable predictions have remained an elusive target largely because volcanologists do not fully understand the natural dynamics and processes of the magma underneath a volcano before it finds its way to
the surface. Now, the results of a new study led by volcanologist Dan Rasmussen, a Peter Buck Fellow at the Smithsonian's National Museum
of Natural History, may bring experts one step closer to accurately
forecasting volcanic eruptions.

The study, published today, March 10, in the journal Science, finds that,
for the world's most common type of volcano, magma with higher water
content tends to be stored deeper in the Earth's crust. The finding
identifies what some scientists expect is the most important factor
controlling the depth at which magma is stored.

"This study connects the depth at which magma is stored to water, which
is significant because water largely initiates and fuels eruptions,"
Rasmussen said. He explained that water drives eruptions analogously to
how carbon dioxide can make a shaken-up soda bottle explode.

"With water dissolved in magma that is stored beneath a volcano, if there
is a sudden decrease in pressure, like when a shaken soda bottle cap is suddenly opened, gas bubbles form and those cause the magma to rise and
jet out the volcano, similar to when a soda shoots out of a bottle top," Rasmussen said.

"More water content in magma means more gas bubbles and potentially a
more violent eruption." "These results move us closer to understanding
the physics and conditions of magma storage beneath volcanoes, and that
is an essential ingredient for the kinds of detailed physics-based models necessary to more accurately forecast eruptions," Rasmussen said.



==========================================================================
The study was completed through new field work and lab analyses in
addition to reanalysis of existing data collected from past volcanic
eruptions tracked by the Smithsonian's Global Volcanism Program.

Rasmussen began his research in 2015 while completing his doctorate at
Columbia University's Lamont-Doherty Earth Observatory with his advisor, volcanologist Terry Plank, who suggested he pursue the still-open question
of why magma storage depth varies from one volcano to the next and what controls that depth.

Along with a team that included geophysicist Diana Roman of the Carnegie Institution for Science, Rasmussen went into the field to collect volcanic material from eight volcanoes located in the rugged and remote Aleutian
Islands of Alaska.

The researchers focused on a particular geological setting when selecting volcanoes for this study: so-called arc volcanoes that occur at the intersection of two converging tectonic plates. Arc volcanoes, like those
found in the Aleutians, are the most numerous type of volcano on Earth
and comprise the entirety of the infamous "Ring of Fire" encircling
the Pacific Plate, making them the most obvious target for improving
predictive capacities.

Using ships and helicopters, the team collected bits of volcanic ash
from these eight volcanoes amid rough seas and, on the island of Unimak,
the threat of giant brown bears. Volcanic ash was the primary target of
the expedition because it can contain green crystals made of olivine --
each one with a diameter of about 1 millimeter, about the thickness of
a plastic ID card.



========================================================================== Underground, these olivine crystals sometimes trap tiny bits of magma
when they form. After an eruption sends these special olivine crystals
to Earth's surface, the magma inside them cools and becomes glass. By
analyzing the chemical composition of these miniscule pieces of cooled
magma from the inside of a volcano, the researchers were able to estimate
the magma's water content.

After estimating the water content from the entrapped pieces of magma
collected from six of the eight Aleutian volcanoes, the team then combined those data with other estimates of magmatic water content taken from
the scientific literature for an additional 56 volcanoes from around
the world. The final list of estimated magmatic water content spanned
3,856 individual samples from 62 volcanoes.

To examine the relationship between the estimated water content of these
magma reservoirs and their respective storage depths, the researchers
scoured the scientific literature and created an accompanying list of
331 depth estimates for 112 volcanoes.

Rasmussen said the Smithsonian's Global Volcanism Program's database
"was key in compiling these lists because it's a really good resource
for eruption history, and we only wanted to consider volcanoes that had recently erupted." Rasmussen and the research team focused on recent
eruptions because magma reservoirs do not appear to move a lot following
an eruption, and so any estimates of depth or water content that were
made using recently erupted material have the highest likelihood of
accurately reflecting the current state of the volcano's magma reservoir.

After years of field work, geochemical analysis and literature review,
the team was able to plot the estimated magma storage depths for 28
volcanoes from around the world against their respective estimated
magmatic water contents.

The results were strikingly clear: a magma reservoir's water content
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 merely correlating to it. The team
showed this causal relationship by detecting the presence of chemical
tracers associated with the formation of water-containing magmas in
Earth's mantle.

"If storage depth determined water content in magma, it could still create
the correlation between water content and depth that we observed, but
it wouldn't produce the chemical tracers of the magma's initial water
content that we found," Rasmussen said.

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 the water mixed in with the magma forms bubbles of
gas. When magma rising through the Earth's crust begins to degas, it
becomes more viscous, which the researchers suggest causes the magma's
ascent to slow and stall.

The evidence that water content largely controls magma storage depth
overturns the most widely accepted explanation in the field today,
which contends that magma rises through cracks in Earth's crust because
the molten rock is more buoyant than the surrounding crust, settling at
its storage depth because it reaches neutral buoyancy where magma is no
more buoyant than its surroundings.

Rasmussen said 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
larger question looms: "If magma water content controls magma storage
depth, what controls magma water content?" Funding and support for
this research were provided by the Smithsonian, the National Science Foundation, the Community Foundation for Southwest Washington and the
U.S. Geological Survey.


========================================================================== Story Source: Materials provided by Smithsonian. Note: Content may be
edited for style and length.


========================================================================== Journal Reference:
1. Daniel J. Rasmussen, Terry A. Plank, Diana C. Roman, Mindy
M. Zimmer.

Magmatic water content controls the pre-eruptive depth of arc
magmas.

Science, 2022; 375 (6585): 1169 DOI: 10.1126/science.abm5174 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220310143705.htm

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