How to better identify dangerous volcanoes

Date:
October 12, 2021
Source:
ETH Zurich
Summary:
The more water is dissolved in the magma, the greater the risk that
a volcano will explode. A new study now shows that this simple
rule is only partially true. Paradoxically, high water content
significantly reduces the risk of explosion.



FULL STORY ========================================================================== Volcanologists have long been troubled by two questions: When exactly
will a volcano erupt next? And how will that eruption unfold? Will the
lava flow down the mountain as a viscous paste, or will the volcano
explosively drive a cloud of ash kilometres up into the atmosphere?

==========================================================================
The first question of "when" can now be answered relatively precisely,
explains Olivier Bachmann, Professor of Magmatic Petrology at ETH
Zurich. He points to monitoring data from the Canary Island of La Palma,
where the Cumbre Vieja volcano recently emitted a lava flow that poured
down to the sea. Using seismic data, the experts were able to track the
rise of the lava in real time, so to speak, and predict the eruption to
within a few days.

Unpredictable forces of nature The "how," on the other hand, is still
a major headache for volcanologists.

Volcanoes on islands such as La Palma or Hawaii are known to be unlikely
to produce huge explosions. But this question is much more difficult to
answer for the large volcanoes located along subduction zones, such as
those found in the Andes, on the US West Coast, in Japan, Indonesia,
or in Italy and Greece. This is because all these volcanoes can erupt
in many different ways, with no way to predict which will occur.

To better understand how a volcano erupts, in recent years many
researchers have focused on what happens in the volcanic conduit. It has
been known for some time that the dissolved gases in the magma, which then emerges as lava at the Earth's surface, are an important factor. If there
are large quantities of dissolved gases in the magma, gas bubbles form
in response to the decrease in pressure as the magma rises up through the conduit, similar to what happens in a shaken champagne bottle. These gas bubbles, if they cannot escape, then lead to an explosive eruption. In contrast, a magma containing little dissolved gas flows gently out of
the conduit and is therefore much less dangerous for the surrounding area.

What happens in the run-up? Bachmann and his postdoctoral researcher Răzvan-Gabriel Popa have now focused on the magma chamber in a
new study they recently published in the journal Nature Geoscience. In
an extensive literature study, they analysed data from 245 volcanic
eruptions, reconstructing how hot the magma chamber was before the
eruption, how many solid crystals there were in the melt and how high the dissolved water content was. This last factor is particularly important, because the dissolved water later forms the infamous gas bubbles during
the magma's ascent, turning the volcano into a champagne bottle that
was too quickly uncorked.



==========================================================================
The data initially confirmed the existing doctrine: if the magma contains little water, the risk of an explosive eruption is low. The risk is also
low if the magma already contains many crystals. This is because these
ensure the formation of gas channels in the conduit through which the
gas can easily escape, Bachmann explains. In the case of magma with few crystals and a water content of more than 3.5 percent, on the other hand,
the risk of an explosive eruption is very high -- just as the prevailing doctrine predicts.

What surprised Bachmann and Popa, however, was that the picture changes
again with high water content: if there is more than about 5.5 percent
water in the magma, the risk of an explosive eruption drops markedly,
even though many gas bubbles can certainly form as the lava rises. "So
there's a clearly defined area of risk that we need to focus on,"
Bachmann explains.

Gases as a buffer The two volcanologists explain their new finding by
way of two effects, all related to the very high water content that
causes gas bubbles to form not only in the conduit, but also down in the
magma chamber. First, the many gas bubbles link up early on, at great
depth, to form channels in the conduit, making it easier for the gas to
escape. The gas can then leak into the atmosphere without any explosive
effect. Second, the gas bubbles present in the magma chamber delay the
eruption of the volcano and thus reduce the risk of an explosion.

"Before a volcano erupts, hot magma rises from great depths and enters
the subvolcanic chamber of the volcano, which is located 6 to 8 kilometres below the surface, and increases the pressure there," Popa explains. "As
soon as the pressure in the magma chamber is high enough to crack the
overlying rocks, an eruption occurs." If the molten rock in the magma
chamber contains gas bubbles, these act as a buffer: they are compressed
by the material rising from below, slowing the pressure buildup in the
magma chamber. This delay gives the magma more time to absorb heat from
below, such that the lava is hotter and thus less viscous when it finally erupts. This makes it easier for the gas in the conduit to escape from
the magma without explosive side effects.



========================================================================== Lockdown opportunity These new findings make it theoretically possible to arrive at better forecasts for when to expect a dangerous explosion. The question is, how can scientists determine in advance the quantity of
gas bubble in the magma chamber and the extent to which the magma has
already crystallised? "We're currently discussing with geophysicists
which methods could be used to best record these crucial parameters,"
Bachmann says. "I think the solution is to combine different metrics
-- seismic, gravimetric, geoelectric and magnetic data, for example."
To conclude, Bachmann mentions a side aspect of the new study: "If it
weren't for the coronavirus crisis, we probably wouldn't have written
this paper," he says with a grin. "When the first lockdown meant we
suddenly couldn't go into the field or the lab, we had to rethink our
research activities at short notice. So we took the time we now had on
our hands and spent it going through the literature to verify an idea
we'd already had based on our own measurement data. We probably wouldn't
have done this time-consuming research under normal circumstances." ========================================================================== Story Source: Materials provided by ETH_Zurich. Original written by
Felix Wu"rsten. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Răzvan-Gabriel Popa, Olivier Bachmann, Christian
Huber. Explosive or
effusive style of volcanic eruption determined by magma
storage conditions. Nature Geoscience, 2021; 14 (10): 781 DOI:
10.1038/s41561- 021-00827-9 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211012112236.htm

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