The silent build-up to a super-eruption
Geologists have developed a technique that makes it possible to estimate
the maximum size of a future super-eruption of Toba volcano in Sumatra.

Date:
November 1, 2021
Source:
Universite' de Gene`ve
Summary:
It is estimated that about 5-10 volcanoes worldwide are capable
of producing a super-eruption that could catastrophically affect
global climate. One of these volcanoes hides below the waters of
Lake Toba in Sumatra and has caused two super-eruptions in the
last one million years.

But when will the next one be? Will there be any warning signs? To
answer these questions, an international team of geologists
developed an analysis of the levels of uranium and lead in zircons
-- a mineral typically found in explosive volcanic eruptions --
to determine how long it took the volcano to prepare for its
super-eruptions.



FULL STORY ==========================================================================
It is estimated that about 5-10 volcanoes worldwide are capable of
producing a super-eruption that could catastrophically affect global
climate. One of these volcanoes hides below the waters of Lake Toba
in Sumatra and has caused two super-eruptions in the last one million
year. But when will the next one be? Will there be any warning signs? To answer these questions, an international team of geologists led from the University of Geneva (UNIGE), Switzerland, and Peking University, China, developed an analysis of the levels of uranium and lead in zircons -- a
mineral typically found in explosive volcanic eruptions - - to determine
how long it took the volcano to prepare for its super-eruptions.

Unfortunately, these results, published in the journal Proceedings of
the National Academy of Sciences (PNAS), refute the notion that unusual geological signs would herald an imminent super-eruption. Instead, the
magma silently accumulated in the magma reservoir until these massive explosions occur.


==========================================================================
The Toba volcano in Sumatra caused two of the largest eruptions known on
the Earth: the first 840,000 years ago, the second 75,000 years ago, each measuring about 2,800 km3, enough to blanket the whole of Switzerland
with 7 cm thick of ash, and representing 70,000 times the amount of
magma erupted to this moment by the ongoing La Palma eruption. Two other smaller eruptions took place, one 1.4 million years ago and the other
500,000 years ago.

Geologists from UNIGE and Peking University are interested in the
Toba volcano because there is no historical record of human response
to a super-eruption of the size that it produced in the past. Such
an event would affect the global climate and pose numerous problems, particularly in terms of food supply, not to mention the migration
of populations. "Toba volcano forms a caldera, meaning that previous
eruptions have created a large depression that is occupied today by
meteoric water," explains Luca Caricchi, professor at the Department
of Earth Sciences at the UNIGE Faculty of Science and co-author of
the study. At the centre of the lake is an island that raised from
the water because of the push of the magma injected in the subvolcanic reservoir. "We can see that this island is gradually increasing in height, indicating that the volcano is active and that magma is accumulating underneath," says Ping-Ping Liu, a professor at the Faculty of Earth and
Space Sciences of Peking University and leading author of the article. But
are we close to the next super-eruption? Measuring uranium and lead in
zircon Zircon is a mineral that is found in the products of explosive
volcanic eruptions. "One of its characteristics is that it takes uranium
within its structure," explains Ping-Ping Liu. Over time, the uranium
decays into lead.

"So by measuring the amount of uranium and lead in zircon with a
mass spectrometer, we can determine its age," says the geologist. The scientists determined the age of a large number of zircons extracted
from the products of different eruptions: the youngest zircon provides information on the date of the eruption and the older zircons reveal
the history of magma accumulation preceding the super-eruptions.

"The first super-eruption occurred around 840,000 years ago after
1.4 million years of magma input, whereas magma fed the second
super-eruption at 75,000 years accumulated only in 600,000 years,"
notes Luca Caricchi. Why was the time of magma accumulation halved even
if the two super-eruptions were of the same size? "This is linked to the progressive increase of the temperature of the continental crust in which Toba's magma reservoir is assembled," explains Ping- Ping Liu. The input
of magma has gradually heated the surrounding continental crust, which
makes the magma cool slower. "This is a 'vicious circle' of eruptions:
the more the magma heats the crust, the slower the magma cools and the
faster the rate of magma accumulation becomes," she says. The result is
that super-eruptions can become more frequent in time.

Estimating the rate of magma accumulation to anticipate the size of the
next super-eruption This technique, based on zircon geochronology,
can also be used to estimate the rate of magma input in a magma
reservoir. "Today, we estimate that about 320 km3 of magma could be ready
to erupt within the reservoir of Toba volcano," says Luca Caricchi. If
such an eruption would occur now, this would be a very catastrophic event
that strongly affect not only the highly populated island of Sumatra but
also the global environment. Geologists have estimated that currently
about 4 km3 of eruptible magma is accumulating within Toba's magma
reservoir every thousand years and that this rate was rather stable
throughout its eruptive history. "The next super-eruption of the size
of the last two would therefore take place in about 600,000 years," he continues. This does not rule out that smaller eruptions could occur in
the meantime.

This innovative method can be applied to any other volcano
globally and could serve to identify which volcano is closest
to a super-eruption. "This is a great advance, because with few
super-eruptions in the last 2 million years, it is not possible for us
to obtain statistically significant values for the frequency of these catastrophic events at a global scale," explains Ping-Ping Liu. "Our study
also shows that no extreme events occur before a super- eruption. This
suggests that signs of an impending super-eruption such as a significant increase in earthquakes or rapid ground uplift, might not be as obvious
as pictured in disaster movies by the film industry. At Toba volcano, everything is happening silently underground, and the analysis of the
zircons now gives us an idea of what is to come," concludes Luca Caricchi.

========================================================================== Story Source: Materials provided by Universite'_de_Gene`ve. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Ping-Ping Liu, Luca Caricchi, Sun-Lin Chung, Xian-Hua Li, Qiu-Li
Li, Mei-
Fu Zhou, Yu-Ming Lai, Azman A. Ghani, Theodora Sihotang, Tom E.

Sheldrake, Guy Simpson. Growth and thermal maturation of the Toba
magma reservoir. Proceedings of the National Academy of Sciences,
2021; 118 (45): e2101695118 DOI: 10.1073/pnas.2101695118 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211101154803.htm

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