Is Vesuvius taking an extended siesta?

Date:
January 20, 2022
Source:
ETH Zurich
Summary:
Located near Naples, Italy, Vesuvius last had a violent eruption
in 1944, towards the end of the Second World War. It could be a few
hundred years before another dangerous, explosive eruption occurs,
suggests a new study by volcano experts.



FULL STORY ========================================================================== Vesuvius is one of Europe's most dangerous volcanoes. More than three
million people live in its immediate vicinity, and in historical and prehistoric times, there were explosive eruptions that destroyed entire settlements and towns in the area.


==========================================================================
So, the pressing question is: When will Vesuvius erupt again and how
strong could the eruption be? To answer this question, a research group
at ETH Zurich, in collaboration with researchers from Italy, has taken
a close look at the four largest eruptions of Vesuvius over the last
10,000 years so that they can better assess whether a dangerous event
might be expected in the foreseeable future.

The four eruptions studied include the Avellino eruption of 3,950 years
ago, which is considered a possible "worst case scenario" for future
eruptions, and the eruption of AD 79 that buried the Roman cities of
Pompeii and Herculaneum.

The latter was documented by the Roman writer Pliny the Younger,
and so all eruptions of this type are referred to as "Plinian"
eruptions. Further, the volcanologists studied eruptions of 472 AD and
8890 y BP. The sub- ​Plinian eruption of AD 472 is the smallest
of the investigated eruptions but still similar in size compared to the
recent Tonga eruption.

Garnets allow precise dating In their study, which has just been
published in the journal Science Advances, the researchers working
with lead author Jo"rn-​Frederik Wotzlaw and ETH Zurich Professor
Olivier Bachmann determined the age of garnet crystals present in the
volcanic deposits. This mineral grows from the magma as it is stored in
the magma chamber in the upper crust beneath Vesuvius. Knowing the age
of these minerals makes it possible to infer how long magma resided in
this chamber before the volcano spewed it out.



========================================================================== Garnet is an unusual choice for determining the age of volcanic ejecta.

Researchers typically use zircons, which are tiny accessory minerals found
in many igneous rocks. Magma from Vesuvius, however, is too alkaline to crystallize zircons, but it is rich in garnet.

To determine the age of the garnets, the researchers used the
radioactive elements uranium and thorium. The crystal structure of garnet incorporates both in small but measurable quantities, with a preference
for uranium. Using the ratio of the isotopes uranium-​238 to thorium-​230, the researchers can calculate the crystallization
age of the minerals.

The garnets for this study all came from material that the ETH team
collected on site with the help of colleagues from the Universities of
Milan and Bari.

For this purpose, they searched for corresponding sites where the
volcanic deposits from the four eruptions mentioned above are exposed
at the surface and are accessible for sampling.

Intervals become shorter By using the crystallization ages of garnets, the researchers can now show that the most explosive magma type at Vesuvius
(so called "phonolitic" magma) is stored in a reservoir in the upper
crust for several thousand years before the influx of more primitive,
and hotter, magma from the lower crust triggers an eruption.



==========================================================================
For the two prehistoric events, the researchers determined that the
phonolitic magma resided in the chamber for about 5,000 years. Before
the eruptions in the historical period, it was stored in this reservoir
for only about 1,000 years.

For all the eruptions, the residence time of the phonolitic magma in
the upper crustal chamber coincides with Vesuvius' quiescent periods.

"We think it's likely that a large body of phonolitic magma in the upper
crust blocked the upwelling of more primitive, hotter magma from deeper reservoirs," Bachmann says. "Vesuvius has quite a complicated plumbing
system," he adds with a grin.

Below the volcano are several magma chambers connected by a system
of pipes.

The top chamber, which is critical for the eruptions, fills with
magma from one of the lower chambers in a fairly short time. In
this colder environment, the magma cools and crystallizes, leading
to chemical changes of the residual melt (a process called "magmatic differentiation"). Experts call the "differentiated" magma of Vesuvius phonolite. At some point (probably at relatively regular intervals), more primitive, or "mafic" magma flows into the upper chamber from greater
depths. This recharge leads to a pressure rise within the chamber,
which can force the phonolitic magma upwards, potentially all the way
to the surface, starting an eruption.

A reservoir of phonolitic magma appears to have almost always existed
beneath Vesuvius for the last 10'000 years. However, the question is
whether one today that could feed a dangerous eruption like the one of
3,950 years ago or the one of AD 79.

Magma build-up rather unlikely Seismic surveys indicate that there is
indeed a reservoir at a depth of about six to eight kilometres underneath Vesuvius. However, the composition of the magma it contains -- i.e.,
whether it is phonolitic, or more mafic -- cannot be determined using
seismic technology. But since Vesuvius has been producing mostly mafic
magma since 1631, researchers believe it is unlikely that differentiated phonolite is currently accumulating. "The last major eruption in 1944 is
now nearly 80 years ago, which may well be the beginning of a prolonged quiescent period during which differentiated magma can accumulate. Still,
a dangerous eruption comparable to the one in AD 79 probably needs the quiescent period to last much longer," Wotzlaw says.

If predominantly mafic magma is ejected in the coming decades, this
could indicate that the magma body detected by seismic surveys is not
composed of differentiated magma and that none is currently present
beneath Vesuvius.

"That's why we think it's more likely that a large, explosive eruption of Vesuvius would occur only after a quiescent period lasting for centuries," Bachmann says. Wotzlaw adds: "However, smaller but still very dangerous eruptions like the one in 1944 or even the one in 1631 can occur after
shorter periods of quiescence. Accurate forecasting of size and style of volcanic eruptions is so far not possible. However, the reawakening of the magma reservoirs beneath volcanoes are now recognizable by monitoring."
Close monitoring To avoid any nasty surprises, Vesuvius and its activity, together with its big brother to the west, the Phlegraean Fields, are
monitored around the clock. For example, Italy's National Institute of Geophysics and Volcanology measures every earthquake around the volcanoes, analyses gases emitted from fumaroles and observes ground deformation,
which are indicators of underground activity.

There is also an emergency plan outlining how to evacuate the greater
Naples area should surveillance conclude that an eruption is imminent.

========================================================================== Story Source: Materials provided by ETH_Zurich. Original written by
Peter Ru"egg. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Jo"rn-Frederik Wotzlaw, Lena Bastian, Marcel Guillong, Francesca
Forni,
Oscar Laurent, Julia Neukampf, Roberto Sulpizio, Cyril
Chelle-Michou, Olivier Bachmann. Garnet petrochronology
reveals the lifetime and dynamics of phonolitic magma
chambers at Somma-Vesuvius. Science Advances, 2022; 8 (2) DOI:
10.1126/sciadv.abk2184 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220120125422.htm

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