On April 21st 2023, at 3:11 pm local time, a change occurred at the highly active Piton
de la Fournaise volcano which had not produced an eruption since October of 2022.
A seismic crisis had begun, as a rapid intrusion of magma was most likely moving towards the
surface, generating earthquakes along the way.
However, this sudden and sustained increase of shallow earthquakes did not occur alongside
an abundance of ground deformation, as if a volcanic eruption was about to occur you
would expect the ground overlying the eventual eruption site to be pushed upwards by several
centimeters at a minimum.
However, only 69 minutes after the seismic crisis began, it ended.
Only two minor earthquakes occurred after this cessation of activity, yet this volcano
is still at a raised alert level.
The reason for this is that generally speaking Piton de la Fournaise does not produce these
strong earthquake swarms unless an eruption is about to occur in the short to mid term
future, meaning on the order of several hours to a few weeks in advance.
This is best showcased via a seismic crisis on December 4 2020 which led to an eruption
3 days later or a seismic crisis on September 7 2022 which was followed by another crisis
and eruption 12 days later.
Of course, not all of these seismic crises result in an eventual eruption, as they can
occur in isolation.
Yet, because of this sudden event, the Piton de la Fournaise volcano has a decent chance
of producing a volcanic eruption at some point in the next two weeks, aka by May 4th although
such an eruption is not guaranteed.
Thus, on my 0 to 10 opinion based volcanic unrest scale I am placing Piton de la Fournaise
at an 8.
For context, Piton de la Fournaise is the easternmost of two volcanoes located on the
French Island of Reunion with this entire island having a population of around 850,000,
and being 750 kilometers east of the eastern coast of Madagascar.
The westernmost Piton des Neiges volcano which is about 3 million years in age, is dormant,
possibly even extinct, and has not erupted for 12,000 years.
Piton de la Fournaise on the other hand is the younger of the two, having been erupting
at a high rate for around 500,000 years, being a large volume shield volcano.
It is the youngest expression of the Reunion Hotspot, which once created the Deccan Traps
flood basalts around the same time of the mass extinction of the non avian dinosaurs
66 million years ago, and has since tracked to the southwest, now being located underneath
the Piton de la Fournaise volcano.
This volcano contains a series of steep ravines which represent the oldest of three collapse
scars where the ancient volcano slid to the east in massive landslides.
These collapse scars formed 250,000 years ago, 65,000 years ago, and most recently in
2700 BCE.
Since then, the latest cone of the shield volcano with an 1170 meter wide summit crater
has grown within the collapse scar with eruptive products being primarily funneled to the east,
which is best shown via this diagram of 1931 to 2019 lava flows which largely resulted
from hawaiian to strombolian type eruptions.
It is quite rare for lava flows to occur via flank cinder cones on rift zones outside of
the caldera, but this has occurred 13 times in the last 2,000 years of which 11 occurred
in the last 200 years.
Flank eruptions are the most potentially worrying as people do not live within the caldera but
they do live on its edges just to the north and south of it.
Luckily, only about 6% of eruptions migrate to the flank of Piton de la Fournaise, and
if an eruption was to occur from this volcano in the next 2 weeks, it does not currently
appear likely that a flank eruption would happen.
Instead, ground deformation sensors seemingly suggest that an eruption might occur in a
1.3 square kilometer region just to the east of the central crater but then again we cannot
predict where it will erupt next with this being merely a guess.
Typical eruptions last anywhere from a few days to several months, and generally do not
produce lava flows that travel longer than 5 kilometers.
Such a small eruption, perhaps on the same order of magnitude to the September to October
2022 eruption of this volcano with its lava flows being outlined on screen might be expected
if an eruption was to occur.
Thus, I would not worry too much about the next eruption of this volcano, at least for
now.
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