On August 19th of 2022 I was checking for signs of ground deformation at several volcanoes
in the Philippines for my weekly news segment.
While performing this task using satellite datasets, I noted a potential uplift signature
coming from a regional volcano whose alert level was not at a raised status, Mount Mayon,
so I made a post about it.
It later turned out that this uplift signal was indeed real, and since Phivolcs had access
to more thorough data, they subsequently raised Mayon's alert level from 0 to 1 on August
21st of 2022.
This change in alert level carried with it a 6 kilometer exclusion zone around its summit
meaning residents should not enter this radius due to the threat of potential rockfalls and
unexpected phreatic eruptions.
Additionally, Phivolcs stated that the lava dome at the top of Mayon has grown by 40,000
cubic meters between January of 2021 and August of 2022.
For context, the Mayon stratovolcano is located on the southeastern edge of the island of
Luzon in the Philippines where it is centered 14 kilometers north-northwest of Legazpi City.
Mayon is one of the most symmetrical volcanoes on the planet, owing in part to its young
age as it has only been erupting intermittently for around 20,000 years.
Since 1900, Mount Mayon has been the most active volcano in the Philippines, with 27
eruptions during this time period.
Its eruptions have consisted of a wide range of activity, including strombolian eruptions
which eject bombs of lava around its summit, vulcanian eruptions which consist of intermittent
larger scale explosions, and deadly plinian eruptions with sustained towering eruption
columns.
Two such large scale plinian eruptions occurred in 1814 and 1897, the latter of which ejected
bombs of lava 10 kilometers away from its summit into the city of Santo Domingo.
However, historically the most dangerous aspect of Mayon's eruptions have been its pyroclastic
flows.
Pyroclastic flows are currents of volcanic gasses and superheated rock fragments which
generally hug the ground and move at speeds of up to 720 kilometers per hour, and have
historically travelled distances on the order of several kilometers.
For example, one larger than average pyroclastic flow in 1993 traveled approximately 6 kilometers
distant.
The current exclusion zone is in place as it represents the longest a pyroclastic flow
would be expected to travel if a phreatic eruption occurred without warning, in a similar
manner to Bulusan's unexpected eruption earlier this year when its alert level was 0.
At a volcano, uplift typically occurs due to an intrusion of magma, which pushes up
overlying ground.
However, it can also occur due to movement or increases in abundance or pressure of gas
or fluids within a volcano's hydrothermal system.
In terms of ground deformation, Mayon's edifice has been the site of minor uplift since April
of 2022.
However, if the cause of this unrest was an intrusion of magma, you would typically expect
the abundance of volcanic gasses at its summit to dramatically increase.
While sulfur dioxide emissions at Mayon's summit have increased slightly, they are still
within normal levels.
In my opinion, I believe that instead this latest unrest was triggered by increasing
gas pressure in the volcano's magma conduit.
My hypothesis is supported by the fact that Mayon's lava dome has grown by 40,000 cubic
meters in the last 19 months, which was not a symptom of ground uplift.
During Mayon's last eruption from 2018 to 2019, viscous lava solidified on the surface
in the shape of a dome while part of this material cooled within its magma conduit.
Some of this material has since been extruded despite the fact that the volcano is not erupting,
as increasing gas pressure in the conduit has pushed the overlying cooled lava upwards,
causing the dome to grow almost as if it was erupting.
While something has clearly changed underneath Mayon, which could all be occurring due to
the movement of magma, there are currently no prominent signs that this volcano will
erupt anytime soon.
However, this could change if Mayon produces higher rates of ground uplift, higher levels
of volcanic gasses such as sulfur dioxide, or an increasing number of volcanic earthquakes.
And, a small phreatic eruption does have an increased likelihood of occurring while the
volcano's unrest continues.
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