On February 4th, at 4:37 pm local time, the towering Mount Mayon volcano unexpectedly erupted,
producing a notable explosion and sending superheated pyroclastic flows down its
southern slopes. While this type of activity is fairly common at certain volcanoes such as Fuego
in Guatemala and sometimes Sakurajima in Japan, some observers might be perplexed as to how Mayon
actually produced an eruption. After all, it produced a five and a half month long eruption
during 2023 which ended around November 30th. No lava flows occurred since,
the amount of gas emissions Mayon produced greatly decreased, and the number of volcanic earthquakes
returned to a weekly average just barely above zero. Yet, I believe that the 249 second long
eruption which occurred was completely phreatic in nature, and the entire sequence including its
pyroclastic flows originated to an aspect of the area's weather. Specifically, heavy
rainfall. During Mayon's 2023 eruption, I had observed that days with higher levels of volcanic
earthquakes correlated strongly to days where it happened to rain. But, I also observed that
occasional ash producing phreatic explosions were seemingly linked with periods of heavy rainfall,
specifically involving days where more than 1.5 inches or 38 millimeters of rain fell.
Explosions would either follow A, within 24 hours of the heavy precipitation event, or B,
sometime between 7 and 10 days later. In the case of Mayon's February 4th 2024 explosion, scenario
B is what occurred. I can attribute it to a heavy rainfall event on January 25th, when approximately
4.5 inches or 11.4 centimeters of rain fell, representing the volcano's heaviest rainfall
in a single day in a little over 2 months. My hypothesis is that around 1.5 inches of rain
is needed to saturate the ground, causing water to pool and then seep deeper into the ground. Within
the next 7 days, groundwater from the recent rains seeps into Mayon's still hot magma conduit. While
no longer erupting, many lava flows can take several years or even decades to fully cool,
so the groundwater may be encountering hot rock at temperatures upwards of 500 degrees Celsius.
Some of the heat from this solidifying magma subsequently transfers into the water, heating it,
and causing it to flash to steam. Since the same mass of steam takes up about 1600 times the volume
of water it once was, this reaction causes an immense amount of pressure to build. Eventually,
a sufficient quantity of steam becomes pressurized just enough that it actually
fractures adjacent rock, causing it to escape via a path of least resistance which happens
to be upwards in a phreatic explosion which does not eject any fresh lava.
In other words, while it truly is possible that the latest eruption of Mayon was caused by a
fresh batch of shallow magma, its geothermal sensor readings suggest a non magmatic cause.
While more explosions could occur in the next few hours to days, it is unlikely that any will
reach the same intensity of the 4:37 pm eruption, which generated a plume of ash
that rose 1200 meters above Mayon's summit. Luckily, no one was harmed by Mayon's latest
eruption, as there is still a 6 kilometer exclusion zone in place, and the pyroclastic
flows it just produced traveled no further than 1.8 kilometers downslope. While cooling magma
retains the potential to produce similar explosions, it is unlikely that Mayon's
exclusion zone will be reduced anytime soon. Looking back at the explosion which occurred,
you can identify isolated ballistic blocks of material which were launched high into
the air before falling back to the ground. These fragments represent blasted portions
of Mayon's 2023 summit lava dome. I estimate that the singular explosion
ejected around 14,400 cubic meters of ash and pyroclastic flow deposits, meaning the February
4th explosion just barely passed the threshold to be assigned a volcanic explosivity index of a 1.
As a final note, I would like to thank my new patron John Mundy for supporting this channel!