Popo is up to something: during most of our stay, the volcano calmly and steadily exhaled a faint white plume of gas. At night, this plume reflected an equally faint reddish glow within the crater – a reminder of the power that lies beneath our majestic mountain. But in the last few days, the number and intensity of small explosions has increased, and the colour of the plumes changed from steam-white to ash-grey.
This development is not unusual for Popo in the last 20 years of its activity. Let’s have a look at why this happens…
Recipe for a lava dome
Since 1994, when Popo woke up after some 70 years of quiescence, magma is more or less constantly pushed towards the surface from deep down, at quite a slow speed. While the magma rises upwards, it degasses and cools down with both processes making the melt more viscous and less elastic. By the time it reaches the surface, the melt becomes basically solid.
In the beginning, the gas that escapes the magma during ascent has no problems finding its way to the atmosphere around and through the solidifying lava at the surface. At this stage, we expect to observe the type of constant degassing that we have experienced during the first week of our stay.
But as more and more of this thick melt arrives at the surface and piles up in the crater, it forms a half-sphere shaped plug (a ‘lava dome’) that effectively closes down many of the pathways through which the volcanic gasses escape: the gas is trapped close to the surface! At the same time, more gas (and more magma) is pushing upwards from deep down, which steadily increases the pressure within the volcano – until it becomes large enough for the trapped gas to fracture the lava dome and escape in an explosion.
This is what we have seen in the last couple of days. The power of the explosion can fragment parts of the lava dome to tiny pieces – this is the volcanic ash that makes the plumes turn grey.
Back to zero
Now, small explosions like this usually relieve the system only for a few hours. Within a short time, the new degassing pathways are closed by the lava, the gas is trapped once more and the game starts again. As the lava dome continues to grow, it becomes more and more effective in closing the volcanic vent, more and more gas is being trapped, and the explosions may become more and more explosive.
At some point, the pressure within the volcano rises to such high levels that when the gas finally forces its way to the surface once more, the energy release is large enough to destroy the whole lava dome! The volcano basically clears its throat and gets rid of the plug, throwing big blocks of the dome out of the crater and onto the volcano’s flanks.
These are the largest eruptions we have seen in the last 20 years, and though they don’t usually represent serious danger to people living around the volcano, they often result in ashfall in closeby villages and cities. Here is a video of a large eruption that occurred in 2013:
After such dome-destroying eruptions, the cycle starts again: magma ascends, degasses, becomes more viscous, forms a lava dome that plugs the vent more and more effectively, until it is being destroyed again. Since 1994, we have observed about 60 of these dome-building and destruction cycles, and there is no sign of Popo planning to stop this behaviour!
A look into the future
There are two ways in which Popo’s behaviour might change in the future: either the rate of magma rising from deep down decreases, which would result in the volcano falling quiet again, potentially with some non-explosive degassing. Or, more gas-rich magma is being fed into the volcano from beneath, which might result in a sudden, large increase of pressure and an eruption of a much bigger dimension than what we saw in the last 20 years… A scary prospect!
It is exactly these potential types of transitions in eruptive behaviour that we are studying, trying to improve our understanding of how and how fast these might happen. Especially the latter question – how fast? – will be crucial in order for us to get better at predicting large eruptions and mitigating the associated risks.
With our fieldwork this year, we have been able to collect more important samples and information about Popo’s stratigraphy, which will help us to address the above questions. Despite the storm, it has been a successful season, and we are pumped to get started working with the new samples.
Last day #Popo2016 packing and a small explosion @NHM_Science @NHM_London @NERCscience @royalsociety pic.twitter.com/FGL2l0t6x2
— Dr Chiara Petrone (@chpet) March 18, 2016
Redux
That’s it for this year – we have returned to London. And what do we have to see just as we are back? Popo’s activity has further increased – the explosions have become more violent, with larger plumes transporting more ash! The explosion of the 28 March was even large enough to make it into the Daily Mail and other international media.
La impresionante explosión de esta tarde del @Popocatepetl_MX vista desde la #SkyAlertCam de Amecameca #EdoMex pic.twitter.com/JaM9CMAmH7
— Webcams de México (@webcamsdemexico) March 30, 2016
So Popo was up to something indeed – it’s just waited until we had left, this mean volcano! The development in the upcoming days and weeks will be very interesting to follow, and I once more encourage you to have an occasional look at one of Popo’s webcams. At the moment, the volcano continues to have bigger, ash-laden explosions, so there is always something to see on the webcams!
… Except there are no scientists from the Museum scrambling its flanks for the next few months …
Explosión del @Popocatepetl_MX 16:41 h.
Explosiones son por destrucción del domo de lava formado meses atrás. pic.twitter.com/8TLwLWyT8T— SkyAlert (@SkyAlertMx) March 29, 2016