Boom! Analysing explosions at Santiaguito, Guatemala

If you have been following myself or Liverpool Volcanology, you will probably know that the group has been carrying out an extensive amount of fieldwork around Santiaguito volcano in Guatemala. You can read about those trips in previous posts here, by Felix on GeoLog, and an amazing article by Nathanial Hoffman. For this post, I'll be writing about the first of hopefully many articles to come out of our efforts, published in Geophysical Research Letters.

A gas-and-ash plume rises from the Caliente vent at Santiaguito in November 2014.

The article, put together by Silvio de Angelis, details how we used infrasound and infrared thermal data to characterise small explosions at Santiaguito. This analysis was also complemented by measurements of ash collected after the explosions.  What we wanted to know was: how much ash is in the explosion plume, and how fast and high is it being injected into the atmosphere?

Silvio, Armando and Andreas deploying a station with one of our infrasound microphones. The active vent is only a few hundred metres away in the background.

Back in November 2014, we successfully deployed four infrasound microphones around the active Caliente vent at Santiaguito. Using this data, we can carry out what's called an 'Acoustic multipole source inversion', which sounds complicated. In a nutshell, it roughly models what happened at the vent to produce the infrasound signals we measured at every infrasound instrument. With this method, we can get a rough guess of how much and how fast material was leaving the vent during the explosion.

What an explosion at Santiaguito looks like through a thermal infrared camera. You can clearly see the hot vent with an explosion plume on the right, and the 2014 lava flow on the left. This image was taken on 29 November 2014. Thanks to Anthony Lamur for the image!

Our next step was to combine these measurements with thermal images collected during the same fieldwork as deployment. From a stunning vantage point on Santa Maria volcano, the 'mother' of Santiaguito, we can look directly into the active vent. This meant that we could record thermal images of volcanic explosions from the moment they begin until the moment the ash begins floating away (e.g. the image above). Using the images we collected, we calculated how fast the explosion plume rises, and the volume of the plume. Combined with the erupted mass measured using infrasound, we can then estimate the bulk density of the plume. Taking it a step further, we had also calculated the density of the ash collected after the explosions, which meant we could estimate how much ash was in the plume itself. Our results showed that the explosion plumes were extremely gas-rich, with only a minor fraction of ash: these explosions did not break apart a lot of magma.

The view from Santa Maria shortly after an explosion from Santiaguito.

So what are the implications of this article? Firstly, we have added to a long known fact that volcanic explosions contain a variable amount of gas and ash, which presents a challenge for those assessing the hazard of an explosion plume. The methodology we outline in the article holds the potential for assessing volcanic ash plumes using instruments already accessible to volcano observatories worldwide. Secondly, and possibly more importantly, we further demonstrate the power of combining measurements from several different aspects of a volcanic explosion to help understand their mechanics.

If you want to read the article you can find it over at Geophysical Research Letters, or contact me or Silvio for a copy.

I should stress that this method was applied to small explosions that occurred in December 2014. Explosions at Santiaguito have turned somewhat more powerful recently. For that reason, we are heading back to Guatemala soon to make sure our network is okay, deploy a few more instruments, and record more thermal images. Keep your eyes peeled on my twitter or on this blog for more details on that!