Sunday, 26 April 2015

Calbuco Volcano - a beginner's guide to its hazards

I'm sure you all have read tons about Calbuco Volcano now, so I'm not going to bore you with the details. Hopefully you've seen some of the stunning photos that have emerged, e.g., the ones on the Flickr stream by the Chilean Geological Service.
I quickly want to talk about hazards though. This volcano has quite the selection of hazards for you to choose from. The explosive eruptions have sent ash more than 15 km high into the air (click on the orange links to learn more about each hazard). This ash is covering a lot of infrastructure, property, and destroying crops. Most of it is being blown to the North-East at this point. With eruptions this explosive there will also be big blocks of rock being thrown out of the volcano, sometimes landing several kilometers away!
If an ash cloud collapses it can produce a pyroclastic flow. The deposits from old eruptions at Calbuco show that pyroclastic flows in the past have reached as far as Puerto Montt, a city with around 200,000 inhabitants around 30 km away from the mountain. For now I would guess that these pyroclastic flows are more likely to go towards the North-East, following the direction of the wind, but there is no way to know for sure, especially if the weather conditions change.
In addition, if ash settles on the mountain and is mixed with water (for example from snow on the top, of if there is a bit more rain over the next few days or weeks), big lahars (mudflows, mixtures of ash, dirt, water, snow, and debris such as trees etc.) can happen and travel down the valleys of some of the many rivers flowing down the slopes of the mountains. These flows can be incredibly powerful and destructive. Lahars can also reach tens of kilometers, so the 20 km exclusion zone they've put up makes a lot of sense.
In addition, some lava fountaining has been seen at Calbuco after the initial, more explosive phase that sent the ash into the skies. This means that some small lava flows can occur on the mountain. And of course, the gases that accompany volcanic eruptions can be quite dangerous too, if you get too close. Better stay at a safe distance. That way it's also much easier to take photos of the entire ash cloud!
It currently looks like the activity is getting a bit weaker: Whereas the Chilean Geological Survey observed more than 1,500 earthquakes between April 22-23, this number went down to just over 1,000 between April 23-24, just over 500 one day later, and to around 300 today. Unfortunately it's very difficult to know whether this number is going to increase again, which could mean another pulse of eruptive activity. For now all we can do is to closely monitor and to keep away from the mountain as much as possible.