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.

View image | gettyimages.com

View image | gettyimages.com

Mini series on volcano hazards - part III: More on flows

As we learned last time, if an ash cloud becomes to heavy it can collapse and generate a pyroclastic flow. Those flows are not the only ones that happen during or after volcanic eruptions.

An obvious one are lava flows. Depending on the type of volcano, lava can be sticky or runny. The sticky lava tends to be able to store more pressure, and erupt more violently when it finally does. That's what happened e.g. when Mount St. Helens erupted on May 18, 1980. More runny lava tends to erupt less explosively, instead we say the eruptions are "effusive". Of course, as always, there are exceptions to those rules, but it's a good big picture way to think about different styles of eruptions. During effusive eruptions, runny lava either just trickles out of a vent, or sometimes fountains out of fissures. That looks just like a fountain in the park, but with lava instead of water. Here's a video from Kilauea on Hawai`i, you can see lava fountaining out of a fissure, and then - curiously - disappearing into a crack in the ground.

Lava flows are a hazard, mainly because there's not that much that you can do when one shows up in your backyard, like in the photo below. Luckily, at least they're usually quite slow, so you should be able to run (or even walk) away and save yourself.

Lava flow in Kalapana, a now mostly abandoned village on the Big Island of Hawai`i (photo:USGS/Wikimedia Commons)

So that's lava flows. But did I mention mudflows, so-called lahars? Those guys can be quite dangerous too. But what are they? Imagine an explosive eruption with a pyroclastic flow. That kind of eruption often happens on steep, high volcanoes which in turn have snow and ice covering them, sometimes all year round. We learned that pyroclastic flows are really hot, right? What happens to some (or all) of the snow and ice when it gets hit by a pyroclastic flow? It melts. Sometimes a lot. So now we have a lahar - a hot mix of ash, lapilli, bombs, and meltwater rushing down the mountain, sometimes as fast as a car. During that process, lahars often take out trees and other "obstacles", and the more material they carry the more obstacles they can take out, like a tsunami on land. That's exactly what happened in 1985 at Nevado del Ruiz, Colombia. The lahar that was caused by a relatively small explosive eruption was so powerful rushing down into the country surrounding the volcano that it killed over 23,000 people, and left another 10,000 injured and/or homeless.

The same thing can happen long after an eruption has stopped. After the famous eruption of Mount Pinatubo in the Philippines in 1991, not snow and ice but heavy rain started to generate mudflows by mixing with ash on the slopes of the mountain. 

Below an image of a house buried by a lahar. Imagine how powerful the flow must have been!

House buried during lahar, Chaiten, Chile, Dec 2009 (photo: Photovolcanica/Richard Roscoe)

Let's keep in mind that these flows can happen a long time after a volcano has stopped erupting, as long as there is enough loose material on its slopes. Now we need to monitor not only the volcano, but also keep an eye on the weather to make sure we're covering all our bases. Luckily, some smart engineering can help us against this hazard. In some places, e.g. at Sakurajima in Japan, they constructed large, concrete flow channels and dams (sabos) to direct lahars away from villages. Even though sabos can't provide a guarantee that a lahar won't sweep away your house, they're a good start at reducing the risk linked to this particular type of volcanic hazard.