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.

Mini series on volcano hazards: Ash and more

Since I was talking about eruption forecasting in the last post I think it's time to talk a bit about why we even care. Ok, we all know that volcanic eruptions can be dangerous, and that people like me are trying to understand them better, but what specifically can be a hazard during or after an eruption?

This is going to be a mini series - each post will cover a new hazard. So let's start with a very obvious one: Volcanic ash, lapilli, and bombs. What do these terms mean?

Explosive eruptions usually send pieces of rock into the air. All of the pieces smaller than 2 mm diameter are called ash. Everything between 2 mm and 6.4 cm is called lapilli, and everything larger than that is called volcanic bombs. Look at the photo below to see an explosion with a bunch of ash and some really large bombs.

The smaller the piece the further it can get away from the volcano - either because of the explosive power of the eruption, or because it gets carried away by wind in the atmosphere. Bombs are really dangerous when you're close to volcanic eruptions - it's probably not very healthy to get hit in the head with a 10 cm or so potentially hot rock that comes flying through the air. 

Explosive eruption at Sakurajima Volcano, Japan, Jul 2013. You can see an ash cloud rising. Can you spot the bombs at the bottom right of the ash cloud? Look at the size of the trees and the mountain, and estimate how large the bombs must be. Definitely wouldn't wanna get too close! Photo: K.Unglert

Ash is obviously also a problem close to the volcano: Imagine a huge sandstorm, but in addition particles in the ash are often also hot, and covered with acids from the gases in the eruption. Getting that in your eyes is inconvenient at best, and once you get the fine particles in your nose or lungs it only goes downhill. If you're exposed to ash from volcanic eruptions for a long time (e.g. many years living close to an erupting volcano) it can cause significant health issues. One way to make it at least a little bit better is to wear a mask that covers your face.

Unfortunately that's not the end of it. Even small layers of fine ash on e.g. air conditioning or air plane turbines cause the parts to corrode really fast - that's why airspace usually gets closed off around volcanic eruptions. Remember the eruption of Eyjafjallajökull in Iceland in 2010? Very fine ash particles got blown towards Europe and a lot of people were stranded in airports for days. 

Now imagine ash fall onto the roof of your house: Even a small layer, say 5 cm, can be really dangerous. Why? Well, ash - just like sand - is just tiny pieces of rock. If your roof is 10 m x10 m and has a 5 cm layer of ash on it that's a total volume of 5 cubic meters. Rock has a density of approximately 2600 kg per 1 cubic meter, so 2600 times 5 is? That's right, really really heavy! Even if the ash isn't as dense (and thus heavy) as solid rock, thin layers of ash add up to a heavy weight quite quickly. That makes building collapse a big danger.

Last but not least, there can be impacts on the economy. Ash fall covering crops can cause entire seasons to be without harvest, and animals don't find plants to feed from. Below is a photo of flowers covered in ash at Sakurajima Volcano, Japan, to give you an idea of what the ash can do. Now imagine thicker layers of ash from a bigger eruption!

Flowers covered in ash after a small explosive eruption at Sakurajima Volcano, Japan, Jul 2013. This was only a small eruption, so imagine what it must be like after a large explosive eruption! Photo: K.Unglert

Now we've learned about the main dangers of ash, lapilli, and bombs being thrown out of volcanic vents during an eruption. When the ash is a bit too heavy too rise, or when the ash and the bigger pieces build up over time, they can cause more hazards (pyroclastic flows and lahars), but we're gonna hear about those another time!