Is the eruption of Chile’s Calbuco volcano a surprise?

It's the first major eruption in decades - but how big is big?

Philip Oyarzo Calisto

Two weeks ago, the Calbuco volcano in southern Chile erupted for the first time in decades. Its last major eruption was back in 1962, but it’s now erupted three times in eight days since April 22nd.

Everyone within 12 miles of Calbuco has been evacuated – over 4,400 people – and happily that seems to have been successful, as there have been no reports of any fatalities.

One of the lines taken up by the media around the time of the first eruption (by all accounts the strongest of the three) is that people were taken by surprise, and that got me thinking.

After 53 years without a big eruption – there was a minor one in 1972 and a burble of gas in 1996 – it’s understandable that the locals might not be expecting it. After all, a lot of them weren’t alive for the last big one.

But then again, if you look where in the world they are, no one can be too surprised. Time for a bit of context.

Where are the active volcanoes?

As part of their Global Volcanism Program, the Smithsonian National Museum of Natural History list every known, suspected and discredited volcanic eruption in history, and it’s their data that’s inside the map.

Not every volcano is shown – just those that are known to have erupted in the last 2,000 years. The darker red the blob, the more eruptions that volcano has seen.

Those reddish outlines running around the map are the tectonic plates of the Earth’s crust, and you can see quite clearly that it’s where these plates meet that volcanoes are found. That’s why we don’t get them in the UK.

Chile on the other hand, home to Calbuco, is right on a plate boundary.

How often do volcanoes erupt?

Every day. Small volcanic eruptions are happening all the time, but they aren’t particularly concerning unless you happen to have a particularly severe case of ifestíophobia.

You might well have noticed if you clicked around the map a bit that an awful lot of the eruptions listed seem to have happened in the last hundred years, and you’d be right. A little under half – 44 per cent – of all recorded eruptions in the last 2,000 years happened in the last hundred, which is really quite frightening… right up until you realise that we’re much, much better at monitoring them now.

It’s impossible to say if volcanoes are erupting more frequently now – all we know for sure is that we’re better at writing them down.

Is Calbuco a big one?

Since 1982, the strength of volcanic eruptions has been recorded on something called the Volcanic Explosivity Index (VEI). It’s pretty self-explanatory really – the scale runs from 0 to 8, with more explosive eruptions higher up the scale. Calbuco’s most recent eruption is yet to be scored (we’re sticking with 1972’s VEI 2 eruption on the map for now).

Don’t waste your time hunting around for an eight. There hasn’t been a VEI 8 magnitude eruption for 26,500 years, which is just as well seeing as the official descriptor is ‘Apocalyptic’. There are a couple of VEI 7 eruptions in there though, if you feel the urge to procrastinate a little.

The National Oceanic and Atmospheric Administration (NOAA) defines an eruption as ‘significant’ if it’s above VEI 6 in magnitude. But as you can see, there have only been 23 of those in the last 2,000 years, and if you take a look at this page you’ll find that the NOAA has recorded 638 significant eruptions in that time.

That’s because, as well a high VEI, the NOAA considers an eruption significant if it causes fatalities, more than $1 million of damage, a tsunami or is associated with an earthquake.

As we’ve already said, Calbuco hasn’t killed anybody. There’s also been no tsunami, and no associated earthquake. However, the NOAA have recorded what they call ‘moderate’ damage of between $1-5 million, which means that – whatever it’s VEI eventually comes out as – Calbuco is officially a big one.