This post will be part of an exciting triple feature on Japanese volcanoes. Why Japanese volcanoes? Because I'm going to Japan! :) There is a conference coming up in July, and I am planning to spend some time afterwards to explore a bit of Japan's volcanoes. I hope I get a chance to visit Sakurajima, Aso and Fuji, all of three of which I'm going to feature here over the next few weeks!
Let's start with an old friend: Aso-san (The Japanese have the awesome tradition of politely addressing their volcanoes just like people). This is the volcano I studied for quite a while during my Master's. Aso Volcano is a large caldera on the island of Kyushu in southwestern Japan. Japan is really close to some major plate boundaries. For those who care: The Philippine Sea Plate is subducting under the Eurasian Plate at an angle, the Pacific Plate is also subducting, and the North American Plate is potentially stuck somewhere in between...... whew, it's really quite complicated! Because of all those plates pushing in different directions, Japan gets tons of earthquakes (as if we didn't know that by now...) and, of course, a bunch of active volcanism. So Aso Volcano sits somewhere behind one of the plate boundaries, and has been sitting there quite happily for more than 300,000 years. Quite happily? Well, it does get upset sometimes, and 4 of those volcanic "rants" have been pretty devastating to the island of Kyushu. Thankfully for the people there, the last big eruption was roughly 90,000 years ago. However, that doesn't mean that it is just quietly sitting there. Within the past 20 years alone there have been 11 "small" eruptions! (What does "small" mean? Click here for some info on what we call "Volcanic Explosivity Index", or VEI.)
Those smaller eruptions, of course, don't make the whole caldera (something like 18 times 25 km) blow up. Instead, in the centre of the caldera there is a bunch of little peaks, called cones. One of those, Nakadake, is the one that's causing most of the trouble nowadays. It's around 1,600 m above sea level, and has a crater lake that apparently looks really cool! Now if there are only small eruptions, why would anybody care? Why did I spent months and months studying it?
Well, one reason why we should care is, of course, the fact that it looks really cool. At a relatively low elevation it's easily accessible (especially in comparison to some other volcanoes). There is a road leading up to the crater lake, and a cable car goes all the way up too! It's a big tourist attraction on the island of Kyushu. Of course one wouldn't want a volcano blow up in the face of some peaceful tourists, so the least we can do is monitor it. Then there is the oh-so-insignificant fact that the island of Kyushu alone is home to over 13 Mio. people!! You can imagine that none of those people would be very excited if the entire caldera decided to blow up at some point out of the blue (except for volcano-crazy people like me, they might get a little bit excited...). So there you have 13 Mio. and some more good reasons to monitor and study what the mighty Aso-san is up to.
There is an additional excitement factor at this volcano: Volcanoes with crater lakes often have what we call "phreatomagmatic" eruptions! These eruptions can be pretty dangerous if you're close by, even when only small amounts of magma come up to the surface. This is because of the water, of course! Have you ever made some pasta and forgot to turn the stove down once the water was boiling? When it flows over the top of your pot and onto the hot plate it usually starts sizzling and evaporates, i.e. gets so hot that it turns into steam and goes up in the air. If hot magma comes up through a volcano neck and finds a lake at the top of that neck, something very similar happens: The water in the lake gets heated up really really fast, and can suddenly turn into steam. But the steam needs much more space than the lake water that it used to be, so if the whole process happens fast enough then the water/steam/magma mixture will push away the surrounding rocks or whatever is in the way. It creates a really big explosion! If we're really unlucky, that mixture can then flow out of the crater and down the slopes of the volcano, taking with it anything that's in the way. These strong flows of water, rocks, and "stuff" are often called lahars. They can even wipe out bridges! (Read the story about the Tangiwai disaster in New Zealand, where a passenger train was wrecked when it crossed a bridge previously damaged by a lahar and collapsed with it on Christmas Eve, 1953)
This is really crazy stuff! And of course I'm very excited to check out the crime scene! Can't wait to see the crater lake after my conference in July!
PS: When I was in New Zealand I went to check out the way the lahar came down the slopes of Ruapehu and took out that train. It must have been so exciting and scary for anybody to witness!