Eruptions Newsletter #11 for May 23, 2025

A bit of news on my front: I am no longer writing Rocky Planet over on Discover. Long story short, I didn’t mesh with the requirements of the new ownership. Such is life! I may occasionally have articles over there and in other venues, but right now, Eruptions is the place to find me writing about, well, volcanoes.

That all being said, I am starting a voluntary subscription for the newsletter! You can still read the newsletter for free, but if you want to chip in as little as $5/month, you can pay for my coffee, newsletter hosting or my time (whichever you choose to imagine). There may be some posts that come between newsletters that will be subscribers only, but I haven’t quite worked that out just yet. Look for more about this next month.

Onto the volcanoes!

Activity News

Sakurajima (Aira), Japan

As many of you know, one of the most active volcanoes on Earth today is Sakurajima, a part of the larger Aira Caldera complex. Located in southern Japan, the cone located in the middle of Kagoshima Bay erupts almost daily, dusting Kagoshima City with ash on a regular basis. These eruptions have been occurring almost continuously since 1955 with only brief breaks of months or years all the way back to 1935.

Thankfully, most of the eruptions during this ~90 years of activity, although explosive, have not been destructive. This doesn’t mean that the Aira Caldera can be ignored as it has produced VEI 4-5 eruptions in 1914, 1779, 1471 and 764 CE. It has a track record for big blasts.

Right now, Sakurajima is pretty quiet, with little in the way of explosions over early May. However, new explosions resumed on May 15 and inflation of the volcano was noted by the Japanese Meteorological Agency (JMA). On May 18, an explosion threw blocks over 1 kilometer from the crater and sent ash ~3.2 kilometers (~10,000 feet) above sea level.

Dr. Masato Iguchi (Sakurajima Volcano Disaster Prevention Research Institute) told TBS News Dig in an interview this week that the volcano needs to be closely watched as the potential for a larger explosion is increasing. We’re not talking 1914 scale according to Dr. Iguchi, but rather just something akin to the bigger explosions during 2019-2020 at Sakurajima. JMA has the alert status for the Aira Caldera at 3 (of 5).

Ontakesan, Japan

Meanwhile, the alert status was raised to Level 2 at Ontakesan, one of the largest volcanoes in Japan. If the name seems familiar, it is likely due to the fatalities at Ontakesan in 2014 when a phreatic (steam-driven) eruption caught hikers by surprise. Over 60 people were killed mainly be flying debris and ash inhalation near the summit of the volcano.

The volcano has been mostly quiet in the intervening decade since that tragedy. Earlier this year, the JMA did briefly raise the Alert Status for this popular tourist destination to Level 2 after slight inflation was measured. However, since then, the inflation has ceased and few, if any, earthquakes were recorded, so the Alert Status has been officially dropped back to Level 1 (the lowest).

This is an excellent example of just how challenging volcano monitoring can be, especially as our ability to measure small changes increases. Just how frequently a volcano like Ontakesan experiences a little bit of inflation and exactly what is driving that inflation is not well understood. That being said, playing things safe is always the way to go when it comes to potentially hazardous or deadly situations.

Volcano Word of the Week

This week marked the 45th anniversary of the eruption of Washington’s Mount St. Helens, so this week’s Volcano Word of the Week is lahar.

A lahar is simple said, a volcanic mudflow. It comes from the Javanese language as lahars are common at volcanoes in Indonesia. Unlike many other volcanic hazards like lava flows or ash fall, a lahar can occur even if the volcano isn’t erupting. That makes monitoring for lahars extremely important in places where they can happen.

What makes a lahar different than a regular mudflow or landslide? The key is the material being moved. If the mudflow is primarily made of volcanic debris like ash and pumice, then it is a lahar. It is the mixture of copious water with this volcanic debris that generates the flow. This water might be from melted snow and ice on the volcano, crater lakes, streams flowing down the volcano, even heavy rainfall.

A lahar will move down stream channels and begin to pick up whatever happens to be in its path. It might look like a muddy river, but the material flowing down the channel (see below) during a lahar is much denser, almost like liquid concrete. This means it has a lot of strength, so it can bowl over and carry very large objects like trees, cars, houses.

Lahars don’t move as fast and aren’t hot like pyroclastic flows, so if you live downslope from a volcano, you do have a chance to get out of their way. Lahars will stay in the stream channels, so you can escape most lahars by moving uphill quickly. At the speed they move, typically nearby people have tens of minutes if not hours before a lahar will hit their location.

What is key is warning. Not all eruptions create lahars and as I mentioned, lahars can happen without an eruption. So, stream channels leading from volcanoes need to be monitored for lahars using webcams, seismometers and trip wires. This allows for early lahar detection that can be transmitted downslope so people can evacuate. Mount Rainier in Washington, a common spot for non-eruption-triggered lahars, has an elaborate lahar early warning system.

Lahars can pack a wallop to infrastructure as all the mud and debris will knock bridges and building down. Once the lahar comes to rest, the water will drain away and you’ll be left with many feet thick of what is almost now solid concrete. This means many times towns hit by lahars, like Armero in Colombia, need to be rebuilt, hopefully out of the path of future lahars.

Odds & Ends

I saw photos this week that I have never seen of the 1980 eruption of Mount St. Helens. These photos, shot from an airplane in vicinity of the volcano when the eruption began, shows how an entire side of the mountain literally exploded. You can see how quickly the pilot turns to get the heck out of there.

The mystery is who shot these pictures. Even though they are available for the public to see and were originally aired on TV stations in Seattle, no one know who took the picture or who the pilot might have been. Someone out there likely knows. [In best Robert Stack voice] Maybe its you?

Sounds of the Week

With all this talk of the 45th anniversary of the Mount St. Helens eruptions, how about a few of my favorite songs from 1980. Mind you, I was all of 3 when the eruption happened, so I wasn’t rocking out to any of these when it happened (to my knowledge).

One of my first concerts was Peter Gabriel at the long-demolished Worcester Centrum during his Us tour. Great show!

The Police were great, even if Sting sometimes sung like he thought he was from Jamaica.

Quite honestly, I have no idea what’s up with Siouxsie’s outfit in this video. Song is a banger, though.

Having grown up listening to WFNX during my formative teen years, I think I may have gotten the impression that more people heard “People Who Died” on a regular basis. It turns out this is wrong.

And as the coup de grâce, you lucky people get Motörhead playing the absolute classic “Ace of Spades” on the absolute classic British TV show The Young Ones. Honestly, I didn’t grow up in England.

Questions? Comments? Thoughts? Feel free to send me a note or follow me on Bluesky (@erikklemetti.bsky.social).