Eruptions Newsletter #18 for September 5, 2025

So, it has been awhile!

Ever put something off so long you get to the point you start worrying you’ve put it off too long, so you don’t do it? That’s where I was. Anyway, I am hoping to get back on track now that the fall semester has begun. I am making a small schedule change — the newsletter will hopefully go out each Wednesday now (it works better with my weekly routine). It has helped that things have been relatively volcanically-quiet for the past few weeks, but still, plenty to discuss.

With that done, allons-y!

Activity News

Kīlauea, Hawai’i

The latest eruption at Kīlauea began on August 22 - the 31st of the current eruptive period that began in December 2024. This one was similar to some of the more recent eruptions, with low-level fountaining and lava flows within the Halema’uma’u Crater. We had a 1,000 foot fountain again this time, but instead of shooting lava straight up, it was an inclined fountain (see above). A long lava flow emanated from the end of the inclined fountain as well, creating a cool lava delta out in the crater (see below).

This eruption lasted ~12 hours, ending in the early morning of August 23. Based on the measurements of inflation and the behaviour across this eruptive period, the USGS Hawai’i Volcano Observatory forecasts that a new eruption might be likely at the end of this week (September 5 or thereabouts). As of the morning of September 2, small overflows (see below) began and the volcano continued to inflate at a rate that it would be surprising if any eruption didn’t start in the near future.

Well, the near future was very near, indeed. At 6:35 am (HST), fountaining began again! The inclined fountain returned, with lava reaching 150 meters (500 feet) high and shooting laterally the same. The fountaining last a little over 13 hours before the eruption ceased.

However, all the news from Kīlauea is not from the current eruption. Repairs to roads destroyed by the 2018 eruption on the far Eastern rift of the volcano have begun. The process involved excavating the pre-existing roads from the lava where possible and will take until at least 2027.

However, parts of the lava delta built on the former site of Vacationland during the 2018 lava flows are showing signs of instability. This is a fairly common occurrence for these lava flow deltas that do not have strong foundations underneath them. Cracks have formed in parts of the coastline, although officials in Hawai’i note that there has not been a major collapse of the 1960 lava delta, suggesting that it may take decades before something significant collapses.

Spurr, Alaska and Rainier, Washington

Two volcanoes that had people’s attention across the spring and summer were Mount Spurr near Anchorage in Alaska and Mount Rainier near Seattle in Washington. Both were experiencing earthquake swarms and the former showed signs of heating within the volcano. The US Geological Survey was concerned enough about a potential new Spurr eruption to raise the alert level there to Yellow. The restlessness at Rainier was notable but never prompted an increase in its alert level.

Now, here in late August, both volcanoes have officially been returned to “background” levels of unrest. This doesn’t mean things could ramp up again, but all the previous signs of potentially activity at both Spurr and Rainier have ceased to the point that they are indistinguishable from the background noise of a potentially active volcano. I’m sure people in Anchorage and Seattle are breathing a sigh of relief on this front. You can read the full updates for Rainier from the Cascades Volcano Observatory and Spurr from the Alaska Volcano Observatory.

Shishaldin, Alaska

Of course, with all the potentially active volcanoes in Alaska, it isn’t surprising as one falls out of the elevated advisory status, another picks up. Shishaldin, located on Unimak Island in the Aleutians has started to show strong signs of renewed activity.

Earthquakes have been increasing at the volcano. Combine that with increases in sulphur dioxide emissions and steam plumes sighted on Shishaldin, it is pretty good sign that something could be brewing. This has caused the Alaska Volcano Observatory to increase the alert status to Yellow.

Shishaldin is one of the more active volcanoes in the Aleutians (and a beautiful conical stratocone shape). Its last eruptive period ended a little less than two years ago. Since 1950, the volcano has had 19 eruptive episodes that vary from VEI 2-3, meaning most of the activity is mild (as volcanoes go).

The activity in 2023 saw lava flows and explosive plumes that reached 14 kilometers (45,000 feet) during cycles of lava flow extrusive and then destruction. This is what we might expect from Shishaldin if it is indeed reawakening after a brief nap.

Volcano Word of the Week

Caldera is the Volcano Word of the Week. For one summer in graduate school, I was a Park Ranger at Crater Lake National Park in Oregon. The lake that sits there today was formed by a collapse of Mount Mazama, a volcano that was likely akin to Mount Hood. One of the most common misconceptions is that Crater Lake got its name from the bowl shaped landform that is now the lake … and that’s where you’d be wrong! In fact, the name comes from the tiny crater on Wizard Island, a small cinder cone inside the caldera. So, what exactly is the difference between a crater and caldera?

This difference comes down to how they form … and it is a little confusing for the non-volcanologist. I mean, craters and caldera both look the same — pits in the ground. Calderas tend to be larger, usually span kilometres or tens of kilometres across while craters might be only tens to hundreds of meters from side to side. Yet, it isn’t the size difference that sets a caldera apart from a crater.

Craters (in volcanology) form through explosion. When volcanic activity blasts materials out, leaving a pit or building up ridges around a central vent, that’s when a crater forms. Eruptions from stratovolcanoes, cinder cones, maars and steam-driven explosions will leave a crater behind, with an apron of debris (ash, bombs, other volcanic material) surrounding it. Even in lava flow eruptions you can get craters to form at the vent as spatter from the lava builds up a structure around the spot where the lava is coming out.

A caldera is not directly formed by an explosion — that is, they are not formed by the excavation caused by an outward blast. However, explosions can be the reason a caldera forms. That might seem confusing, but the key is taking and making an empty space under the land’s surface so that it can’t support itself anymore.

In a massive explosive eruption like what happened at Mount Mazama ~7,700 years ago, so much magma was evacuated from under the volcano so quickly (hours to days) that little was left under it. This meant that you literally pulled the foundation out from under Mazama, causing the volcano to collapse into the open space like a piston. The maximum depth of the lake is ~1,900 feet, so, the volcano lost something like 7,000 feet of elevation, going from ~11,000 feet to 4,000 feet above sea level.

You don’t need a volcanic edifice to form a caldera, either. The caldera at Yellowstone formed from the collapse of the land surface in the area where the eruption happened. No pre-existing mountain stood there, but the land itself lost its support when the massive blast happened.

Even non-explosive eruptions can form calderas. Rapid evacuation of magma in the form of lava flows can form more slowly (and less dramatic) calderas to form in places like Kīlauea. Here, the land surface slowly slumped down into the space formed by all that basalt lava erupting in the form of voluminous lava flows.

Long and short: craters are explosive, calderas are depressing. That is a mnemonic device that could help you remember which is which.

Odds & Ends

The media seem to have made “eruption any time now at Axial seamount” a thing. I feel like every few weeks we get another wave (no pun intended) of these breathless reports. However, what they fail to mention is that there is a 99.999% chance that no one on shore would even notice if an eruption happened. This is not Tonga 2.0. For one, Axial Seamount is a different type of volcano, erupting lower viscosity (thus less explosive) magma than Hunga Tonga and for another, it is in much deeper water, which acts to suppress any explosively in the first place.

And what in the fresh heck is this AI video of a Mount Fuji eruption. I realise that local authorities want people to take the potential threat to the region that Fuji poses seriously, but using AI is just all sorts of wrong. Remember folks, there is no ethical uses of AI!

And speaking of nonsense, what do you even do with a “news” article with a headline like this: “Underwater Volcano Eruption Spews Over a Million Giant Eggs as Marine Experts Warn Oceans May Hide Life We Cannot Understand” I won’t honour it with a link but wow.

Finally, there has been some buzz about an earthquake swarm over in eastern Nevada near Elko. This swarm began in late August, with dozens of earthquakes, some of which were as large as almost M5. Now, some sources want to link the swarm with potential volcanic activity, but to say that is a stretch is a massive understatement. As Nevada state seismologist Christine Rowe emphasises, the state is full of faults that can occasionally reactivate. Although the swarm is near a hydrothermal area, not all hydrothermal areas are linked to volcanism. The whole Basin & Range province that spans from eastern California to Colorado is full of hot springs, where water flows up and down faults, getting heated up deep underground.

Sounds of the Week

I am unaware of many songs about calderas, but there is one of my favourite songs that features the word in a volcanic context. So, that is our sound of the week. Enjoy!

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

Be sure to check out my podcast, 5 Minute Volcano and the Patreon page for the Eruptions Newsletter/5 Minute Volcano Podcast.