BJ Deming

Tengger Caldera/Bromo/Semeru: Part 2 Semeru (Dec. 11, 2025, 1615 UTC)

December 11, 2025, 8:15 a.m., Pacific: Semeru has settled back into its typical activity level again, as this tweet shows:

https://platform.twitter.com/widgets.js

November 20, 2025, 0927 UTC: Darwin VAAC, two and a half hours ago, reported continuous emissions to a much lower level — much closer to but not quite yet at typical Semeru levels, in this layperson’s opinion — and that the high cloud (to 45,000 feet) had dissipated.

Reuters reports that Semeru erupted ten times yesterday. They don’t mention casualties one way or the other; I hope, if it had to be, that it was only those who stayed on the bridge, like rabbits fascinated by a snake. About 900 villagers reportedly were evacuated. Also some 170 people were stranded ON the mountain, but reportedly they are all okay and are being rescued at last report (about an hour ago).

Per someone local, lahars are already occurring.

November 19, 2025, 1508 UTC: Semeru seems to be having a big eruption. Some sources I read attribute it to lava dome collapse, but Darwin VAAC puts the ash cloud to 59,000 feet.

Large pyroclastic flows have hit the towns of Supit Urang and Gladak Perak Bridge —

https://platform.twitter.com/widgets.js

— and tephra appears to be collapsing roofs in town, too.

There are casualties.

This is only about five hours old. I am trying to get more information and will update below periodically.

Needless to say, Aviation Code is Red.

1519 UTC: Looking over the VONAs (Volcano Observatory Notice to Aviation), Semeru — which typically has a more or less steady low-level activity — had four more powerful blasts on the 18th.

Here is the current VONA, from about six hours ago.

1529 UTC: Not verified but reasonable, in my lay opinion:

https://platform.twitter.com/widgets.js

X translation: “Hot cloud pyroclastic flows from Mount Semeru volcano extended up to a radius of 13 kilometers, heading toward two river flows of the Curah Kobokan River and Kali Lanang or Besuk Lengkong, Supit Urang Village, Pronojiwo District, Lumajang Regency, Wednesday (November 19, 2025).”

1537 UTC: This English-language account is always a reliable one for events in the area.

The behavior below is what people do when a constantly active volcano suddenly ramps up — they stand there and watch the cloud coming right at them.

https://platform.twitter.com/widgets.js

It seems to have worked okay for these people (? for those at the other end of the bridge), but many died in Guatemala that way when Fuego suddenly went briefly berserk one day a few years ago.

Don’t ever get used to a volcano, even if you’ve known it all your life.

1547 UTC: Don’t trust everything you see on X, either. Some AI fakery is already appearing.

This account I’ve followed for a while, and this post from about an hour ago shows what appear to be roofs collapsed from ash (it is raining heavily there and the weather had affected flights even before this happened at Semeru, whatever “this” is — dome collapse or significant eruption):

https://platform.twitter.com/widgets.js

(Embed corrected!)

A lot of that ash, and any more coming, is likely to be remobilized by the rain and water flows as lahars, which are always a problem at Semeru (per the original post below).

Even if Semeru stops all trouble-making right now (a big “if”), this is going to be a very long night for everyone there, residents and emergency responders alike.

1606 UTC: As of right now, this is the only information I have on casualties:

https://platform.twitter.com/widgets.js

X translation: “A number of victims from the Mount Semeru eruption have already been treated at the Candipuro health center.
( Source: Disaster Volunteers )”

1622 UTC: Posted an hour ago from an account I’m familiar with. The Regency has set up an emergency response: it’s a lengthy post, so please let X or your preferred online translator work on it.

https://platform.twitter.com/widgets.js

Of note, it’s almost midnight there. This is not routine. I worry about what daylight will bring. 😦

1645: Ash still to 59,000 feet per latest Darwin VAAC advisory:

FVAU02 ADRM 191600
VA ADVISORY
DTG: 20251119/1600Z
VAAC: DARWIN
VOLCANO: SEMERU 263300
PSN: S0806 E11255
AREA: INDONESIA
SUMMIT ELEV: 3657M
ADVISORY NR: 2025/1322
INFO SOURCE: CVGHM, HIMAWARI-8, WEBCAM
AVIATION COLOUR CODE: RED
ERUPTION DETAILS: VA TO FL590 LAST OBS AT 19/1240Z MOV WSW
EST VA DTG: 19/1540Z
EST VA CLD: SFC/FL590 S0725 E11307 – S0907 E11349 – S1036
E11136 – S0925 E10908 – S0726 E11016 MOV SW 25KT
FCST VA CLD +6 HR: 19/2140Z SFC/FL590 S0707 E11324 – S0932
E11422 – S1214 E11209 – S1159 E10834 – S1022 E10658 – S0708
E10917
FCST VA CLD +12 HR: 20/0340Z SFC/FL590 S0707 E11324 – S0942
E11441 – S1358 E11249 – S1356 E10758 – S1101 E10505 – S0708
E10748
FCST VA CLD +18 HR: 20/0940Z SFC/FL590 S0707 E11324 – S1410
E11450 – S1617 E11138 – S1506 E10626 – S1101 E10332 – S0712
E10631
RMK: VA NOT IDENTIFIABLE ON SATELLITE IMAGERY DUE TO MET
CLOUD. CVGHM VONA REPORTED ERUPTION AT 19/0849Z. HEIGHT
BASED ON IR TEMP AND SURABAYA 19/1200Z SOUNDING. MOVEMENT
BASED ON SATELLITE IMAGERY AND DISPERSION MODEL GUIDANCE.
NXT ADVISORY: NO LATER THAN 20251119/1900Z=

This layperson doesn’t think a dome collapse could continue to generate that much ash, but I know little. Still awaiting official word; officials are probably doing their darnedest to see Semeru through darkness, weather clouds, and intense ash.

1705 UTC: Not new, just some more footage from a reliable source:

https://platform.twitter.com/widgets.js

X translation: “Semeru’s Hot Cloud Eruption Reaches 14 Km, Residents Begin to Evacuate”

1716 UTC: From a couple of hours ago, an official summary (this online image translator only did one page before paywalling it; I’ll upload that after the tweet):

https://platform.twitter.com/widgets.js

1753 UTC: I’m not sure how reliable this source is, as they include one of the pyroclastic surge videos that looked fake to me in their first-tweet montage (of course, that doesn’t mean it was fake).

However, the last post in this thread is cool, showing authorities cleaning ash off what I assume is the bridge we saw those people standing on, in an earlier tweet, watching the flow come at them.

And, sadly, a married couple got horribly burned on this bridge, according to another tweet in this thread, and died a few hours later in the hospital. (They might be the casualties shown up above in another tweet.)

https://platform.twitter.com/widgets.js

1958 UTC: Per The Guardian about half an hour ago:

…No casualties have been reported. More than 300 residents in the three villages most at risk in the district of Lumajang were evacuated to government shelters, said National Disaster Mitigation Agency’s spokesperson Abdul Muhari.

Videos showed ash sweeping through valleys.

Local media reported that authorities are struggling to rescue about 178 people stranded on the 3,676m mountain at the Ranu Kumbolo monitoring post. The group includes 137 climbers, 15 porters, seven guides and six tourism officials, according to an official with the Bromo-Tengger-Semeru National Park, TNBTS.

“They are currently safe at Ranu Kumbolo monitoring post,” Endrip Wahyutama, a TNBTS spokesperson, said in video statement, adding that the post is located 4.5km from the crater on the northern slope of the mountain, which is not in the path of the hot cloud flow that was observed moving to south-southeast. Bad weather and rain forced them to spend the night there, he said.

I don’t think that the images above of injured people were fake; perhaps they were the only direct victims of yesterday’s flows.

But what is the volcano doing? The latest VAAC advisory is pretty much a repeat of the one above, and they are working relatively blindly because of weather clouds.

Original post, not updated.

Distracted by the Iceland eruption, I didn’t add this slightly edited repost, too. Sorry!

Semeru — the erupting volcano in the background of this image — rises south of the Tengger caldera, which we explored last week.

Bromo — where the steam is rising in the foreground — is the only other active volcano in this complex.

Tengger’s volcanoes and Semeru are connected by a 12-mile-long massive jumble of igneous rock, mostly in the form of calderas and cones.

ESCapade via Wikimedia, CC BY-SA 3.0

Taken together with its wild areas, lakes, and streams, this all is an Indonesian national park and a UNESCO biosphere reserve, even though millions of people live less than a hundred miles away.

Unfortunately, it also contains a geological serial killer, hiding in plain sight.

Semeru has killed more than 800 people since the 1800s, when record-keeping began.

While Semeru’s murder tools include explosions, gas, and pyroclastic flows, most victims have died in lahars (cement-like floods of watery ash, rocks, and debris):

This nonlethal early 2020 lahar is one of the small ones that happen weekly at Semeru during the rainy season.

The volcano doesn’t have be having an intense eruption for a lahar to happen. Monsoonal rains can easily wash down loose ash and rock from the steep upper slopes.

There’s plenty of material up there from Semeru’s frequent, though usually small, vulcanian blasts.

One reason why so many people endure the rugged two-day climb up to Semeru’s complex summit is that eruption close-ups like these are almost guaranteed. At night, you can even see this tiny lava dome “breathe.”

Other material contributors to Semeru’s lahars are old lava flows, as well as debris left over from pyroclastic flows that happened during larger eruptions in the past.

Most of the time, things just tumble around in a lahar. That still makes it destructive and dangerous, but it’s really bad news when a lahar gets cohesive enough for gravity to act as though it is a single body rather than a particle collection.

Per Gomez and Lavigne, this area gets up to 145 inches of rain annually. About every five years, monsoonal rainfall can be as much as 8 inches a day; every fifty years or so, that doubles to 16 inches a day. (Image: Oberoy/Shutterstock)

This produces large-volume, long-distance lahars. Just one of these flows at Semeru can be big enough to fill the Great Pyramid at Gizeh more than twice.

At least five enormous lahars, each containing millions of cubic meters of thick mud and heavy debris, have roared down Semeru’s steep slopes since 1884.

In 1909, per Photovolcanica, after very heavy rainfall, such a lahar happened, reaching and heavily damaging the city of Lumajang, about 20 miles out on the plain east of Semeru.

Along the way, it wiped out 38 settlements and killed hundreds of people.

Authorities put up sediment dams and other barriers. These did help protect Lumajang in 1981 during another gigantic flow. Nevertheless, almost 300 people died.

Here is an Indonesian-language short news film from Lumajang in 2017 that shows the sabo dams around Semeru. The sediment does build up and must be removed.

Google translation of accompanying text: “Entering the rainy season, the volcanic ash pile of Mount Semeru is prone to flooding when it rains. Officials from the regional disaster management agency, Lumajang, East Java, appealed to residents to raise awareness of the dangers of Semeru’s cold lava flood, which is prone to casualties.”

There is a positive side to Semeru’s lahar-producing ability.

Most of the time, these lahars are small enough to study safely, as well as frequent during the monsoon.

Semeru is one of the few “natural labs” available to geoscientists who want to know how lahars happen, how they move, and how people can be protected from them.

Location:

8.108° S, 112.922° E, in Malang and Lumajang Regencies, East Java Province on the island of Java, Indonesia. The GVP Volcano Number is 263300.

Nearby Population:

Per the Global Volcanism Program (GVP):

  • Within 5 km (3 miles): 2,686
  • Within 10 km (6 miles): 8,375
  • Within 30 km (19 miles): 1,022,197
  • Within 100 km (62 miles): 20,098,931

Current Status:

Level 2 on a four-point scale. Aviation Code Orange.

Eruptions:

  • Eruption styles: Small to moderate explosions from the summit crater, with occasional lava flows from the summit or a flank vent. Activity increases every five to seven years on average, with lava dome formation. Ash can fall up to 25 miles away during these episodes. Pyroclastic flows occur from either eruptions or lava dome/front collapses. (Solikhin et al.)
  • Biggest recorded event: The Global Volcanism Program doesn’t list any eruption higher than VEI 3, with the majority recorded since 1818 being VEI 2.
  • Most recent eruption: Ongoing since 2014, per the GVP. Semeru’s last major eruption was in 2002-2003, when pyroclastic surges damaged crops and forced evacuations near the district capital of Pronojiwo. Fortunately, there were no casualties.

    Semeru’s two most recent lava flows happened in 1941-1942 from a flank vent and in 1895 from the summit. This thick, gooey “gray lava” only travelled a few miles.

  • Past history: See the GVP for details. They also have an archive of bulletins on Semeru’s activity from 1972 to April 2020.

    Monitoring:

    Semeru’s latest activity notices.

    Darwin Volcanic Ash Advisories

    Featured image: Jan-Pieter Nap via Wikimedia, CC BY-SA 3.0.

    Sources:

    Brown, S.K.; Jenkins, S.F.; Sparks, R.S.J.; Odbert, H.; and Auker, M. R. 2017. Volcanic fatalities database: analysis of volcanic threat with distance and victim classification. Journal of Applied Volcanology, 6: 15.

    Gomez, C., & Lavigne, F. (2010). Transverse architecture of lahar terraces, inferred from radargrams: preliminary results from Semeru Volcano, Indonesia. Earth surface processes and landforms, 35(9), 1116-1121.

    Oregon State University: Volcano World. n.d. Semeru. http://volcano.oregonstate.edu/semeru Last accessed August 24, 2020.

    Photovolcanica. n.d. Semeru. http://www.photovolcanica.com/VolcanoInfo/Semeru/Semeru.html Last accessed August 24, 2020.

    Siswowidjoyo et al Siswowidjoyo, S., Sudarsono, U., & Wirakusumah, A. D. (1997). The threat of hazards in the Semeru volcano region in East Java, Indonesia. (Abstract only) Journal of Asian Earth Sciences, 15(2-3), 185-194.

    Solikhin et al Solikhin, A., Thouret, J. C., Gupta, A., Harris, A. J., & Liew, S. C. (2012). Geology, tectonics, and the 2002–2003 eruption of the Semeru volcano, Indonesia: Interpreted from high-spatial resolution satellite imagery. Geomorphology, 138(1), 364-379.

    Starheim, C. C., Gomez, C., Davies, T., Lavigne, F., & Wassmer, P. (2013). In-flow evolution of lahar deposits from video-imagery with implications for post-event deposit interpretation, Mount Semeru, Indonesia. Journal of volcanology and geothermal research, 256, 96-104.

    Thouret, J. C., Lavigne, F., Suwa, H., & Sukatja, B. (2007). Volcanic hazards at Mount Semeru, East Java (Indonesia), with emphasis on lahars. (Abstract only) Bulletin of Volcanology, 70(2), 221-244.

    Wikipedia. 2020. Semeru. https://en.wikipedia.org/wiki/Semeru Last accessed August 24, 2020.

    Wikipedia (Indonesian). 2020. Gunung Semeru. https://id.wikipedia.org/wiki/Gunung_Semeru Last accessed August 24, 2020.

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