Tengger Caldera/Bromo/Semeru: Part 2

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.


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.


  • 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.


    Semeru’s latest activity notices.

    Darwin Volcanic Ash Advisories

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


    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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