Semeru Erupts Again: Pyroclastic Flow Runs 5.5 Kilometres Toward Besuk Kobokan

Mount Semeru, East Java's highest volcano, erupted again in the first minutes of Saturday, November 7. The eruption began at 12:11 a.m. local time (WIB), according to Sigit Rian Alfian, an official at the Semeru Volcano Observation Post, and sent a pyroclastic flow — a fast, hot avalanche of gas and rock — 5.5 kilometres (3.4 miles) toward Besuk Kobokan, the drainage channel on the volcano's southeastern flank.

The ash column rose about 1 kilometre (0.6 miles) above the summit. The eruption lasted about six minutes, with a maximum seismic amplitude of 33 millimetres, monitoring data showed.

The overnight flow was the longest reported in a week of escalating activity. It came a day after a Friday afternoon eruption — at 3:48 p.m. local time on November 6, Indonesia's Geological Agency said — sent a pyroclastic flow 5 kilometres toward Besuk Kobokan, with an ash column also observed at about 1,000 metres above the summit and a seismograph record of 23 millimetres maximum amplitude over 744 seconds.

Before that, Semeru erupted six times on Thursday, November 5, with flows travelling between 3 and 4.5 kilometres from the summit, according to the East Java Disaster Management Agency's Operations Control Center. The volcano, on the border of Lumajang and Malang regencies, stands 3,676 metres above sea level and remains at Alert Level III (Siaga).

Local authorities reported no direct impact on daily activities from the overnight eruption. Isnugroho, head of the Lumajang district disaster management agency, said residents' activities were proceeding normally and no direct effects had been reported.

That does not mean the mountain is being treated casually. The Lumajang agency urged people near the volcano to remain alert for further eruptions, and rapid-response personnel, along with Indonesian military and police officers, were stationed at the Curah Kobokan and Oro-Oro Ombo observation posts to monitor conditions.

The Geological Agency's guidance, carried on the Magma Indonesia monitoring site, is specific. People should not undertake any activities in the southeastern sector along Besuk Kobokan up to 13 kilometres from the summit, and beyond that distance should stay at least 500 metres from the riverbanks along Besuk Kobokan, because pyroclastic flows and lahars could spread as far as 17 kilometres from the summit.

Nobody should enter a 5-kilometre radius of the crater because of the risk of ejected rocks. Officials also warn of pyroclastic flows, lava avalanches and lahars along the rivers and valleys that originate at the summit — Besuk Kobokan, Besuk Bang, Besuk Kembar and Besuk Sat — and in smaller rivers that feed into Besuk Kobokan, a risk that grows in rainy weather, days after heavy rain triggered lahar flooding around the volcano on November 2.

Semeru is one of Indonesia's most persistently active volcanoes, and this week's sequence — six eruptions on November 5, a 5-kilometre flow on November 6, a 5.5-kilometre flow in the first minutes of November 7 — shows an edifice that is repeatedly clearing its throat rather than settling.

Pyroclastic flows of this kind are the volcano's signature hazard: they follow the southeastern drainage because that is the flank the current dome and crater geometry feed, which is exactly why the exclusion guidance is drawn so tightly around Besuk Kobokan and its tributaries.

WorldEye's volcano webcams collection gathers live views of the world's restless mountains in one place — the same network of 74,229 cameras that carries summit views of volcanoes from Hawaii to Iceland — and the earthquake tracker shows the seismic activity that so often accompanies an eruption sequence like this one.

For Semeru itself, the authoritative word remains Indonesia's Geological Agency and its Magma Indonesia monitoring service: alert levels and exclusion zones there override any single dramatic video from the flank.