Mount Semeru erupts repeatedly, officials warn of hot ash and lahars

Mount Semeru, Java’s highest volcano, erupted seven times within five hours on 4 May 2026, sending ash plumes up to 1,200 m high. A hot ash cloud (APG) travelled about 2 km southeast toward the Besuk Kobokan river, prompting alerts for severe heat and rock fallout.

Authorities placed the volcano on Level III (Alert) and issued strict safety zones: no activity within 13 km of the crater, a 5 km radius around hot rock fallout, and a 500 m buffer along the riverbanks. Residents were urged to stay alert for potential lahars, especially during rain, and to avoid the southeast river valleys that channel the hot material.

The repeated eruptions follow weeks of heightened activity, with ash columns regularly reaching several hundred metres. Ongoing monitoring continues as officials caution that the situation could evolve quickly.