Eruption on Mount Semeru.

People living close to Mount Semeru, a 3676 m high stratovolcano (cone-shaped volcano made up of layers of ash and lava) close to the city of Lumajang in East Java, have been advised to avoid going near the volcano, following an eruption on Wednesday 19 November 2025. The volcano began erupting at about 4.00 pm local time (about 9.00 am GMT) producing an ash column about 5.6 km high, and triggering a series of pyroclastic flows (avalanches of hot rock, ash and gas) which reached as far as 7.00 km from the volcano.

A column of Ash above Mount Semeru on 19 October 2025. Badan Geologi/AP.

Mount Semeru forms part of the Tengger Volcanic Complex is a string of five overlapping stratovolcanoes (cone-shaped volcanoes) forming a single massif, extending northward from Semeru. The whole massif sits within a vast caldera (volcanic crater), left by the explosion of an ancient volcano about 150 000 years ago. The volcanoes are at the center of this caldera, surrounded by a vast sea of sand, the Tengger Sand Sea.

The approximate location of Mount Semeru. Google Maps.

The Indo-Australian Plate, which underlies the Indian Ocean to the south of Java, is being subducted beneath the Sunda Plate, a breakaway part of the Eurasian Plate which underlies Java and neighbouring Sumatra, along the Sunda Trench, passing under Java, where friction between the two plates can cause Earthquakes. As the Indo-Australian Plate sinks further into the Earth it is partially melted and some of the melted material rises through the overlying Sunda Plate as magma, fuelling the volcanoes of Java and Sumatra.

Subduction along the Sunda Trench. Earth Observatory of Singapore.

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Indonesan authorities begin evacuations after eruption on Mount Semeru, East Java.

Authorities in East Java, Indonesia, have begun evacuating communities close to Mount Semeru, a stratovolcano (cone-shaped volcano made up of layers of ash and lava) close to the city of Lumajang which forms the highest point on the island, following a sudden eruption on Saturday 4 December 2021. The eruption produced an ash column that rose to about 15 000 m, leading to heavy ash-falls in several local communities. There are no reports of any casualties at this time, although a bridge connecting Lumajang to Malang has been either damaged or swept away, according to posts on social media. The Darwin Volcanic Ash Advisory Center has reported that the ash cloud appears to have become detached from the volcano, and is drifting to the southwest.

 

An ash cloud generated by an eruption on Mount Semeru, East Jave, on 4 December 2021. Daily Sabah/Twitter.

Mount Semeru forms part of the Tengger Volcanic Complex is a string of five overlapping stratovolcanoes (cone-shaped volcanoes) forming a single massif, extending northward from Semeru. The whole massif sits within a vast caldera (volcanic crater), left by the explosion of an ancient volcano about 150 000 years ago. The volcanoes are at the center of this caldera, surrounded by a vast sea of sand, the Tengger Sand Sea.

 

The approximate location of Mount Semeru. Google Maps.

The Indo-Australian Plate, which underlies the Indian Ocean to the south of Java, is being subducted beneath the Sunda Plate, a breakaway part of the Eurasian Plate which underlies Java and neighbouring Sumatra, along the Sunda Trench, passing under Java, where friction between the two plates can cause Earthquakes. As the Indo-Australian Plate sinks further into the Earth it is partially melted and some of the melted material rises through the overlying Sunda Plate as magma, fuelling the volcanoes of Java and Sumatra.

 

Subduction along the Sunda Trench. Earth Observatory of Singapore.

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