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.
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 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.
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