ByDavid Bressan,
Senior Contributor.
Semeru volcano — the highest volcano on Java — erupted on the afternoon of November 19, 2025, with pyroclastic flows racing at high speed more than 8.5 kilometers down its slopes, as shown by video footage widely shared on social media platforms like X.
Mount Semeru is one of fifteen active volcanoes on Java, one of Indonesia’s most densely populated islands. In 2010, an eruption of Mount Merapi generated a series of deadly pyroclastic flows that killed 350 people and forced more than 10,000 residents to flee.
Pyroclastic flows are avalanches of superheated ash, gas, and volcanic rock fragments, with temperatures that can reach up to 1,000 degrees Celsius. They can travel down a volcano’s slopes at speeds over 99 mph (160 km/h), destroying everything in their path. These flows are lethal even at distances of six to nine miles (10 to 15 kilometers) from the volcano.
