Mount Semeru

Semeru has been in a state of near-continuous eruption since 1967 — a period of over 58 years that makes it one of the longest-active volcanoes on Earth.

At 3,657 m (11,998 ft), Semeru is the tallest peak on the island of Java — home to over 150 million people — and the tallest volcano in Indonesia outside of Papua.

During active phases, Semeru's summit crater Jonggring Seloko produces Strombolian explosions every 15–40 minutes, generating hundreds of individual explosions per day.

The December 4, 2021 pyroclastic flows killed at least 51 people and traveled up to 11 km from the summit, making it one of Indonesia's deadliest volcanic events of the 21st century.

Semeru's name derives from Mount Meru, the cosmic mountain at the center of the universe in Hindu-Buddhist cosmology — the gods are said to have placed a fragment of Meru on Java to stabilize the island.

The volcano lies within Bromo Tengger Semeru National Park, which receives over 500,000 visitors annually, primarily for the iconic sunrise views over Mount Bromo and the Tengger Sand Sea.

Semeru's Jonggring Seloko crater opens toward the southeast, creating an asymmetric hazard profile that channels pyroclastic flows directly toward the densely populated lowlands of Lumajang regency.

The 1967–2009 eruption at Semeru lasted approximately 42 years, ranking among the longest continuous eruption episodes documented anywhere in the world during the 20th century.

Three separate eruptive episodes at Semeru since 2014 have reached VEI 4, indicating an escalation in explosive capability compared to the predominantly VEI 2–3 activity of the 19th and 20th centuries.

Semeru's 66 recorded eruptions since 1818 give it an average of roughly one eruption every three years, though the volcano's persistent activity means true repose periods are rare.

The Tengger people who live on Semeru's highland slopes are one of the few remaining Hindu communities on predominantly Muslim Java, and they maintain volcanic rituals including the annual Kasada ceremony.

Rising from sea level to 3,657 m, Semeru encompasses climate zones from tropical lowland to alpine tundra — climbers experience temperature changes of 25°C or more during the summit trek.