A high-level eruption took place at Semeru volcano, East Java, Indonesia at around 07:00 UTC on December 4, 2021. The Aviation Color Code was raised to Red.

At 09:20 UTC, the ash cloud was estimated rising up to 15.2 km (50 000 feet) above sea level, according to the Darwin VAAC VA Advisory released at 09:48 UTC.1

"Volcanic ash observed on latest RGB and Visible satellite imagery moving SW of the summit. Ash seems to have detached from the summit and continues to move SW. IR temperature of -85 °C (-121 °F) obtained correspond to 15.2 km (50 000 feet) a.s.l."

Ash has reportedly completely blocked out the Sun over the area.

Some users on social media are reporting fast-flowing lahars.