Semeru - also known as "The Great Mountain" - is the highest volcano in Java and one of the most active. It is also one of Indonesia's most popular tourist hiking destinations.
The volcano previously erupted in December, when about 550 people were evacuated.
Within the last week, Indonesia has endured multiple landslides, a deadly earthquake on Sulawesi island, and the loss of a Sriwijaya Air Boeing 737 with 62 people on board.
Indonesia sits on the Pacific "Ring of Fire" where tectonic plates collide, causing frequent volcanic activity as well as earthquakes.