Earlier this year a plume of volcanic ash and gases raised concerns about an imminent eruption at Mount Merapi on Java. Java is one of Indonesia’s most densely populated island and hosts fifteen active volcanoes. The Merapi is one of the most dangerous because of its explosive eruption style. In 2010 a series of eruptions killed 350 and led to 10,000 people fleeing their homes, one of the most deadly volcanic eruptions in recent years. According to the local Geological Agency (GA), on Monday a series of pyroclastic flows, avalanches composed of hot rock fragments and gases, traveled over one mile down the slopes of the mountain.

On Saturday, Mount Semeru, the highest volcano on the island of Java, spewed volcanic ash as far away as 4.5 kilometers (nearly 3 miles). There were no immediate evacuations, but the National Disaster Mitigation Agency warned people who live in the villages on the slopes of the mountain to be vigilant, as loose ash deposits could form mudflows during a storm. The volcano began erupting in May 2020, forming a 3,000 meters (9,800 feet) high plume of volcanic ash in early December, triggering panic among villagers.

Indonesia frequently experiences earthquakes and volcanic eruptions because it lies near an intersection of shifting tectonic plates, including the Pacific plate, Eurasian plate, Australian plate and Philippine plate. As parts of the plates are pushed into Earth’s mantle and partially melt, large blobs of magma rise towards the surface, feeding more than 120 active volcanoes.