A cycle of creation and destruction
At the northern end of Indonesia's Siau Island, the stratovolcano Karangetang, also known as Api Siau ("Fire of Siau"), is in a constant state of transformation. This volcano, standing 1,784 meters high, is one of Indonesia's most active, with a history of more than 40 recorded eruptions since 1675. The reason for its behavior lies in the chemical makeup of its magma. The magma is primarily andesitic, meaning it has a high silica content. This high concentration of silica creates strong bonds within the molten rock, making it extremely viscous and thick—too sticky to flow easily like more fluid lavas.
Instead of producing rivers of lava, this viscous magma pushes slowly out of the volcano's vents, piling up at the summit to form a steep, unstable mound called a lava dome. Karangetang has five summit craters aligned along a north-south line, and dome growth can occur in any of them, sometimes in two at once. This process is effusive, it is a relatively quiet extrusion of lava. However, the domes cannot grow indefinitely. As more magma pushes from below, the dome inflates and its slopes become dangerously over-steepened. Eventually, gravitational forces overwhelm the strength of the cooling, brittle lava, and the dome collapses.
The consequences of collapse
The collapse of a lava dome is a violent event. The unstable mass of hot rock and solidified lava disintegrates into a pyroclastic flow—a superheated avalanche of ash, gas, and rock fragments. These flows are the primary hazard at Karangetang. They can travel down the volcano's flanks at high speeds, destroying everything in their path. The volcano's frequent activity has produced numerous such events; a pyroclastic flow in 1997 killed three people, and another in 2010 killed four.
Since 1675, Karangetang has had at least 59 historically documented eruptions, averaging one every 5.9 years. In reality, the volcano is almost perpetually active, with periods of dome growth and collapse being a regular feature. Eruptions in 2015, 2019, and 2023 all produced significant pyroclastic flows from the reactivation of its southern and northern craters. Constant monitoring by Indonesia's Center for Volcanology and Geological Hazard Mitigation (PVMBG) tracks seismic activity, gas emissions, and ground deformation to provide warnings. Exclusion zones, typically a 2.5-kilometer radius from the craters, are often established to protect the island's residents.