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.