Indonesia is located on a subduction zone, which the Indo-Australian plate subducts under the Eurasian plate. At subduction zones, water that flowed in the hot mantle cause the mantle to partially melt. Eventually, the magma cooled on the surface and created a volcano. Since the volcano spewed out pyroclastic flow, this indicates that the magma was high in silica and therefore high in viscosity. Explosive volcanoes typically occur at subduction zones. At subduction zones, the magma doesn’t quickly reach the surface and because it sits under the continental crust and melts. This may have caused the magma to have a lot of silica. Another reason, why the magma might have had an excessive amount of silica is because the initial (basalt) magma slowed