When the Earth's crust is pushed deep into the Earth, the rocks melt and some of them come out through cracks and fissures in the Earth as lava.
In addition, the subduction zone changes direction between Sumatra and Java. At the point where it bends, enormous stresses are created. As a result, powerful fissures are formed, through which magma escapes to the surface in a powerful stream, causing gigantic eruptions. Such fissures were found in the area of the Kratakau volcano.
