Indonesia is at the same time one of the most populous (5th in the world after PR China, India, USSR, USA), and one of the most volcanic, areas in the world, lying as it does at the intersection of three major plates. (The Geological background has been covered by Dr. Barber's contribution).
From the last two eruptions of Mt.Kelut, i.e. in 1951 and 1966, remarkable changes of the crater bottom were recorded. After the 1951 eruption a lowering of 79 m and after the 1996 eruption up upheaval of 43 m was found to have taken place, as measured at the deepest sections of the crater.