Lurking under the clouds in this image is Anak Krakatau, 'Child of Krakatau'. Anak rose from the caldera left by the explosion of Krakatau and is now beginning to spew an ash cloud high into the atmosphere. In 2009, an eruption started and you'll find some spectacular images here .
Sumatra is the site of Lake Toba, the largest volcanic lake in the world. It was formed when a supervolcanic eruption occurred and is considered the largest explosive event in the last 25 million years leading to a volcanic winter with temperatures declining 5 to 15 degrees at the highter latitudes.
The tectonic plates seem to be shifting under the Indonesian Islands.