On June 12, 2009, Russia's Sarychev Volcano erupted in spectacular fashion, issuing plumes of ash skyward in a phenomenal display of geophysical force. It was the first time Sarychev had erupted in over twenty years – and it happened, rather serendipitously, just as the International Space Station was flying overhead.
There's a lot going on in this brief sequence. The billowing plume casts a black shadow on the lands below. Dense, dark ash – most likely a pyroclastic flow – can be seen racing down the slopes of Sarychev's summit. The smooth, milky cloud situated atop the largest ash-plume, uncannily white given the circumstances, is called a "pileus cloud," and is thought to form transiently as the air above the ash column rises and cools, condensing the air above it into a lenticular cap of moisture. Already it appears the underlying plume is beginning to force its way through the cap from below.