English: The thermal signature of hot ash and rock and a glowing lava dome on Mount Merapi. The thermal data is overlaid on a three-dimensional map of the volcano in this image to show the approximate location of the flow. The Center of Volcanology and Geological Hazard Mitigation reported that two pyroclastic flows moved down the volcano on October 30, 2010. A pyroclastic flow is an avalanche of extremely hot gas, ash, and rock that tears down the side of a volcano at high speeds.