Via Erik Klemetti's Eruptions blog: Extraordinary Video of Pyroclastic Flows From the Eruption of Sinabung. Click through to see the video and read Klemetti's whole post. Excerpt:
In terms of the destruction they wreak upon the landscape, few volcanic hazards can top pyroclastic flows (or more precisely calling pyroclastic density currents). They hug the ground as they flow down the slopes of the volcano.
These at Sinabung were generated by the collapse of the extruding lava dome at the summit of the volcano. As it crumbles due to gravity, the hot lava fractures into boulders, smaller debris and ash, which then flows downhill at remarkable speeds — in the hundreds of kilometers per hour.
The volcanic debris is still hot, so not only is it moving quickly, it is seemingly hot, probably well over 400°C (~800°F) — hot enough to carbonize trees that are caught in the flow.
Now, try to picture any human structure withstanding being hit by a flow of superheated rocks moving faster than an airplane — this is why pyroclastic flows are the most destructive volcanic hazard during these types of eruptions (note: a close second would be lahars — volcanic mudflows — generated by remobilizing volcanic debris).