The kinetic energy of the moving cloud will flatten trees and buildings in its path . The hot gases and high speed make them particularly lethal , as they will incinerate living organisms instantaneously or turn them into carbonized fossils : Testimonial evidence from the 1883 eruption of Krakatoa , supported by experimental evidence , shows that pyroclastic flows can cross significant bodies of water . However , that might be a pyroclastic surge , not flow , because the density of a gravity current means it can not move across the surface of water . One flow reached the Sumatran coast as much as 48 km ( 30 mi ) away
1883 eruption of Krakatoa
1883 eruption of Krakatoa
Pyroclastic flow