As shown by lateral facies transitions in pyroclastic flow deposits , pyroclastic flows and surges commonly develop simultaneously from the same flow and evolve one from the other. Quarry within pyroclastic deposits of Laacher See volcano, Germany. Pyroclastic flow deposits are the thick, massive beds. The pyroclastic surge deposits are the duned and cross bedded deposits. Field study with Professor Hans-Ulrich Schmincke and students in 1980.
Pyroclastic flows are one kind of sediment gravity flow.
Sediment gravity flows are unique fluids because properties such as density and viscosity can change as they move, unlike fluids such as water or air within which density and viscosity change very little, if at all, during movement. The driving force of a sediment gravity flow is the action of gravity upon the particles. Particles with different settling velocities, however, behave differently within the mass. Some pyroclastic flows and surges, however, originate without accompanying vertical eruption columns. They are able to surmount high mountain ranges, as illustrated by the Campanian Ignimbrite located in the Naples region of Italy.