Pyroclastic surges are less dense than pyroclastic flows, but are highly turbulent mixtures of gas and rock that flow at rapid velocities just above the ground surface.
Due to their lower density and higher gas content, pyroclastic surges can rise over ridges and hills rather than being primarily confined to valleys like many pyroclastic flows. They are less constrained by topography and may climb or overtop valley walls. For example, pyroclastic surges on May 18,1980 surmounted a 150–meter–high (nearly 500 ft) ridge north of the volcano, even though the related pyroclastic flows were deflected by the steep, north- facing escarpment of the ridge. Despite these key differences the hazards associated with pyroclastic surges also include igniting objects on fire, destruction by high-velocity ash-laden winds, impact by rock fragments, burial by deposits, exposure to noxious gases, and asphyxiation.