A pyroclastic surge is a dilute, turbulent cloud of gases and rock debris that moves above the ground surface at great speeds. Pyroclastic surges form in a similar way to pyroclastic flows, but their effects are more widespread as they are less confined by topography and may therefore sweep across ridges and hills as well as down valleys. Pyroclastic surges can be either hot (several hundred ⁰C) or cold (< 100 ⁰C). Cold pyroclastic surges are generally known as base surges, and are commonly generated by phreatic and phreatomagmatic explosions. The hazardous aspects of both types of surges include the destruction of vegetation and structures, impact damage by rock fragments, and burial by ash and rock debris. People in the path of a hot pyroclastic surge can also be killed by asphyxiation, heat and noxious gases. Pyroclastic flows and surges from Mt. Pelée completely destroyed the town of St. Pierre in Martinique in 1902, killing about 30,000 people