The velocity of the head of pyroclastic density currents has been measured directly via e.g. photography as for the case of Mount St. Helens, reaching . Estimates for other recent eruptions range from less than 10 m/s up to 100 m/s. Pyroclastic flows may generate surges. For example, the city of Saint-Pierre in Martinique was overcome by a pyroclastic surge in 1902. Pyroclastic surges can be subdivided into three types: base surge, ash-cloud surge, and ground surge.