The aim of this lecture is to present on How do Pyroclastic Flows cause devastation? The direct force of a pyroclastic flow traveling at 10’s of metres per second and carrying boulders as large as houses is extremely damaging. Heavy rain after an eruption or hot volcanic activity melting snow and ice will provide a large volume of water that will flow down the sides of the volcano. This water picks up the newly erupted material forming fast flowing torrents of water, mud, ash, rock and debris.