Lahars almost always occur on or near stratovolcanoes because of their tendency to erupt explosively and their tall, steep cones are either snow covered, topped with a crater lake, constructed of weakly consolidated rock debris that is easily eroded, or internally weakened by hot hyrothermal fluids. Lahars are also common from the snow- and ice-covered shield volcanoes in Iceland where eruptions of fluid basalt lava frequently occur beneath huge glaciers. The scenarios listed below illustrate most of the mechanisms by which lahars are generated. For convenience, we've grouped the mechanism according to whether a volcano is erupting, has erupted, or is quiet.