An avalanche is a rapid flow of snow down a slope such as a hill or mountain. Lahars can form during or after eruptions or even during periods of inactivity. Geography Notes On Tectonic Landforms And Weather Geography Especially Physical Geog Is Actually Really Interesting Geography School Notes Notes Inspiration Lahars can occur with little to no warning and may travel great distances at high. . Avalanches can be set off spontaneously by such factors as increased precipitation or snowpack weakening or by external means such as humans animals and earthquakesPrimarily composed of flowing snow and air large avalanches have the capability to capture and move ice rocks and trees. They are among the greatest threats volcanoes pose to people and property. Lahar an Indonesian word for volcanic mudflow is a mixture of water mud and volcanic rock flowing swiftly along a channel draining a volcano.