Rain, snow, or hail may then fall from the clouds on top of these mountains. Clouds form at the summit of Mt Ruapehu. A lahar from the September '95 eruption on the west side of Mt Ruapehu. When a volcano erupts, snow on the tops of the volcano quickly melts and flows off down the slope with great speed. This river of snowmelt and volcanic debris is called a lahar .
Lahars are incredibly fast moving and cause great damage .
Lahars would also be formed if a crater lake was to overflow, or if part of the dam wall of a crater lake collapsed. A snow-capped Mt Ruapehu. The volcanoes of the Central North Island are called mountains. Much of the precipitation that falls on these mountains is as snow. During the colder seasons the snow builds up and is stored until spring and summer when it begins to melt.