"Explosive eruptions have occured frequently over a long period at Ruapehu from many vents. A crater lake about 500 metres wide and containing about 10 million cubic metres of water usually occupies the current vent which is believed to have been active for about 2500 years. The summit lake is heated volcanically yet is surrounded by glacial ice and snow. The temperature of the lake fluctuates, depending mainly on what is happening in the vent beneath." Excerpt from Karen Williams, Ruapehu Erupts.

Ruapehu's crater lake during the 1995 eruption. The glacier's ice cliff can be seen at the top of the picture. Photo from Ruapehu Erupts.

"Lahars are flowing mixtures of water and volcanic debris generated during eruptions. Explosions through Crater Lake at Ruapehu eject large quantities of hot lake water onto the glaciers and snow fields. The hot water melts and mixes with snow to form the lahar which typically carries with it ash and blocks thrown out by the explosions. As lahars move down valley they pick up and incorporate more snow and rock, drastically increasing the volume of the lahars. Other lahars at Ruapehu have formed when heavy rainfall affects thick unstable ash deposits high on the flanks of the volcano." Excerpt from Eruption! Mt Ruapehu Awakes.

This lahar occured on 23 September 1995, and travelled down the north-western side of Ruapehu at 15-20 metres per second.
Photo Bruce Houghton, from Eruption! Mt Ruapehu Awakes.

explosive, ash, gases, magma
lahars