Mt. Ruapehu is a stratovolcano on the North Island of New Zealand. It has erupted about 50 times since 1861. At the top of the volcano there is an acidic crater lake. Most eruptions are phreatic, involving steam explosions caused by the contact of lava and water.
Mt. Ruapehu has a summit plateau which consists of several craters covered under a central glacier. The active vent contains a crater lake. It is about 400 m wide, 600 m deep and contains highly acidic hot water. The lake drains through an ice cave just off the tip of the wing. The color ranges from green to muddy grey, and earlier this century the water sometimes had frozen or even disappeared.
From time to time, steam explosions (phreatic eruptions) cause hot water to spill over the flanks. The water melts snow and ice and carries rocks with it. These so called lahars pose a danger on skiiers. Earlier this century, a lahar carried away a nearby train bridge. Today, an automatic warning system has been installed which warns Skiiers on the Whakapapa Ski Field and stops trains on threatened routes.
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