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Name: Samantha
Status: Student
Grade: 9-12
Location: N/A
Country: New Zealand
Date: Summer 2009

What causes a lahar on new Zealands Mt Ruapehu?


Lahars are not unique to New Zealand. Lahars are mud flows that originate on volcanoes. These viscous flows are produced when heat from a volcano melts snow near the vent, thus producing water. The water mixes with the pyroclastics (rock fragments previously ejected from a volcano) on the flanks of a volcano to form a mud flow (otherwise known as a lahar).

Ruapehu is a stratovolcano which means that the flanks of the volcano are represented by numerous, repeating layers of lava and pyroclastics. The summit elevation is 2707 meters, so the vents are commonly surrounded by snow.

Common eruptive materials from volcanoes include lava, pyroclastics, gases, and water. Water eruptions are known as phreatic eruptions and Ruapehu has a lake at its summit, known as Crater Lake. The lahars of Ruapehu have an abundant source of water (Crater Lake and snow) to mix with ample pyroclastic materials and produce a lahar.

Have you ever considered why snow is more common on the top of a volcano neat the vent?


Leslie Kanat, Ph.D.
Professor of Geology
Department of Environmental Sciences
Johnson State College

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