Samstag, 15. November 2008

Volcano-Ice Interactions

A lahar is a general term for a type of rapidly flowing mudflow / landslide composed of an water-satured (at least 40-80 weigth% ) mixture of volcanic deposits. The term 'lahar' originated in the Javanese language of Indonesia, meaning “wave”.

Lahars can have four main causes:

-Snow and glaciers can be melted during an volcanic eruption
-(Crater-)Lakes breakout, triggered by collapse of natural dams (lava flows, ash deposits etc.)
-Heavy rainfall, caused by precipitation from eruption cloud
-General remobilizing of volcanic deposits without implications of eruptions

Considering mainly the first category –two main premises have to be considered. The volcano, if he is not located in the Arctic or Antarctic realm, must be high enough to possess a snow cover or enable glacier formation, and he must be active in historic times.
Several mountains in the world, including Mount Rainier, Mount Shasta and formerly Mount St. Helens in the Cascadian Range, Mount Ruapehu in New Zealand, Popocatepetl in central Mexico, different Volcanoes in the Andes (like Nevado del Ruiz) are considered particularly dangerous due to the risk of lahars.

The lahars from the Nevado del Ruiz (5.321m a.s.l.) eruption in Colombia in 1985 caused the Armero tragedy (13.11.), which killed an estimated 23.000 when the city of Armero was buried under 5 metres of mud and debris. Pyroclastic flows melted ice and snow at the summit, forming four thick lahars that rushed down several river valleys. Historic lahar-events date back to the 16th century.

Popocatepetl volcano (5450m a.s.l.) is probably the most active volcano in central Mexico, and threatens more than 40 million people livingin the Mexico City area. The principal danger is rappresented by laharic events, that following the main river, can reach zones distant up to 15km from the volcano. The Ventorillo Glacier, located on the northern flank of the volcano, is the main source of meltwater during eruptions. Popocatepetl got active after 50 years of quiescence in 1994, and on 31. June 1997 a lahar with an estimated volume of 1x10^7m^3 formed from the tongue of the glacier and deposited 3.3x10^5 m^3 of material in the circumstandig river valleys.

Mount Ruapehu, or just Ruapehu (consisting of three major peaks Tahurangi, 2.797m, Te Heuheu, 2.755 m and Paretetaitonga 2.751 m), is the largest active stratovolcano at the southern end of the Taupo Volcanic Zone in New Zealand. The North Island's major skifields and only glaciers are on its slopes.
In recorded history, major eruptions have been about 50 years apart, in 1895, 1945 and 1995-1996. Minor eruptions are frequent, with at least 60 since 1945. Some of the minor eruptions in the 1970s generated small ash falls and lahars that damaged ski fields.

Between major eruptions, a crater lake, damned by volcanic ash and rocks, forms, fed by melting snow. The collapse of this natural dam, blocking the outlet of Mount Ruapehu´s crater lake, caused in the past (and presumably will cause in the future) catastrophic lahars.

Fresh lahar channels scar Ruapehu's eastern slopes (27.03.2007, wikipedia)

December 24., 1953, a lahar destroyed the Tangiwai rail bridge, causing the derailment of a train, and the dead of 151 of the 285 people aboard the train- the worst train accident in New Zealand. Until then, the danger of lahars was underestimated in the public view, and only after the tragedy a monitoring program was installed on the volcano.
The eruption in 1995 closed the ski season for that year and was followed by some more eruptions in 1996. During March 18., 2007 a lahar, with estimated 1.4 million cubic metres of mud and rocks, was documented by a film crew.

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