Monday March 19, 2007

Lower water level in New Zealand volcano's crater lake sparks volcanic eruptions

WELLINGTON, New Zealand (AP): A drop in the water level in the crater lake at New Zealand's Mount Ruapehu volcano after a sudden outflow of mud and acidic water has led to fresh volcanic eruptions, a seismologist said Monday.

Sensors detected four or five volcanic eruptions under the lake since it partially emptied during the mud flow, or lahar, volcano surveillance coordinator Brad Scott said.

The lower water level in the lake has changed the balance between water and steam, "and that allows more steam to be produced,'' he said.

The phenomenon is common after a volcanic lake empties and the hydrothermal system under the lake adjusts to the lower weight of lake water, Scott said in a statement.

The adjustment period can take several weeks, he said.

"We're talking about eruptions that may dome up the middle of the lake by 15-20 meters (yards). It's like a geyser,'' he said.

Such small eruptions "could generate waves large enough to over-top the hard rock rim of the lake, resulting in a small lahar,'' Scott said.

Sunday's flow out of the volcano's lake released 1.3 million cubic meters (1.70 million cubic yards) of water and debris, which had accumulated behind a wall of soft rock built up by eruptions in 1995. The outflow reduced the lake level by about 6 meters (20 feet), he said.

The mud flow triggered an alarm system, and police and civil defense workers immediately closed roads and the nation's main rail track near the southern base of the mountain on New Zealand's North Island. No one was injured in the violent mud flow.

A lahar that tore down the same volcano in 1953 killed 151 people when it washed away a rail bridge, plunging a passenger train into the raging torrent of liquid mud.

More than 60 lahars have been recorded in the Whangaehu Valley below the mountain since the 1860s.

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