Thursday, March 23, 2006

Raoul Island is not done with volcanic activity

Signs of Increased Pressure at Raoul, Volcanologist Says

Scientists who flew over Raoul Island on Tuesday say the hydrothermal system under the island is showing signs of over-pressuring, and further eruptions cannot be ruled out.

“ From our aerial observations, it is clear that the heat, gas and water that are discharging into Green Lake are making this part of the volcano’s hydrothermal system unstable,” said GNS Science volcanologist Bruce Christenson.

Several new steam vents had opened in and around Green Lake during the eruption and some old ones had reactivated. Many of these have since been drowned as Green Lake has risen more than seven metres since last Friday’s eruption.

Dr Christenson was one of two volcanologists from GNS Science who joined DoC and airforce personnel on a RNZAF Orion reconnaissance flight over the island on Tuesday.

“One explanation for the increased hydrothermal activity is that it is being driven by the intrusion of magma at depth.”

Heightened activity is not confined to the lake. At Denham Bay, on the western side of the island, there is evidence of hot fluids seeping into the sea along the 2.5km-long beach.

The seepage was causing a milky discolouring of the seawater. This observation indicates that hydrothermal fluids under parts of the island are now rising to shallower levels in the volcano than in the recent past.

“ Everything we saw during the aerial surveillance suggests that last Friday’s eruption was hydrothermal in nature, originating from a reasonably shallow source beneath Green Lake.
“ Whether this is pre-cursory to magmatic events to come is not clear at this time, but with Green Lake continuing to rise and multiple steam vents remaining active, the probability of further hydrothermal eruptions is high.”

Volcanic debris blanketing parts of the island is largely made up of rocks and mud ejected from the bottom of the lake and from nearby vents.

Seismic activity under the island has declined steadily during the past week. The seismometer on the island is currently recording between 20 and 30 earthquakes a day, which is significantly above the normal background level for Raoul Island.

Dr Christenson said the Green Lake crater should not be entered while the present level of activity continued. The volcanic alert level remains at 2.

Raoul Island is an active volcano that has erupted 15 times in the past 3600 years.

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