Rising Magma at Raoul Island

Gas from magma (molten rock) triggered the volcanic eruption at Raoul Island on March 17th, reports Geonet.

This means that the likelihood of magma-related activity at the volcano is now higher than originally thought.

The eruption of March 17th released 200 tonnes of sulphur dioxide gas according to satellite data provided to GNS Science by the University of Maryland, NASA and contributing agencies. The presence of sulphur dioxide confirms that the eruption was triggered by a release of magmatic gases, instead of a change in the large body of boiling water underneath the island. This indicates that magma has risen to a shallow depth.

Gas discharge ceased after the eruption, probably due to the water absorbing the gas as it rose in the vent. The water level in Green Lake, which rose 6 – 8 metres after the eruption, has continued its slow rise and is now 10 metres higher than its pre-eruption level.

Earthquake activity declined after the eruption from a peak of 200 earthquakes per day to 10 or 20 per day on the 22nd and was down to 5 to 10 per day on the 23rd. Few of the quakes showed the low-frequency component that indicates magma movement – usually referred to as “volcanic tremor” by scientists.

The declining earthquake activity and absence of volcanic tremor indicates that the volume of magma that rose to a shallow depth under Raoul may be small and is not being replenished by flows from a deeper level at this time.

However, as the volcano moves to a state of equilibrium, the earthquakes caused by the movement of the Pacific and Australian tectonic plates will be of concern to vulcanologists. With a volcano primed, an increase in earthquake activity nearby could re-activate the eruption sequence. The magnitude 5.3 earthquake, 55 km deep that struck 80 km south-west of Raoul Island at 3:35 p.m. on Thursday was one such earthquake.

Department of Conservation (DOC) worker Mark Kearney was inside the volcanic vent testing lake water when it erupted on the 17th. The high lake level and access restrictions in the area have prevented a search for his body. The boat which returned evacuated DOC staff and took scientists and a police recovery team to the island earlier this week is now returning to New Zealand.

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