Informations scientifiques ŕ propos de l'éruption d'Ambae eruption

A small-scale volcanic eruption continues from an active vents within the 
summit crater lake (Lake Vui) at Ambae. Molten material is entering the crater 
lake and reacting with the water to produce small explosive eruptions and a 
plume of steam and gas. The eruption has built a cone around the active vents, 
enclosing them on 3 sides, forming an island about 200m across and 50-60m high 
in the lake. There are two vents, one is erupting water, rocks and mud and the 
other is producing a tall column of steam and gas. To date the eruption has 
had little effect outside the crater lake (minor ashfall occurred only in the 
first three days of the eruption).
 
Five days of seismic recordings show a moderate level of seismic activity 
(mostly volcanic tremor), which is typical of an eruption of this size. There 
has been one measurment of the volcanic gas output measured, showing a 
moderate level of sulphur dioxide (SO2) gas (about 2000 t/d) from the active 
vents. This is typical of erupting volcanoes, but not of large eruptions. We 
have seen no change in the level of Lake Vui. There is also no evidence of 
ground uplift or cracking near the lake, which suggests there is no large 
volume of magma close to the surface.
 
As the volcanic activity is contining at a low level, we recommend that the 
Vanuatu Volcano Alert Level remains at Level 2.
 
The current activity appears to be similar in both style and size to that 
which occurred in 1966 and 1870 (when islands also formed in the lake). We 
think that the existing islands in the lake were formed by similar eruptions 
in the past.
 
Based on our observations and the history of Ambae we think there are four 
possible scenarios for future activty:
1. The eruption will continue with the same style and at a similar level for 
the next few days or weeks (most likely).
2. The cone currently forming in the lake continues to grow and starts to 
produce ash which falls on the island (as the cone grows this becomes as 
likely as number 1). This may also continue for a few days or weeks.
3. The level of actvity increases significantly producing larger ash falls and 
possibly lahars (less likely than number 2).
4. An eruption occurs to the northeast or southwest of the Lake Vui on the 
volcanic rifts and activity declines in the summit crater (less likely than 
numbers 1 and 2, but happened in 1870).
We recommend that that the volcano should continue to be monitored and in 7-10 
days time a detailed assessment is made of the activity. Should the activity 
remain low it may become possible for people to return to their villages after 
the assessment.
 
Charlie Douglas, Morris Harrison (DGMWR)
Brad Scott, Steve Sherburn (GNS Sciences, NZ)


***Ambae Volcano Update:10 Dec 2005***
 
The small scale eruptions continue in Lake Vui, building a volcanic cone in 
the lake and producing a tall (8-10,000') steam and gas plume. Views and video 
obtained on December 9 by a TVNZ film crew show the cone is similar to that 
observed on December 8. The active vent is surrounded on 3 sides. The cone is 
not complete on its eastern side, but many small rocks could be seen breaking 
the lake surface, indicating it is very shallow there.  Only 4 or 5 explosions 
were seen by the film crew, and these appeared weaker than those we saw on 
December 8 (however they only saw the activity for a very short time).  
The eruptions are coming from a volcanic cone in the lake, ejecting water, mud 
and rocks, and a large (tall) plume of steam and gas. There continues to be 
two vents active, one producing the small explosions, and the second the steam 
and gas emissions. No volcanic ash is present in the steam plume.
The eruptions are not having any immediate effect beyond Lake Vui. The level 
of the lake appears to remain stable, no large scale changes are apparent.
The level of eruptive activty remains at a similar level to that we have 
observed over the last week.
The style of the eruption may change over the next 1-2 weeks if the cone 
continues to build, and the active vents become isolated from the lake. The 
eruptions will become dryer, and volcanic ash will probably be produced.
We recommend that that the volcano should continue to be monitored and in 7-10 
days time a detailed assessment is made of the activity. 
The Vanuatu Volcanic Alert Level should remain at Level 2.

Brad Scott
Steven Sherburn
GNS Sciences, NZ