Apr 21 1999, 4:30 pm - White Island Volcano

Explosive eruptive activity at White Island on April 17 has re-excavated Metra Crater, destroying the lakelet that had recently occupied it. Ballistic eject has fallen out to over 450 m from the vent area. The explosive eruptions were accompanied by long duration seismic signatures (07:05 and 07:23); moderate volcanic tremor followed, lasting for about 4 hours. A small vent has also formed east of PeeJay vent.

Outlined below are observations by M. Rosenberg and D. Keen following a visit on Tuesday (April 20) to service the seismic installation and assess the effects of the explosive eruptions. This visit confirms activity within Metra Crater has now recommenced. PeeJay vent remains active, but was not emitting ash during Tuesday’s visit. These are the largest explosive eruptions since September 1992.

Observations

The whole of the island is thinly blanketed with light grey coloured ash – no single direction of dispersal is apparent though ground thickness suggests towards east. There is no longer a lake present in Metra Crater. While at the island a steam plume was rising 750 - 900 m and travelling to the south-east.

Ballistic blocks and bombs have been ejected to approximately 450 m and judging from changes in crater size and shape, they have come from Metra Crater. Dispersal of the largest is more towards the south and south-west. Abundant centimetre-sized fragments have impacted into new ash to at least 600 m radius, though distribution thins towards the north-east. At least 10 cm of fresh ash has fallen within a 200 m radius of the main crater (there had been rain and therefore some erosion and consolidation). Approximately 12 cm is present at Peg Z, but only 2 cm at Peg M.

The floor of 1978/90 Crater Complex has been covered with ash and ballistic ejecta. Much of the original Metra Crater area has been excavated. Metra is steaming but only gently and contains a few small puddles of yellow-green coloured brine. Mud bubbling could be heard distinctly. No ash fall was experienced and the PeeJay discharges are white steam and gas. Collapse of the 1978/90 Crater Complex floor has occurred, especially in an area between PeeJay and Donald Mound, leaving concentric cracks around the slumped margins of Metra Crater. A large fumarole has also formed east of PeeJay vent.

Ballistic blocks reach an estimated maximum size of 2m x 2m. Most of the large ones are fresh black highly vesicular andesite, sometimes with internal crystal (plagioclase) rich and crystal-poor banding. No evidence of plastic deformation was seen and most have an outer rim of red "baked" ash. The largest blocks have shattered on impact. Dense (older, altered) lava is relatively minor and blocks are perhaps <500mm. Lithified crater-fill sediment blocks are relatively common and comprise either dark grey soft sandstone or harder red/yellow/pale grey (hydrothermally altered) material.

Ejection of the ballistics occurred largely after the main ash falls - as evidenced by the thin layer of ash coating the blocks and further away from vent, the clearly visible impact craters from small lapilli.

Output of main fumaroles on the western or eastern walls didn’t appear to be changed greatly. Noisy Nellie seems to be producing reasonably high temperature steam and gas (almost colourless at immediate vent). Gas emissions around Donald Mound are weak.

Summary

The explosive eruptions that excavated Metra Carter are typical of activity at Alert Level 2. However the short duration of the explosive eruptions (7 - 8 minutes), and rapid decline to present conditions of predominantly steam with little or no ash emission means that the current Alert Level of 1 remains in force.

It is likely that similar explosive eruptions will occur again, with little or no warning, while Metra Crater is open. The amount of ash fall, and the size and number of ballistic blocks ejected on April 17 show that eruptions of this type present a potentially lethal hazard for visitors who approach within a few hundred metres of the edge of 1978/90 Crater Complex.

Brad Scott
Volcano Surveillance Manager