Sep 17 1999, 1:30 pm - Ruapehu Volcano

In response to observed steam plumes and recent increases in volcanic tremor levels the Crater Lake was visited this morning by a scientific party to assess its status, collect water and gas samples, and measure its temperature. Following is a summary of their observations:

"The lake is a dirty grey colour with considerable convective movement of lake waters resulting in a lot of suspended material causing the water discoloration. Sulphur slicks are present on the surface. A maximum temperature of 58°C was measured, which is similar to measurements on August 27. The steam plumes are therefore consistent with both this morning’s clear and cool weather and warm temperatures of the lake. There is no evidence of any eruptive activity occurring at the lake."

Since the major 1995 - 1996 eruptions the occurrence of volcanic earthquakes and tremor levels at Ruapehu have stabilised at levels higher than those prior to the major eruptions have. During the 1997 - 1999 period there have been intervals where volcanic tremor has intensified, and during these intervals minor hydrothermal eruptions have occurred in the Crater Lake. Recently elevated levels of volcanic tremor have occurred between August 6 and 21 and this culminated in a sequence of volcanic earthquakes on August 21. No eruptive activity occurred. Since September 13 volcanic tremor levels have again risen.

The elevation of volcanic tremor levels over the last 2 - 3 weeks does indicate a slight increase in the probability of further minor activity occurring at Ruapehu, especially in the form of hydrothermal eruptions in the Crater Lake. Presently there is no reason to change the Alert Level from its current level.

The Alert Level remains at Level 1.

Brad Scott
Duty Volcanologist