Crater rim hazard

Ruapehu Bulletin, 24 June 1997, Page 6

Crater rim hazard

The southeastern crater rim is composed of interbedded lavas, which dip at 5°-15° away from the crater, and non-welded and welded volcanic ash (tuff), scoriaceous blocky (agglomerate) layers. The tuff and agglomerate beds dip into and away from the crater at 30°-35°. The slopes on either side of the rim are relatively steep, varying from 30°-40° near the crest, steepening to 45°55° lower down. The 1995-96 eruptions eroded and thinned the lower crater wall by 20%, reduced the height of "Stump Saddle" by 2025m, but only about 5-10 m of material was lost at the former lake level. Because the crater rim still has a broad cross section, and is composed of relatively strong materials, including several supporting bands of strong blocky lava, the changes are believed unlikely to have significantly reduced the rim's strength and stability. Based on observations of geology and topography, and the rim's performance over the last 2000 years, the potential for collapse of the

southeastern crater rim (or the broader eastern rim) is considered to be low, assuming "that future eruptions are similar to the historical events. However, the southern side of the pr'esently-active Crater Lake vent has collapsed within the last 2000 years, forming the low outlet channel area, possibly during an extremely large volcanic eruption. Prior to that event the entire eastern side of the older South Crater has been destroyed and the Crater Lake vent formed in its place. Therefore, the possibility of another large crater rim collapse cannot be ruled out. The southeastern crater rim could possibly be destabilised by dyke or plug intrusion occurring to the east of the presently active vent conduit beneath Crater Lake. Dyke intrusion appears to have caused a major collapse of the Pinnacle Ridge area of Ruapehu about 10,000 years ago, forming the prominent "lahar" mounds on the road to the Chateau. Also, plug intrusion within Mount St Helens in 1980 caused massive cone collapse. Because a large-scale failure of the crater rim seems likely only during an extreme event such as a very large explosive eruption or magma intrusion, installation of an elaborate real-time monitoring and warning system on the southeastern crater rim does not appear to be justifiable at present. However, annual monitoring by visual inspection, photographic and surveying methods would allow early detection

of any obvious changes in rim stability. In addition, computer modelling and further studies of prehistoric crater rim collapses of Ruapehu would provide a better understanding of the possible causes and effects of future rim collapses, and identification of realistic hazard mitigation measures.