The geology of the summit area, Mt Ruapehu

Abstract

Explosive and effusive deposits erupted from the Crater Lake vent in the past c.1800 years have been mapped and a relative stratigraphy produced. A revision of the current stratigraphic framework is proposed and all deposits are from the Crater Lake vent are now considered part of the Crater Lake Formation. Crater Lake Formation eruptives are characterised by both geochemistry and the dominant eruptive style from which the deposits were formed. Genetically related lavas and agglutinated spatter units are correlated using geochemistry and petrology. Discrimination of phreatomagmatic and magmatic tephra deposits is used to infer the amount of water in the Crater Lake at the time of their eruption, this required field observation, and stereoscopic and SEM analysis of the pyroclast morphology. Analysis of the crater rim geology and an understanding of the eruptive history of the Crater Lake vent can be used to predict potential hazards in the future. The outlet is shown to have alternated between its current position and the area of the current J-Peak­ Dome ridge. Active faults may promote further collapse. The 6 m tephra dam that occupies the current outlet area is considered the greatest immediate hazard. Due to the dynamic nature of the crater rim this hazard should be mitigated through a monitoring programme on the flanks of Ruapehu.