Physical controls and processes associated with the 1975 eruption of Ngauruhoe

Abstract

Ngauruhoe is a basaltic andesite to andesite composite cone, and one of the most active vents of the larger Tongariro Volcanic Complex. This research focuses on the pre-eruptive conditions, triggers, and eruption mechanisms associated with eruption and emplacement of the 1975 inner crater rim deposit. It is hoped that the improved knowledge regarding magmatic and volcanic processes associated with this eruption will assist with more reliable interpretation of monitoring data prior to and during such eruptions (both at Ngauruhoe and other basaltic-andesite volcanoes worldwide). This has significant implications to the safety of hundreds of people walking the Tongariro Alpine Crossing each day. 

This study has demonstrated the importance of crystal fractionation and magma mixing in the evolution of the magma that erupted during 1975. In particular, the transporting magma may have entrained exotic crystals from a cooler, more evolved crystal mush zone formed by an antecedent magma. Further, magma ascent was associated with efficient gas segregation and transport to the surface along preferential percolation pathways. Efficient degassing in low viscosity magmas typically results in effusive activity; however, this study shows that the high explosivity of the climactic 19th February 1975 eruption was linked to the presence of a coherent, degassed, and hydrothermally altered caprock resulting in accumulation and coalescence of pressurised gas beneath the plug. This study discounts the previously ascribed theory that magma-water interaction controlled the high explosivity of this event. Instead, limited magma-water interaction likely produced the steam observed in the eruption column. 

Based on the interpretations outlined above, it is now possible to explain the inner crater stratigraphy. The 1975 inner crater rim deposit is now divided into four facies, A through D, based on textural characteristics and componentry. Facies A (comprising dense sub-angular lapilli and vesicular scoria lapilli) and B (comprising black scoria lapilli/bombs and dense lapilli) were emplaced during vulcanian activity associated with disruption of a caprock, leading up the climactic 19th of February 1975 eruption. Facies C (comprising loose scoria, dense lapilli, and large irregular domains of densely coalesced/agglutinated pyroclasts) and D (comprising dense lapilli to blocks, scoria lapilli, and bombs) were deposited during the continuous ‘sub-plinian’ eruption on the 19th of February 1975, from strombolian fire-fountaining activity at the base of the large eruption plume. Evidence for this is recorded by the mixture of scoria, agglutinated spatter, and dense clasts throughout these facies; which are indicative of a spatter-fed eruption.