|dc.description.abstract||The Tongariro Volcanic Centre (TVC) is New Zealand's most recently active volcanic centre and comprises two large active andesitic cones, Ruapehu and Tongariro. Tongariro is a dominantly andesitic cone complex, yet located at its centre is Red Crater, a basaltic andesite vent with an eruptive nature in striking contrast to the rest of Tongariro. The main aim of this research was to reconstruct the eruption history of this vent and provide the basis to model the impact of a range of future eruption scenarios from Tongariro.
The Red Crater basaltic andesite vent occupies a small scoria cone (~ 0.3 km³) which has also erupted eleven lava flows including the single largest preserved lava flow on Tongariro, with a length of ~ 7 km and a volume of ~ 0.3 km³. This flow was erupted between 9.7 ka and 3.4 ka, providing a maximum age for the vent. A distinctive feature of this vent is the exposed drained dike in the eastern wall.
Clast density (vesicularity), SEM, grain size, and petrographic analysis were undertaken on the deposits erupted from Red Crater, and used to constrain the timing of the lava flows in relation to the construction of the scoria cone. Average vesicularities for the Red Crater scoria deposits, exposed in the eastern and southern crater walls in particular, range from 51.5% to 76.8%, while the range of individual clasts is from 30.5% to 82.1 %. These values classify Red Crater scoria as highly vesicular. This scoria section is ~ 60 metres thick and possibly erupted within a 48 hour period. The five lava flows erupted prior to this scoria cone construction (pre 1.85ka Taupo eruption) are andesitic blocky flows, with lengths up to ~ 7 km and thicknesses up to ~ 50 metres. Five basaltic andesite aa flows were erupted post 1.85ka and coincided with the construction of the scoria cone. Maximum thicknesses are ~ 5 metres with lengths up to 0.98 km. The change in composition of the lava flows at this time is reflected by a change in the eruptive processes. Effusive activity dominated pre 1.85 ka while strombolian style eruptions producing discrete gas bursts dominated during the scoria cone construction post 1.85 ka. Two dikes are intruded into the scoria cone; one is the eastern wall and the other in the western wall.
The eruptive history of Red Crater can be divided into three phases. Phase one was probably initiated with a violent phreatic eruption caused by the interaction of an intruding dike with groundwater. The loss of volatiles during this phreatic eruption and through the permeable country rock lead to sustained effusive activity which produced the five andesite flows into Oturere Valley. Phase two began when more basaltic magma was injected into the system, with strombolian eruptions rapidly building the scoria cone and erupting the remaining six basaltic andesite lava flows. The basaltic andesite flow (flow 9) into Central Crater has the most mafic composition of any lava flow from the entire Tongariro cone complex. Phase three began when the withdrawal of magma from the dike caused a series of phreatic explosions originating from the deeper section of the NE-SW orientated dike, which violently excavated the NE trending Emerald Lakes explosion pits, the northern section of the scoria cone and the explosion pit on the south face of the cone. Minor ash and steam eruptions have been observed at the end of the last century, but it is the active fumaroles surrounding the vent which reveal Red Crater's currently active state, and potential for future eruptions.||