Andesitic glaciovolcanic interactions at Tongariro and Ruapehu volcanoes, New Zealand

Abstract

Glaciers that cap or flank volcanoes have a major control on the eruption style and distribution of volcanic material. Ancient deposits that were formed by the interaction of lava and ice inform our understanding of glaciovolcanic eruption dynamics, as well as the eruptive environment. To date, most studies have come from basaltic or rhyolitic volcanic areas covered by thick continental ice sheets (e.g., Iceland, North America and Antarctica). A primary aim of this thesis is to present new maps and emplacement models for previously undescribed proximal deposits from Tongariro and Ruapehu volcanoes, New Zealand; two mid-latitude, andesitic volcanoes where past and present ice has been in the form of waxing and waning alpine glaciers throughout the Late Pleistocene. The results presented here contribute to paleoenvironmental reconstructions for the central North Island of New Zealand, and support inferences about early edifice construction at both volcanoes. The mapped deposits include a variety of fragmental units, which have been sparsely documented from intermediate-composition volcanoes where coherent lava flows dominate. These unusual deposits have implications for the availability of meltwater and its effects during the deposition of voluminous fragmental andesite. At Tongariro volcano, many lithofacies were mapped, including a ≥ 100 m thick hyaloclastite deposit, ice-confined fractured lavas and channelised water-lain primary volcaniclastic deposits, all of which indicate the presence of water. The approximate ages of these deposits coincide with the variably cool last interglacial period (marine isotope stage 5) through the early stage of the last glacial period (marine isotope stage 4). A summit glacier of varying thickness is interpreted to have capped the edifice during this time, which produced and confined meltwater. The waterlain clastic deposits are interpreted as having originated from explosive eruptions into meltwater, which drained from the edifice along subglacial channels and deposited the eruption-fed tephra. These deposits have not yet been reported from andesitic interactions with other alpine glaciers. Their deposition along a high ridgeline is inferred to have been aided by confining ice that filled the adjacent valleys and prevented meltwater from flowing down the steep topography either side. Cool emplacement of these deposits in waterlain flows is supported by temperature determinations from paleomagnetic data. Randomly oriented magnetisation directions for lithic and possible juvenile clasts provide deposit temperature estimates of < 150 °C, which is strongly indicative of efficient mixing of tephra with water. On Ruapehu volcano, a ≥ 150 m thick and ≥ 1 km wide hydroclastic breccia deposit, analogous to submarine volcanic breccias, is exposed near the summit. The breccia is interpreted to have formed from non-explosive fragmentation by quenching and dynamic stressing of lava erupted into accumulated meltwater. The minimum age of the deposit coincides with the penultimate glacial period (marine isotope stage 6). The thickness of the glacier at that time is estimated to have been 150 m or more. Based on the emplacement models determined from field evidence, a new experimental technique was developed to investigate whether the thermal properties of andesitic lava control the availability of meltwater at the eruption site, a paucity of which has been called on for minimal evidence of andesite lava-water interaction. Preliminary data indicate that the thermal efficiency of andesite for ice melting is comparable with that of basalt and that meltwater production is not limited by a lower heat capacity. In addition, the experimental results show that heat transfer and meltwater production rates increase with applied force during dynamic lava-ice interaction. This effect should be taken into account when modelling the thermodynamics of a flowing or inflating lava that interacts with ice. Field mapping and experiments support inferences that retention of meltwater at the eruption site is probably the main control for the emplacement and preservation of fragmental material at andesitic volcanoes, which usually host ice-bounded lava flows controlled by meltwater drainage down steep slopes and where thick ice is hard to accumulate. At Tongariro and Ruapehu, it is likely that a composite arrangement of vents has existed at the edifices since early in their evolution, and allowed thick ice and meltwater to accumulate on steep topography during periods of ice advance.