LAVA-ICE INTERACTION ON A LARGE COMPOSITE VOLCANO: A CASE STUDY FROM RUAPEHU, NEW ZEALAND

Ice exerts a first-order control over the distribution and preservation of eruptive products on glaciated volcanoes. Defining the temporal and spatial distributions of ice-marginal lava flows provides valuable constraints on past glacial extents, and is crucial for understanding the eruptive histories of such settings. We present a case study of effusive ice-marginal volcanism from Ruapehu, a glaciated andesite-dacite composite cone in the southern Taupo Volcanic Zone, New Zealand. Flow morphology, fracture characteristics, whole-rock major element geochemistry and ⁴⁰Ar/³⁹Ar geochronological data indicate that lavas erupted between ~51 and 15 ka interacted with large valley glaciers on Ruapehu. Ice-marginal lava flows exhibit grossly overthickened margins adjacent to glaciated valleys, are intercalated with glacial deposits, and are commonly ridge-capping due to their exclusion from valleys by glaciers. Fracture networks produced via interaction with ice and associated meltwater define the margins of these lava flows and include columnar joints, pseudopillow fracture systems and kubbaberg joints. Fine-scale crease structures are present in association with these fracture types and are recognised as resulting from lava-ice interaction here for the first time. Platy joints commonly define the eroded margins of ice-marginal flows, and were formed inward of the lava-ice interface due to endogenous flow parallel to chilled margins. New and existing ⁴⁰Ar/³⁹Ar eruption ages for ice-marginal lava flows indicate that glaciers with inferred maximum extents during the Last Glaciation (71–14 ka) were present on Ruapehu between ~51–41 ka and ~27–15 ka. Younger lava flows located within valleys are characterised by blocky flow morphologies and fracture networks indicative of only localised and minor interaction with ice and/or snow, especially in their upper reaches at elevations of ~2600–2400 m. Based on regional paleoclimate reconstructions that indicate rapid warming from ~18 ka, valley-filling lavas are interpreted as post-glacial flows that were emplaced after glaciers had largely retreated. An ⁴⁰Ar/³⁹Ar eruption age of 9 ± 3 ka (2σ error) determined for a valley-filling flow on the northern flank of Ruapehu indicates that glaciers had retreated to near-historical extents by 12 ka.