A late Quaternary stratigraphic framework for the northeastern Ruapehu and eastern Tongariro ring plains, New Zealand

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Abstract
The northeastern Ruapehu and eastern Tongariro ring plains record a complex sequence of episodic lahar sedimentation. Andesitic and rhyolitic tephrostratigraphy reveals 15 lahar episodes in the northeastern Ruapehu ring‐plain record ranging in age from >65 to 5 ka, and five in the Tongariro ring‐plain record ranging in age from >23 to 14 ka. The most voluminous and widespread lahar deposition occurred during cool and stormy climatic periods equivalent in age to marine δ18O Stages 2 and 4. In these periods, accelerated physical weathering on the volcanoes supplied erosion debris, while large areas of snow and ice acted as sources of water to form lahars triggered by a variety of mechanisms. Lahar distribution after c. 22.5 ka was affected by two landform changes in the area at about this time. First, a large lava flow was emplaced along the boundary between Ruapehu and Tongariro ring plains shortly before 22.5 ka, effectively separating the two ring plains since. Second, Last Glacial moraines along the Whangaehu and Mangatoetoenui Rivers have blocked direct drainage from the Ruapehu summit region to a large sector of the northeastern ring plain, including the Upper Waikato Stream, formerly an important lahar path. These moraines have directed subsequent lahars (after c. 15 ka) along their current routes (active in 1995) in the Whangaehu and Mangatoetoenui catchments. Lahar deposition in the study area during this time is linked to large, explosive andesitic eruptions impacting on catchments where retreating glaciers provided water for lahar generation.