Holocene lahar deposits in the Whakapapa catchment, northwestern ring plain, Ruapehu volcano (North Island, New Zealand)

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Abstract
The Holocene sedimentary record on the northwestern ring plain of Mount Ruapehu records intermittent volcanic activity over the past 9500 years that is not preserved by primary pyroclastic deposits. The sedimentary record of eruptive activity comprises packages of lahar deposits, representing rapid aggradation of the ring plain in response to increased sediment influx. During these times, large debris flows (calculated volume of some deposits > 1.4 5 104 m3) transported coarse sediment to the ring plain; in some, boulders up to 1.3 m in length were carried over 15 km from source. Eruptive periods were followed by long‐term quiescence, when sediment influx to the ring plain decreased markedly. Dissection during this time produced large‐scale, catchment‐wide erosion surfaces. Abundance of aggradational packages varies throughout the Whakapapa catchment, reflecting changes in the locus of lahar activity through time. Prior to 3400 yr B.P., four major aggradational episodes occurred in the Whakapapanui Stream, indicating a source area on the central Summit Plateau of Ruapehu, north of Crater Lake. Between 3400 and 2000 yr B.P., activity moved to a vent on the northern Summit Plateau. Relatively small eruptions from this source area emplaced lahars mostly in the Wairere Stream. Development of Crater Lake and modern lahar routes occurred between 2000 and 1800 yr B.P. This final change in summit configuration is marked by a thin aggradational episode limited to the Whakapapaiti Stream. Following deposition of the Taupo Pumice (1800 yr B.P.) a series of major eruptions triggered aggradational episodes in both the Whakapapaiti and Whakapapanui Streams, although the Whakapapaiti Stream was the major route taken by lahars off the northwestern flank of the volcano.