40Ar-39Ar Geochronology Of Magmatic Activity, Magma Flux And Hazards At Ruapehu Volcano, Taupo Volcanic Zone, New Zealand

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Journal Article: 40Ar-39Ar Geochronology Of Magmatic Activity, Magma Flux And Hazards At Ruapehu Volcano, Taupo Volcanic Zone, New Zealand

Abstract
We have determined precise eruption ages for andesites from Ruapehu volcano in the Tongariro Volcanic Centre of the Taupo Volcanic Zone (TVZ) using 40Ar/39Ar furnace step-heating of separated groundmass concentrates. The plateau ages indicate several eruptive pulses near 200, 134, 45, 22 and <15 ka and, based on our and previous field mapping confirm the lavas of the Te Herenga Formation as the oldest exposed part of the volcanic edifice. The pulse at ~134 ka includes an entire >300-m section of lavas in Whangaehu gorge as well as some lavas in Ohinepango and Waihianoa catchments on eastern Ruapehu, and this suite of lavas belongs to the Waihianoa Formation. This pulse of activity is not represented on nearby Tongariro volcano, indicating that the two volcanoes have independent magmatic systems. A younger group of lavas yields dates between 50 and 20 ka and includes lava flows from the Turoa skifield and in the Ohinepango and Mangatoetoenui catchments and is consistent with two pulses of magmatism around the time of the last glacial maximum, relating it broadly to the Mangawhero Formation. Syn- and post-last glacial activity lavas, with ages <15 ka are assigned to the Whakapapa Formation, and include the voluminous flows of the Rangataua Member on southern Ruapehu. Magma flux, integrated over 1000-yr periods, averages 0.6 km3 ka-1 assuming a volcano lifespan of 250 ka. Fluxes for the Te Herenga, Waihianoa and Mangawhero Formations are consistent at 0.93, 0.9 and 0.88 km3 ka-1, respectively. These fluxes are broadly comparable with those measured at other modern andesite arc volcanoes (e.g. Ngauruhoe, 0.88; Merapi, 1.2 and Karymsky 1.2 km3 ka-1). The relatively low flux (0.17 km3 ka-1) calculated for the Whakapapa Formation may derive from underestimates of erupted volume arising from an increase in phreatomagmatic explosive eruptions in postglacial times. However, using volume estimates for the 1995-1996 eruptions and a recurrence interval of 25 yr has yielded an integrated 1000-yr flux of 0.8 km3 ka-1 in remarkable agreement to estimates for the prehistoric eruptions. Overall, Ruapehu shows consistency in magma flux, but at time scales of the order of one hundred to some thousands of years, field evidence suggests that short bursts of activity may produce fluxes up to twenty times greater. This is significant from the perspective of future activity and hazard prediction.

Authors 
John A. Gamble, Richard C. Price, Ian E. M. Smith, William C. McIntosh and Nelia W. Dunbar








Published Journal 
Journal of Volcanology and Geothermal Research, 2003





DOI 
Not Provided
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Citation

John A. Gamble,Richard C. Price,Ian E. M. Smith,William C. McIntosh,Nelia W. Dunbar. 2003. 40Ar-39Ar Geochronology Of Magmatic Activity, Magma Flux And Hazards At Ruapehu Volcano, Taupo Volcanic Zone, New Zealand. Journal of Volcanology and Geothermal Research. (!) .