Understanding the Holocene explosive eruption record of the Tongariro Volcanic Centre, New Zealand

Abstract

The Tongariro Volcanic Centre has experienced many VEI 1-4 eruptions over the last 12000 cal. yrs. B.P., dominantly from Ruapehu, Ngauruhoe and Red Crater. The historic record of 150 years alone is insufficient to provide a robust understanding of future volcanic hazard, necessitating a quantification of eruption parameters from the geological record. The major obstacle to this is untangling a complex sequence of interdigitating, fine-grained and poorly distinguishable tephras from the three source volcanoes. With detailed mapping and using volcanic glass chemistry, tephras from the three sources were discriminated. This has led to a revision of the age of Ngauruhoe to be at least 6500 cal. yrs. B.P. Ruapehu and Ngauruhoe tephras were characterised by initial phreatomagmatic explosions that transformed into dry magmatic (strombolian) phases. Magma-water interaction is shown by basal layers of pale-brownish-grey fine ash, containing blocky glass shards with small isolated spherical vesicles, and exhibiting surface conchoidal and step-like fractures. The magmatic phase ash is microlite-rich with dark glass containing elongate vesicles with thin bubble walls and irregular surfaces. The largest eruption recognised from Ngauruhoe, produced a distinct purple tephra, with a well-constrained volume of 26.6 x 106 m3, and a probable eruption column height of about 15km. The total tephra volume from Ngauruhoe is estimated to be 952 x 106 m3, around 50% of the known lava volume. A climactic eruption period on Ngauruhoe occurred between ~2900 and 2700 cal. yrs. B.O., during which 64% of its known explosive eruptions occurred, including its largest known events. This phase, representing 3% of the volcano's lifespan, produced 57% of its pyroclastic output. Over the last 12000 cal. yrs. B.P., the frequency of Ruapehu eruptions appears to have increased about 200 yrs B.P., but this may reflect better preservation and exposure of the more recent tephras. Bursts in Ruapehu explosive activity have occurred out of phase with those from Ngauruhoe. The minor pyroclastic cone of Red Crater represents an eruption site that was active for at least ~4000 cal. yrs. B.P., and has mainly been characterised by effusive events. Since around 900 cal. yrs. B.P. minor explosive events have occurred from this location, increasing in magnitude from 400 cal. yrs. B.P.