| dc.description.abstract | The Tongariro Volcanic Centre has experienced many VEI 1-4 eruptions over the last
12 000 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., around 4000 years earlier than previously thought. It also
provides the most detailed explosive eruption frequency and magnitude record from the
area since 12 000 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 dark
purple tephra, with a well-constrained volume of 26.6 x106 m3, and a probable eruption
column height of about 15 km. 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 of
Ngauruhoe occurred between ~ 2900 and 2700 cal. yrs. B.P., 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 12 000 cal. yrs. B.P., the frequency of Ruapehu eruptions appears to have
increased about 2000 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. | en_US |
