The volcanic history of Ruapehu during the past 2 millennia based on the record of Tufa Trig tephras |
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Authors: | S L Donoghue V E Neall A S Palmer R B Stewart |
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Institution: | (1) Department of Geography and Geology, University of Hong Kong, Pokfulam Road, Hong Kong Fax: +852 2559 8994, HK;(2) Department of Soil Science, Massey University, Private Bag 11222, Palmerston North, New Zealand Fax: +64 6350 5632, NZ |
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Abstract: | Tufa Trig Formation comprises a sequence of at least 19 andesitic tephras erupted from Mt. Ruapehu (Tongariro Volcanic Centre,
New Zealand). Tephras of Tufa Trig Formation are the most recent eruptives from Ruapehu, dated between ca. 1850 years B.P.
and the present. Members of the Formation show restricted dispersals, principally to the east of Mt. Ruapehu. Volumes calculated
for the most widespread members are all less than 0.1 km3. Compared with other Mt. Ruapehu eruptives, Tufa Trig Formation tephras represent small eruptions that have contributed little
tephra to the ring plain. They do, however, show a greater frequency of eruption with one event occurring on average every
100 years. Tufa Trig Formation members Tf3–Tf18 are black to dark grey, vitric, coarse-ash and lapilli-grade tephras which
mantle the relief. They contain juvenile vitric particles which exhibit varying degrees of vesicularity, together with free
crystals of pyroxene and feldspar, and few lithic fragments. Several morphological types of vitric pyroclasts are recognised
in these tephras, the dominant type being of equant blocky morphology with fracture-bound surfaces (type-1 morphology). Field
characteristics, tephra distributions, and the morphologies and textures of constituent pyroclasts suggest that these members
(Tf3–Tf18) are the products of small-volume hydrovolcanic eruptions resulting from the interaction of fresh magma and meteoric
water. We propose that a source of this water was an ancestral crater lake which formed within the late Holocene ca. 3000
years B.P. The morphological, compositional, and chemical (major-element) characteristics of three Tufa Trig Formation Tephras
are compared with those of two new tephras erupted from Ruapehu Volcano during the October 1995 eruptions which comprise part
of a newly defined member (Tf19) of Tufa Trig Formation. The comparisons support our interpretation that the majority of the
Tufa Trig Formation tephras are primarily the products of hydrovolcanic eruptions. Other members of the Formation (Tf1 and
Tf2) are coarse-grained scoriaceous tephras and are interpreted to be the products of strombolian events.
Received: 14 September 1996 / Accepted: 6 June 1997 |
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Keywords: | Ruapehu Tufa Trig Formation Holocene Tephra Hydrovolcanic Pyroclast morphology Crater Lake Marker beds |
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