THE VOLCANIC HISTORY OF RUAPEHU DURING THE PAST 2 MILLENNIA BASED ON THE RECORD OF TUFA-TRIG TEPHRAS

Tufa Trig Formation comprises a sequence of at least 19 andesitic teph ras erupted from Mt. Ruapehu (Tongariro Volcanic Centre, New Zealand). Tephras of Tufa Trig Formation are the most recent eruptives from Rua pehu, dated between ca. 1850 years B.P. and the present. Members of th e Formation show restricted dispersals, principally to the east of Mt. Rua pehu. Volumes calculated for the most widespread members are all less than 0.1 km(3). Compared with other Mt. Ruapehu eruptives, Tufa T rig Formation tephras represent small eruptions that have contributed little tephra to the ring plain. They do, however, show a greater freq uency 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 con tain juvenile vitric particles which exhibit varying degrees of vesicu larity, 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 ch aracteristics, tephra distributions, and the morphologies and textures of constituent pyroclasts suggest that these members (Tf3-Tf18) are t he products of small-volume hydrovolcanic eruptions resulting from the interaction of fresh magma and meteoric water. We propose that a sour ce of this water was an ancestral crater lake which formed within the late Holocene ca. 3000 years B.P. The morphological, compositional, an d chemical (major-element) characteristics of three Tufa Trig Formatio n Tephras are compared with those of two new tephras erupted from Ruap ehu Volcano during the October 1995 eruptions which comprise part of a newly defined member (Tf19)of Tufa Trig Formation. The comparisons su pport our interpretation that the majority of the Tufa Trig Formation tephras are primarily the products of hydrovolcanic eruptions. Other m embers of the Formation (Tf1 and Tf2) are coarse-grained scoriaceous t ephras and interpreted to be the products of strombolian events.