Understanding the evolution of a small-volume silicic fissure eruption: Puketerata Volcanic Complex, Taupo Volcanic Zone, New Zealand

Abstract

The Puketerata Volcanic Complex formed as a result of a series of rhyolitic eruptions along a 2.5km long NE-trending eruptive fissure at the southern margin of Maroa Volcanic Centre (Whakamaru caldera) at 16.5ka. Initial deep-seated explosions were excavated and widen the pre-existing volcanic structures associated to earlier basaltic activity. The following emplacement of two lava domes and associated shallow-seated phreatomagmatic activity produced a widespread tephra blanket characterized by alternating surge and fall dominated pyroclastic units. The silicic magma (73% SiO2) erupted during the Puketerata activity was highly degassed, where explosive fragmentation was driven only by the interaction with external water. The lava domes and ejecta ring makes up more than two-thirds (98×106m3 DRE) of the total eruptive volume (~143×106m3 DRE). The compound ring structures surrounding the larger dome totally overlap the underlying volcanic landforms that formed during the initial phase. The ejecta ring around the larger dome represents a complex landform, which predominantly consists of outwardly thinning pyroclastic breccias forming fan-shape bodies relating to the series of destruction of the larger dome. Despite the relatively large combined dispersal area of the pyroclastic deposits generated by the entire activity (235km2), the volume of single fall beds do not suggest individual explosive events exceeded VEI 2–3. The volcanic hazards associated with such small-volume volcanoes are largely ignored despite these eruptions being more frequent than eruption characterized by caldera collapses. The example of Puketerata suggests that small-volume volcanism often exhibits activity producing fissures, whereas the varied advancement of dykes influenced by external factors makes the course of the eruptions unpredictable in terms of geometry of vents and eruption styles.