Reconstructing eruption processes of a Miocene monogenetic volcanic field from vent remnants:: Waipiata Volcanic Field, South Island, New Zealand

The Miocene Waipiata Volcanic Field, New Zealand, is an eroded phreatomagmatic intracontinental volcanic field formed during a period of weak lithospheric extension. The field includes remnants of at least 55 volcanoes in an area of similar to 5000 km(2). Vent-filling deposits comprising predominantly lava (e.g. plugs, necks, lava flows, or dykes), often associated with thin basal phreatomagmatic pyroclastic deposits, were classified as type I vents and are inferred to be the remnants of scoria cones. Vents represented by predominantly pyroclastic infill are classified as type 2 vents and are inferred to have been the substructures of phreatomagmatic tuff ring and/or maar volcanoes. Type 3 vent complexes are groups of closely spaced or overlapping vents, with voluminous preserved lava flows; they are inferred to be the remnants of volcanoes comprising adjoining to coalescing maars and tuff rings with magmatic explosive and effusive products. Pyroclastic rocks of most of the Waipiata vents record initial phreatomagmatic explosive activity fuelled by groundwater, followed by strombolian-style eruptions. Aligned and clustered vents are accommodated to structural features of the regional basement rock (Otago Schist). (C) 2003 Elsevier Science B.V. All rights reserved.