A basaltic phreatomagmatic eruptive centre at acacia bay, taupo volcanic centre

Abstract

Basaltic tuffs seen in shoreline exposures at Acacia Bay township on Lake Taupo are interpreted as the heavily eroded relicts of a phreatomagmatic tuff ring and its associated outlying deposits. The tuff ring consists largely of ejected pumiceous lake sediments, which incorporate 15-20 volume % of juvenile basalt whose residual heat has caused the material to become highly indurated by cementation. The outlying tuffs are composed largely of juvenile basalt, are weakly to nonindurated and were deposited by both airfall and surge mechanisms. The juvenile magma is a plagioclase plus clinopyroxene phyric high-alumina basalt, broadly similar to that erupted elsewhere in the rhyolite-dominated central part of the Taupo Volcanic Zone. However, the Acacia Bay basalt is sufficiently distinct from that erupted from the nearby K-Trig centres to preclude their common origin. The preservation state of the deposit suggests an age of 100-200,000 years, and it represents an eruptive style and hazard not previously recognised at the Taupo Volcanic Centre. © 1985 Journal of the Royal Society of New Zealand.