The Eruptive History of the Atiamuri Pyroclastics, Maora Volcanic Centre, New Zealand : a project submitted in partial fulfilment of the requirements for the Degree of Bachelor of Science with Honours in Geology at the Department of Geological Sciences, University of Canterbury.

Abstract

The Atiamuri pyroclastic sequence (229 + 12 ka) is part of the Maroa Volcanic Centre, Taupo Volcanic Zone (TVZ), New Zealand. Data from stratigraphy, petrography, geochemistry and comparisons with other research have been gathered to produce a description of the Atiamuri deposits and their eruptive history. Units from this pyroclastic sequence occupy an area of <1 km3 and includes basal phreatomagmatic airfall layers, three ignimbrite units with different degrees of welding separated by shower beds, and overlying airfall layers with alternating ash and lapilli. The Atiamuri pyroclastics are suggested to have erupted from a single vent beneath Mandarin Dome from a small, un-zoned discrete chamber, or from partial evacuation of a larger chamber. Comparisons with other known small ignimbrite-forming events (in particular, the ~AD 1315 Kaharoa eruptive episode at nearby Tarawera Volcanic Centre) have determined that although the Atiamuri is similar in extent and was followed by dome extrusion, it differs in lithic content, absence of block-and-ash flows, interaction with water, general chemical homogeneity and lack of evidence for a mafic eruption trigger. A suggested model for the eruptive sequence at Atiamuri is as follows: (a) Small phreatomagmatic airfall eruption into a swampy lakeshore environment with possible early extrusion of a small proto-dome. (b) Emplacement of pyroclastic flows by a small laterally directed eruption from the side of the proto-dome. Pyroclastic flows are of HARI-type (high aspect ratio) and are topographically controlled. (c) Post-ignimbrite airfall with alternating small eruptions from the vent. Some further minor interaction with water possible. (d) Extrusion of Mandarin Dome from vent, pushing through the ignimbrite deposits