PARASITIC ERUPTION DYNAMICS OF THE AKAROA VOLCANIC COMPLEX: LE BONS BAY SCORIA CONES, BANKS PENINSULA

Flank scoria cones can be used to understand the eruptive history of volcanic systems as they reflect conditions under which the volcano was active (i.e. magmatic systems and paleo-topography). A lineament of scoria cones is exposed on the shore platform of Le Bons Bay, Banks Peninsula, New Zealand, each reflecting a different phase in volcanic growth. Each cone, now dissected and exposed along the shore platform, provides insight into eruptive facies, magmatic systems, and the evolution of the Miocene Akaroa Volcanic Complex. This study combines detailed geological mapping, sampling, and analysis, to interpret eruptive facies and dynamics of the largest scoria cone in this sequence.

Six facies are identified in the cone, defined by clast concentration, size, shape and degree of welding; bomb-rich, ash-rich, mixed scoria, densely welded scoria, agglutinated lava, and ash beds. These units reflect scoria cone stages of early formation and emergence with phreatomagmatic eruptions, the change to Strombolian and Hawaiian during cone building, and central vent ash fall. Post eruption the cone was overtopped by central Akaroa lava flows which surrounded and partially buried the scoria cone, promoting preservation from subsequent seaward erosion. The cone was also intruded by multiple dykes. The geomorphology of this cone is expressed in the current topography, with an arcuate shape and dip slopes evident in the hill slope and on the shore platform. By incorporating these features with mapped facies, cone dimensions are hypothesised as 820m in diameter and reached a height of 60-70m.

This scoria cone reflects the once outer flanks of the Akaroa Volcanic Complex, and through the analysis of bounding lava flows the relationship of parasitic eruptions to central vent eruptives can be further investigated.