Shallow-seated controls on styles of explosive basaltic volcanism: a case study from New Zealand

The pyroclastic deposits of many basaltic volcanic centres show abrupt tran sitions between contrasting eruptive styles, e.g., Hawaiian versus Strombol ian, or 'dry' magmatic versus 'wet' phreatomagmatic. These transitions are controlled dominantly by variations in degassing patterns, magma ascent rat es and degrees of interaction with external water. We use Crater Hill, a 29 ka explosive/effusive monogenetic centre in the Auckland volcanic field, N ew Zealand, as a case study of the transitions between these end-member eru ptive styles. The Crater Hill eruption took place from at least 4 vents spa ced along a NNE-trending, 600-m-long fissure that is contained entirely wit hin a tuff ring generated during the earliest eruption phases. Early explos ive phases at Crater Hill were characterised by eruption from multiple unst able and short-lived vents; later, dominantly extrusive, volcanism took pla ce from a more stable point source. Most of the Crater Hill pyroclastic dep osits were formed in 3 phreatomagmatic (P) and 4 'dry' magmatic (M) episode s, forming in turn the outer tuff ring and maar crater (P1, M1, P2) and sco ria cone 1 (M2-M4). This activity was followed by formation of a lava shiel d and scoria cone 2, Purely 'wet' activity is represented by the bulk of P1 and P2, and purely 'dry' activity by much of M2-M4. However, M1 and parts of M2 and M4 show evidence for simultaneous eruptions of differing style fr om adjacent vents and rapid variations in the extent and timing of magma:wa ter interaction at each vent. The nature of the wall-rock lithics, and thes e rapid variations in inferred water/magma ratios imply interaction was occ urring mostly at depths of less than or equal to 80 m, and the vesicularity patterns in juvenile clasts from these and the P beds imply that rapid deg assing occurred at these shallow levels. We suggest that abrupt transitions between eruptive styles, in time and space, at Crater Hill were linked to changes in the local magma supply rate and patterns and vigour of degassing during the final metres of ascent. (C) 1999 Elsevier Science B.V. All righ ts reserved.