Shallow-seated controls on styles of explosive basaltic volcanism: a case study from New Zealand |
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| Affiliation: | 1. Dipartimento di Scienze, Università di Roma Tre, Largo San L. Murialdo 1, 00146 Roma, Italy;2. Dipartimento di Scienze della Terra, Sapienza Università di Roma, Piazzale Aldo Moro, 5, 00185 Roma, Italy;3. Istituto Nazionale di Geofisica e Vulcanologia, sede di Roma, Via di Vigna Murata 605, 00143 Roma, Italy;4. Escuela Centroamericana de Geología & Red Sismológica Nacional, Universidad de Costa Rica, Ciudad Universitaria Rodrigo Facio, Costa Rica;1. Department of Geological Sciences, University of Oregon, Eugene, OR 97403, USA;2. Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, USA;1. Université de Paris, Institut de physique du globe de Paris, CNRS UMR 7154, F-75005 Paris, France;2. Observatoire volcanologique et sismologique de Guadeloupe, Institut de physique du globe de Paris, F- 97113 Gourbeyre, France;3. Laboratoire Magmas et Volcans, OPGC, Clermont-Ferrand, CNRS UMR 6524, France;1. School of Earth and Environmental Sciences, The University of Manchester, UK;2. School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA;3. Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, Italy;1. Section of Earth and Environmental Sciences, University of Geneva, Switzerland;2. Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna, Italy |
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| Abstract: | The pyroclastic deposits of many basaltic volcanic centres show abrupt transitions between contrasting eruptive styles, e.g., Hawaiian versus Strombolian, or `dry' magmatic versus `wet' phreatomagmatic. These transitions are controlled dominantly by variations in degassing patterns, magma ascent rates and degrees of interaction with external water. We use Crater Hill, a 29 ka explosive/effusive monogenetic centre in the Auckland volcanic field, New Zealand, as a case study of the transitions between these end-member eruptive styles. The Crater Hill eruption took place from at least 4 vents spaced along a NNE-trending, 600-m-long fissure that is contained entirely within a tuff ring generated during the earliest eruption phases. Early explosive phases at Crater Hill were characterised by eruption from multiple unstable and short-lived vents; later, dominantly extrusive, volcanism took place from a more stable point source. Most of the Crater Hill pyroclastic deposits were formed in 3 phreatomagmatic (P) and 4 `dry' magmatic (M) episodes, forming in turn the outer tuff ring and maar crater (P1, M1, P2) and scoria cone 1 (M2–M4). This activity was followed by formation of a lava shield 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 from adjacent vents and rapid variations in the extent and timing of magma:water interaction at each vent. The nature of the wall-rock lithics, and these rapid variations in inferred water/magma ratios imply interaction was occurring mostly at depths of ≤80 m, and the vesicularity patterns in juvenile clasts from these and the P beds imply that rapid degassing 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. |
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