Shifting eruption styles during the emergence of Akaroa strato-shield volcano, Banks Peninsula, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The uniquely eroded harbour of Akaroa volcano provides a rare opportunity to study the dissected core of the basaltic to trachytic strato-shield volcano. With key exposures of both the early phase trachytic to basaltic eruptive deposits and the later phase voluminous basaltic deposits (9.6 – 8.6 Ma) that make up the majority of the emergent volcanic flank. This thesis aims to illustrate the volcanic processes that dominate the diverse early stages of emergent volcanism. We have built on previous research to produce detailed maps and stratigraphic logs of key extrusive and intrusive sequences. We further identify eruptive packages and their facies to identify eruptive centres, and correlate early stratigraphy. This study reveals the emergence of local volcanic centres with differing eruptive styles, chemistry and volumes. An explosive phreatomagmatic trachyte tuff ring and dome/flow complex dominated early eruptions. This extensive trachytic centre had multiple phases of activity, displaying both temporal and spatial transitions in style. Temporal facies transitions consisted of changes from a subaqueous to emergent hyaloclastite dome, to an explosive phreatomagmatic trachyte tuff ring to an effusive trachytic dome (Fig 7.1 – Fig 7.7). Whereas spacial transitions consisted of lateral facies variations within the pyroclastic surge and air fall deposits of the tuff ring reflecting 'en route' changes in deposition. Smaller volcanic centres migrated around the margins of this larger trachytic centre (Fig 7.1 – Fig 7.7). These eruptions variously interacted with seawater forming small tuff and scoria cones. Generally, these smaller eruptions show a progression from low angle phreatomagmatic palagonite rich lapilli tuffs to steeper bedded spatter and bomb dominated deposits. This transition in facies likely represents the emergence of individual volcanoes with later deposits showing less evidence for interaction with seawater. These discrete centres later coalesced through deposition of the more extensive lava flows forming the early volcanic complex of Akaroa Volcano. Erosional processes dramatically reshaped the volcanic complex. Bays of the present day harbour represent eroded basaltic explosive centres, as pyroclastic units are preferentially eroded by the sea. Whereas more coherent lavas tend to form headlands within the harbour. In summary, the findings of this research have refined maps, stratigraphy, lithologic descriptions and facies interpretations of Akaroa volcanoes emergent deposits. Thus enabling the development of a geological formation model for the early to emergent stages of Akaroa Volcano. Furthering the understanding of early to emergent, shallow magmatic, volcanologic processes, associated with alkali basaltic and trachytic eruptions at Akaroa Volcano.