Groundmass crystallinities in the Auckland Volcanic Field as indicators of flow emplacement rates
1Rachel Teasdale , 1Brady Giles, 2Michael Rowe
Affiliations: 1Earth & Environmental Sciences, California State University, Chico US; 2School of Environment, University of Auckland, Auckland NZ
Presentation type: Poster
Presentation time: Thursday 16:30 - 18:30, Room Poster Hall
Poster Board Number: 13
Programme No: 3.7.20
Abstract
The Auckland Volcanic Field (AVF) includes 53 small basaltic cones that erupted from 193- 0.55 ka [1]. With an increased frequency of activity over the last 60 ka around the city limits of Auckland (population, 1.7 million), activity in the AVF could be "in its infancy" [2]. Groundmass crystallinities from samples along the lengths of basaltic flow units from Rangitoto (504 ya [3], the youngest in the AVF, located 13 km from city center) and from Te Kōpuke/Mount St. John (75 ka [2], within Auckland city limits, and the longest flow in the AVF), are used to constrain groundmass crystal growth during lava flow emplacement. Distal samples in 'a 'ā flows from both volcanoes (2 km from the vent at Rangitoto [4] and 9.3 km from the vent at Mount St. John [4]) have groundmass plagioclase crystallinities that are more abundant, have larger crystal lengths and areas, and are blockier (lower aspect ratios) than in samples closer to the vents. Ongoing work will use cooling experiments to reproduce observed crystallinities to constrain the cooling rates of lavas as they were emplaced. Results can then be used to approximate the timing of emplacement, potentially for use by civil defense/emergency management in establishing evacuation protocols in planning for possible eruptive activity. [1] Hopkins et al., 2020; [2] Leonard et al., 2017; [3] Needham et al., 2011; [4] Rhodé, 2016