Comparisons and Contrasts Between Subaerial and Submarine Explosive Volcanic Processes: Case Studies From the Kermadec Arc

Tuesday, 31 January 2017
Marina/Gretel (Hobart Function and Conference Centre)
Melissa D Rotella1, Simon J Barker1, Colin J N Wilson2, Ian Craig Wright3 and Richard J Wysoczanski4, (1)University of Auckland, Auckland, New Zealand, (2)Victoria University of Wellington, Wellington, New Zealand, (3)University of Canterbury, Christchurch, New Zealand, (4)NIWA National Institute of Water and Atmospheric Research, Wellington, New Zealand
Abstract:
We compare and contrast pyroclasts from subaerial and submarine explosive eruptions at Raoul, Raoul SW, Macauley, Healy and Havre volcanoes (Kermadec arc). These volcanoes share tectonic settings, and have erupted Holocene dacite-rhyodacite magma with similar chemical and physical properties. By minimising the variables of magma chemistry and tectonic setting, and considering subaerial eruptions versus submarine eruptions that occurred over a range of water depths, we can constrain the processes of submarine explosive volcanism. We quantify the pyroclasts using density data to screen the clast population, then have undertaken 2-D bubble size and number quantification.

Pumices from six subaerial eruptions from Raoul show a scarcity of pyroclasts with ~65-75% vesicularity with most deposits having pyroclasts with ~82% vesicularity modes. The 65-75% vesicularity clasts, however, have the highest bubble number density (BND) values, with a large population of small bubbles (<20 μm diameter), regardless of eruption intensity, style or degree of interaction with water. We suggest that this 65-75% vesicularity range is pivotal in magma fragmentation, with higher vesicularity clasts preserving varying degrees of post-fragmentation bubble growth and coalescence, and hence decrease in BND values. The implication is that modal density clasts do not represent magma at fragmentation, but instead preserve some degree of post-fragmentation expansion prior to quenching. This brings into question their suitability for interpreting fragmentation processes.

Comparisons between subaerial and submarine explosively-erupted pyroclasts (sampled via dredging) yield inferences about the role of eruption rate and water depth on eruption dynamics. At high eruption rates, fragmentation occurs within the conduit and the higher dynamic pressure of a significant water column inhibits rapid decompression and post-fragmentation expansion of clasts. If the eruption jet breaches the sea surface, the rapid decrease in pressure causes an additional homogenous nucleation event, seen as zones of abundant small bubbles within the clast groundmass textures. These distinctive differences open up the possibility of being able to fingerprint subaerial versus submarine erupted pyroclasts in ancient volcaniclastic sequences.

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