Abstract
Shallow to emergent basaltic subaqueous explosive eruptions, here referred as Surtseyan, are a source of pyroclastic material expelled from subaqueous vents and transferred to the water column and atmosphere. They can threaten coastal communities in different ways. One of these is through the generation of pyroclastic density currents travelling over water once the eruption becomes subaerial, and which may be preceded or accompanied by eruption-fed currents moving along the sea/lake floor; another potential danger offered by these eruptions is from the subaerial eruption, which can deliver hot fragments locally and abrasive ash over hundreds or thousands of km2. Both for their potential hazard, and to better understand the mechanics of such eruptions as one class of volcanic thermodynamics, these volcanos have been the subject of significant investigations for more than 50 years, particularly since the start of the Surtsey eruption in 1963. One approach for studying the early stages of these eruptions is through analysis of their proximal, edificebuilding deposits, but these are commonly either inaccessible (under water) or poorly preserved by the time they are exposed subaerially. The sites selected for this work, Pahvant Butte, Utah, and Black Point, California, USA, overcome these limitations. These volcanos were formed in the late Pleistocene by eruptions at hundred meter depths in the giant former Lake Bonneville and Lake Russell respectively. The water is now drained and all the deposits (both edifice-forming and at medial distances on the old lake floors) are easily accessible and maintain a good level of preservation. This allowed me to investigate the eruption dynamics and conduit conditions of these volcanoes, potentially extendable to other Surtseyan volcanoes.
The combination of field-based work, granulometry, geochemical analysis of majorelements (glass, minerals and melt inclusions) and dissolved-water content of tephra glass, and particle tomography allow me to reconstruct the pre-eruptive and syn-eruptive dynamics for Pahvant Butte and Black Point. Overall, the results suggest that the explosivity of these two monogenetic volcanoes was mainly driven by hydromagmatic explosions, with gas expansion only playing a secondary role. Inferred eruption dynamics are consistent with those inferred by previous authors, and the eruptions yielded ash dispersed both by eruption-fed density currents subaqueously, and subaerially by wind. This study demonstrates that eruption-fed density currents generated prior to eruptive emergence can travel up to > 20 km on a nearly horizontal lake floor (i.e., Pahvant Butte), even in early stages of the subaqueous eruption. In this work, it is shown that geochemistry can be a powerful tool for correlating edifice-building tephra with ash deposited in proximal/medial locations, based on the magma evolution recorded in the glass.
Another way to study shallow subaqueous explosive eruptions is through experimental investigation. I carried out underwater experiments at bench scale to investigate the behaviour of particles transferred to the water column from underwater explosive bursts. In the natural counterpart, there are two different types of particles involved in the explosions, juvenile particles, directly produced by the magma fragmentation, and non-juvenile particles (sea/lake floor sediments or country rock sediments). The former ones are initially dry, while the latter are wet, and these conditions were investigated independently during the experiments. The results show that these two conditions impart different particle behaviours. Wet particles are better coupled with tank water than are dry particles, and this behaviour can be associated with a more efficient transfer in nature from eruption bursts into eruption-fed subaqueous currents.