Evidence for the relative depths and energies of phreatomagmatic explosions recorded in tephra rings

Abstract

Experimental work and field observations have inspired the revision of conceptual models of how maar-diatreme eruptions progress and the effects of variable energy, depth, and lateral position of explosions during an eruption sequence. This study reevaluates natural tephra ring deposits to test these new models against the depositional record. Two incised tephra rings in the Hopi Buttes Volcanic Field are revisited, and published tephra ring stratigraphic studies are compared to identify trends within tephra rings. Five major facies were recognized and interpreted as the result of different transportation and depositional processes and found to be common at these volcanoes. Tephra rings often contain evidence of repeated discrete phreatomagmatic explosions in the form of couplets of two facies: (1) massive lapilli tuffs and tuff breccias and (2) overlying thinly stratified to crossstratified tuffs and lapilli tuffs. The occurrence of repeating layers of either facies and the occurrence of couplets are used to interpret major trends in the relative depth of phreatomagmatic explosions that contribute to these eruptions. For deposits related to near-optimal scaled depth explosions, estimates of the mass of ejected material and initial ejection velocity can be used to approximate the explosion energy. The 19 stratigraphic sections compared indicate that near-optimal scaled depth explosions are common and that the explosion locations can migrate both upward and downward during an eruption, suggesting a complex interplay between water availability and magma flux. Reverse to normal coarse-tail graded tuff breccias inferred to be the product of closely timed phreatomagmatic explosions, and deposits of magmatic gas-driven explosions, were observed interspersed with discrete explosion deposits. This study not only provides a framework for more detailed interpretations of eruption sequences from tephra rings but also highlights the gap in our understanding of syn-eruptive hydrology and variations in magma flux that enables phreatomagmatic explosions.

Insights into the dynamics of mafic magmatic-hydromagmatic eruptions from volatile degassing behaviour: The Hverfjall Fires, Iceland

“…Interestingly, the depth distribution for lithostatic pressure is consistent with experimentally-determined optimum explosion depths of ~100 m, and maximum depths for subaerial tephra ejection of ~200 m (Graettinger et al, 2014;Valentine et al, 2014). Similar explosion depths of 30-115 m were derived for large blocks found in ejecta within natural deposits, assuming near-optimal scaled depths (0.004 m/J 1/3 ) and explosion energies determined from the deposit volume and calculated ejection velocities (Graettinger and Valentine, 2017). Notably, these depth estimates are sensitive to both assumptions made during the calculation of ejection velocities and uncertainties on the mass ejected, and so the observed ejecta could easily reflect deeper source locations by 10s of metres (Graettinger and Valentine, 2017), thus potentially improving further the correspondence with the saturation pressures calculated here.…”

“…Similar explosion depths of 30-115 m were derived for large blocks found in ejecta within natural deposits, assuming near-optimal scaled depths (0.004 m/J 1/3 ) and explosion energies determined from the deposit volume and calculated ejection velocities (Graettinger and Valentine, 2017). Notably, these depth estimates are sensitive to both assumptions made during the calculation of ejection velocities and uncertainties on the mass ejected, and so the observed ejecta could easily reflect deeper source locations by 10s of metres (Graettinger and Valentine, 2017), thus potentially improving further the correspondence with the saturation pressures calculated here. However, dissolved volatile concentrations remain elevated throughout the eruption stratigraphy (Fig.…”

Review of Explosive Hydrovolcanism

“…In the case of magma and groundwater explosive interaction excavation can also incorporate abundant country rock fragments derived from the conduit. While the type of country rock and their relative abundance in the forming pyroclastic deposits are commonly used as a proxy to determine the explosion depth [186,189], recent large scale experiments on cratering suggest that the appearance of different lithologies in the eruptive products do not directly relate to explosion depth as multiple explosive bursts can gradually push deep-seated rock fragments closer to the surface where a successful explosive event may propel them out to be deposited with the pyroclastic succession [190][191][192][193][194][195][196][197][198]. It is also important to highlight that maar volcanoes generally refer to a morphological feature (depression, commonly lake-filled) (Figure 3c-g).…”

Hydrothermal eruption dynamics reflecting vertical variations in host rock geology and geothermal alteration, Champagne Pool, Wai-o-tapu, New Zealand

“…Here, we compare the Champagne Pool eruption to studies of natural explosion craters (Yokoo et al 2002;Graettinger and Valentine 2017;D'Elia et al 2020), and field-based explosion experiments (Goto et al 2001;Ohba et al 2002;Taddeucci et al 2013;Graettinger et al 2014;Sonder et al 2015;Macorps et al 2016). The experiments show that eruption dynamics and Fig.…”