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

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 cross-stratified 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.