Maar‐diatreme geometry and deposits: Subsurface blast experiments with variable explosion depth

Abstract

Basaltic maar-diatreme volcanoes, which have craters cut into preeruption landscapes (maars) underlain by downward-tapering bodies of fragmental material commonly cut by hypabyssal intrusions (diatremes), are produced by multiple subsurface phreatomagmatic explosions. Although many maardiatremes have been studied, the link between explosion dynamics and the resulting deposit architecture is still poorly understood. Scaled experiments employed multiple buried explosions of known energies and depths within layered aggregates in order to assess the effects of explosion depth, and the morphology and compaction of the host on the distribution of host materials in resulting ejecta, the development of subcrater structures and deposits, and the relationships between them. Experimental craters were 1-2 m wide. Analysis of high-speed video shows that explosion jets had heights and shapes that were strongly influenced by scaled depth (physical depth scaled against explosion energy) and by the presence or absence of a crater. Jet properties in turn controlled the distribution of ejecta deposits outside the craters, and we infer that this is also reflected in the diverse range of deposit types at natural maars. Ejecta were dominated by material that originated above the explosion site, and the shallowest material was dispersed the farthest. Subcrater deposits illustrate progressive vertical mixing of host materials through successive explosions. We conclude that the progressive appearance of deeper-seated material stratigraphically upward in deposits of natural maars probably records the length and time scale for upward mixing through multiple explosions with ejection by shallow blasts, rather than progressive deepening of explosion sites in response to draw down of aquifers.

The effects of the host-substrate properties on maar-diatreme volcanoes: experimental evidence

“…In Pad 2d, the structure of the visible disruption presented in Fig 5c is quite similar to what could be expected and interpreted for a 'natural' small maar-diatreme (e.g. Miyakejima example in Geshi et al 2011;Graettinger et al 2014). …”

“…They control eruptive jet behavior and consequently the ejecta distribution and proportion Graettinger et al 2014Graettinger et al , 2015. Nevertheless, the host substrate does have an influence on crater morphology for primary blasts.…”

“…The experiments consisted of shallow-buried explosions (blasts) into layered granular media (pads), in order to mimic the effects of discrete explosions, resulting in the formation of a crater and its subsurface structure. The reader is referred to Ross et al (2013), Graettinger et al (2014), and Valentine et al ( , 2015, for a detailed description of the experiments, and for information on energetics of phreatomagmatic explosions in forming maar-diatremes. The blasts were performed using chemical explosives of known size and energy density, emplaced at defined scaled depths (scaled…”

“…The variability in depth increase from the second blast is due to the variability in the pre-existing craters. The presence of a crater before an explosion has been proven to influence the jet shape which affects proximal deposition and fallback Graettinger et al 2014). The pre-existing crater walls limit initial lateral expansion of the jet, resulting in a lower and more vertically focused jet and thus more fallback in case of blasts deeper than optimal scaled depth .…”

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

“…Scaled depth can be divided into regimes where explosions are close to optimal (excavate the greatest volume of ejecta for a given energy) and those that are either shallower than, or deeper than, that optimal scaled depth . Experiments have constrained an optimal scaled depth for discrete explosions through debris to~0.004 m/J 1/3 and the depth where explosions are confined, and no ejecta is produced, to 0.008 m/J 1/3 (Goto et al 2001;Graettinger et al 2014;Valentine et al 2014;Sonder et al 2015). Explosions above the confinement depth transport material out of the vent, but the scaled depth and overlying topography (e.g., craters produced by previous explosions) influence how much material makes it out of the vent and the how it is transported away from the vent (Graettinger et al , 2015a.…”