Mid-loaf crisis: Internal breadcrust surfaces in rhyolitic pyroclasts reveal dehydration quenching

Abstract

Breadcrust bombs are pyroclasts displaying fractured, dense surfaces enveloping expanded interiors, and are associated with Vulcanian explosions. We document pyroclasts from the 2008–2009 CE eruption of Chaitén (Chile) that are internally as well as externally breadcrusted. The pyroclasts are cut by intersecting micrometer- to millimeter-thick tuffisites with dense glassy walls, which grade into strongly inflated pumiceous material. We find H2O diffusion gradients proximal to the breadcrusted surfaces, such that H2O is depleted from far-field magma (0.68 ± 0.04 wt%) into dense, fractured vein walls (0.2–0.3 wt%), indicating a spatial association between H2O mass transfer within the pyroclast interior and both suppressed vesiculation and breadcrusting. We experimentally confirm that diffusive H2O depletion suppresses bubble growth at shallow conduit conditions. Therefore, we interpret the breadcrust formation to be induced by H2O diffusion and the associated rise in viscosity rather than by cooling in the classical breadcrust-formation models. We posit that a “dehydration quench” is important as degassing continues to very low H2O contents in shallow-conduit magma that continues to vesiculate.

Pre‐Eruptive Outgassing and Pressurization, and Post‐Fragmentation Bubble Nucleation, Recorded by Vesicles in Breadcrust Bombs From Vulcanian Activity at Guagua Pichincha Volcano, Ecuador

“…proposed in other studies (Tuffen et al, 2022;Wilding et al, 1996). In the core, growth rate decreases as water is lost by nucleation and growth, and newly nucleated bubbles are therefore growing progressively more slowly as vesiculation continues.…”

“…In the rim, growth rate decreases mostly due to cooling whereas it decreases by degassing in the core (Figure 9b), as a consequence of the viscosity evolution. We therefore distinguish a dominant thermal quenching in the crust and a vesiculation‐induced quench in the core as proposed in other studies (Tuffen et al., 2022; Wilding et al., 1996). In the core, growth rate decreases as water is lost by nucleation and growth, and newly nucleated bubbles are therefore growing progressively more slowly as vesiculation continues.…”

Evidence for the formation of silicic lava by pyroclast sintering

“…The 2008 eruption of Volcán Chaitén and the 2011-12 eruption of Cordón Caulle, both in Chile, have reframed our understanding of silicic volcanism. Direct observations from those iconic and hazardous events showed that silicic eruptions can exhibit simultaneous and sustained effusive lava production alongside explosive activity producing pyroclastic deposits [1][2][3][4][5][6][7][8][9][10] . Such sustained simultaneity of explosive and effusive dynamics at the same vent has challenged traditional conceptions of the shallow volcanic sub-surface and the physical processes that are inferred.…”