What controls the formation of vulcanian bombs? A case study from the 1 February 2014 eruption of Tungurahua (Ecuador)

Abstract

Vulcanian eruptions are very common at many volcanoes around the world that erupt intermediate to silicic magmas. This type of eruption generates a wide variability of bombs and blocks preserving information onto the conduit processes that strongly control the dynamics of these events. After 84 years of repose, a new cycle of eruptive activity of Tungurahua volcano (Ecuador) started in October 1999, consisting of recurrent low-to-moderate explosive phases, which included sporadic stronger Strombolian to sub-Plinian pulses, as well as Vulcanian outbursts. The 1 February 2014 eruption was one of the most important Vulcanian events and was characterized by highly energetic explosions resulting from a plug conduit failure that generated a ∼ 9 km-high eruptive column and fallback pyroclastic density currents. Four different types of blocks and bombs were found in the deposits of the pyroclastic density currents: dense fragments (DB), breadcrust bombs (BCB), cauliflower bombs (CFB) and foliated, banded bombs (FB). All the different types of bombs have homogenous andesitic bulk-rock compositions but different, highly evolved matrix glass compositions ranging from rhyolitic for BCB to dacitic for CFB and DB, suggesting the occurrence of contrasting shallow crystallization processes within the conduit. The wide variability of the bombs in terms of patterns of surface cracks, external morphologies, internal density and vesicularity gradients records different conditions of formation from a same magma composition.The combination of morphological measurements, compositional data and textural analysis allow us (i) to infer the pre-eruption physical and rheological state of the conduit plug, (ii) to reconstruct the short-scale lateral and vertical gradients characterizing the plug, and (iii) to clarify the specific mechanism controlling the formation conditions of CFB, still poorly defined in the literature.