Abstract
Abstract. The ongoing eruptive cycle of Tungurahua volcano (Ecuador) since 1999 has been characterised by over 15 paroxysmal phases interrupted by periods of relative calm. Those phases included one Subplinian as well as several Strombolian and Vulcanian eruptions and they generated tephra fallouts, pyroclastic density currents (PDCs) and lava flows. The 1 February 2014 eruption occurred after 75 days of quiescence and only 2 days of pre-eruptive seismic crisis. Two short-lived Vulcanian explosions marked the onset of the paroxysmal phase, characterised by a 13.4 km eruptive column and the trigger of PDCs. After 40 min of paroxysm, the activity evolved into sporadic Strombolian explosions with discrete ash emissions and continued for several weeks. Both tephra fall and PDCs were studied for their dispersal, sedimentology, volume and eruption source parameters. At large scale, the tephra cloud dispersed toward the SSW. Based on the field data, two dispersal scenarios were developed forming either elliptical isopachs or proximally PDC-influenced isopachs. The minimum bulk tephra volumes are estimated to 4.55 × 106 m3, for an eruption size estimated at volcanic explosivity index (VEI) 2–3. PDCs, although of small volume, descended by nine ravines of the NNW flanks down to the base of the edifice. The 1 February 2014 eruptions show a similar size to the late 1999 and August 2001 events, but with a higher intensity (I 9–10) and shorter duration. The Vulcanian eruptive mechanism is interpreted to be related to a steady magma ascent and the rise in over-pressure in a blocked conduit (plug) and/or a depressurised solidification front. The transition to Strombolian style is well documented from the tephra fall componentry. In any of the interpretative scenarios, the short-lived precursors for such a major event as well as the unusual tephra dispersion pattern urge for renewed hazard considerations at Tungurahua.
Highlights
In comparison to Strombolian or Plinian fallout deposits, Vulcanian fine-grained fallout deposits often lack a sufficient preservation potential for extensive studies from proximal to distal portions (Rose et al, 2008)
Vulcanian eruptions can be associated with the generation of pyroclastic density currents (PDCs; e.g. Brown and Andrews, 2015), especially if they trigger moderate andesitic eruptions, which are more prone to produce PDCs than other eruption types (Bernard et al, 2016)
This study focuses on the pyroclastic deposits associated with the eruptions from 1 to 14 February 2014 of Tungurahua volcano (Ecuador)
Summary
In comparison to Strombolian or Plinian fallout deposits, Vulcanian fine-grained fallout deposits often lack a sufficient preservation potential for extensive studies from proximal to distal portions (Rose et al, 2008). These eruptions produce predominantly fine-ash to lapilli-sized tephra which is dispersed to heights < 20 km and last on the order of minutes (Morrissey and Mastin, 2000). Tephra fallout and PDC erupted material are described and used to infer the style transition, eruption magnitude and source parameters. The meaning of this eruption is put in perspective with the context of Tungurahua’s ongoing cycle
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