Dynamics and style transition of a moderate, Vulcanian-driven eruption at Tungurahua (Ecuador) in February 2014: Pyroclastic deposits and hazard considerations

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 PDCinfluenced 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.