Resumen
The Tungurahua volcano (Northern Andean Volcanic Zone) has been erupting since 1999, with at least
four eruptive phases up to present. Although a dozen of research focuses in tephra fall deposits during this period, none
of them cover the full eruptive cycle. We investigated the eruptive mechanisms and tephra fall deposition processes at
Tungurahua between 1999 and 2014, through systematic analyses of tephra samples collected westward of the volcano
using mechanical sieving grain size analysis, lithology, scanning electron microscopy, X-Ray fluorescence and X-Ray
diffraction. Tephra is compounded by varying amounts of scoria (black and brown), lithics, hydrothermally altered fragments,
pumice, glass shards and free crystals. Textural analyses of juvenile grains (scoria, pumice and glass shards) revealed
a diversity of features concerning to their vesicularity, shape and surface/perimeter. Initially, tephra was characterized
by hydrothermally altered fragments related to a phreatic phase which then evolved to a pure magmatic activity with
Strombolian eruptions. A homogeneous andesitic composition was observed between 1999 and 2003; however silicarich
compositions occurred later in 2006. Similarly, the mineral assemblage contained plagioclase, pyroxene and olivine,
but magnetite and akermanite were then included during 2006, thus indicating the eruption of a new, probably mixed
magma. As consequence, Plinian activity occurred in August 2006. Further activity in 2007 ejected notable amounts (40-
65%) of recycled material during Vulcanian eruptions. New eruptions occurred between 2008 and 2010, and juvenile ash
revealed the interplay between brittle and ductile fragmentation through ash explosions, jetting events and Strombolian
activity. The activity between 2010 and 2012 incorporated hydrothermally altered material at time that eruptive silences
became longer and frequent, thus suggesting the development of a sporadic hydrothermal system. Finally, between 2013
and 2014 a series of Vulcanian events occurred. Observed grain size distributions allow us to propose three different
processes occurring during tephra deposition: 1) deposition of multiple ash plumes, 2) contributions from elutriated
pyroclastic density currents or grain size mixing due to major eruptions, and 3) the aggregation of particles due to rain
and/or lighting. From mineralogy and grain size we infer that exposition to ash may produce acute human health effects.