Tephra fallout from the long-lasting Tungurahua eruptive cycle (1999-2014): Variations through eruptive style transition and deposition processes

ABSTRACT 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 me...

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Autores principales:	Bustillos A.,Jorge, Romero,Jorge E., Guevara C.,Alicia, Díaz-Alvarado,Juan
Lenguaje:	English
Publicado:	Servicio Nacional de Geología y Minería (SERNAGEOMIN) 2018
Materias:	Eruptive mechanism Tephra fall deposits Andesitic volcanism Tungurahua volcano
Acceso en línea:	http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0718-71062018000100047
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spelling	oai:scielo:S0718-710620180001000472018-02-22Tephra fallout from the long-lasting Tungurahua eruptive cycle (1999-2014): Variations through eruptive style transition and deposition processesBustillos A.,JorgeRomero,Jorge E.Guevara C.,AliciaDíaz-Alvarado,Juan Eruptive mechanism Tephra fall deposits Andesitic volcanism Tungurahua volcano ABSTRACT 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 silica-rich 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.info:eu-repo/semantics/openAccessServicio Nacional de Geología y Minería (SERNAGEOMIN)Andean geology v.45 n.1 20182018-01-01text/htmlhttp://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0718-71062018000100047en10.5027/andgeov45n1-3036
institution	Scielo Chile
collection	Scielo Chile
language	English
topic	Eruptive mechanism Tephra fall deposits Andesitic volcanism Tungurahua volcano
spellingShingle	Eruptive mechanism Tephra fall deposits Andesitic volcanism Tungurahua volcano Bustillos A.,Jorge Romero,Jorge E. Guevara C.,Alicia Díaz-Alvarado,Juan Tephra fallout from the long-lasting Tungurahua eruptive cycle (1999-2014): Variations through eruptive style transition and deposition processes
description	ABSTRACT 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 silica-rich 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.
author	Bustillos A.,Jorge Romero,Jorge E. Guevara C.,Alicia Díaz-Alvarado,Juan
author_facet	Bustillos A.,Jorge Romero,Jorge E. Guevara C.,Alicia Díaz-Alvarado,Juan
author_sort	Bustillos A.,Jorge
title	Tephra fallout from the long-lasting Tungurahua eruptive cycle (1999-2014): Variations through eruptive style transition and deposition processes
title_short	Tephra fallout from the long-lasting Tungurahua eruptive cycle (1999-2014): Variations through eruptive style transition and deposition processes
title_full	Tephra fallout from the long-lasting Tungurahua eruptive cycle (1999-2014): Variations through eruptive style transition and deposition processes
title_fullStr	Tephra fallout from the long-lasting Tungurahua eruptive cycle (1999-2014): Variations through eruptive style transition and deposition processes
title_full_unstemmed	Tephra fallout from the long-lasting Tungurahua eruptive cycle (1999-2014): Variations through eruptive style transition and deposition processes
title_sort	tephra fallout from the long-lasting tungurahua eruptive cycle (1999-2014): variations through eruptive style transition and deposition processes
publisher	Servicio Nacional de Geología y Minería (SERNAGEOMIN)
publishDate	2018
url	http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0718-71062018000100047
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_version_	1714205856582598656

Tephra fallout from the long-lasting Tungurahua eruptive cycle (1999-2014): Variations through eruptive style transition and deposition processes

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