The "ashfall" coming from Irazú volcano (Costa Rica) on december 8th 1994: a phreatic explosion?

The "ashfall" coming from Irazú volcano (Costa Rica) on december 8th 1994: a phreatic explosion?

During the night of December 8th, 1994, along an unstable area at the northern flank of Irazú volcano, Costa Rica, a small volcanic debris avalanche and subsequent debris flows took place, followed by an “ashfall” that reached 30 km southwest from the volcano. At that time, these processes were inte...

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Translated title: La caída de "ceniza" proveniente del volcan Irazú (Costa Rica) el 8 de diciembre de 1994: ¿una explosión freática?
Journal Title: Revista geológica de América Central
First author: Guillermo E. Alvarado
Other Authors: Mauricio M. Mora;
Andrés Ulloa
Traslated keyword:
Language: Undetermined
Get full text: https://revistas.ucr.ac.cr/index.php/geologica/article/view/12241
Resource type: Journal Article
Source: Revista geológica de América Central; No 48 (Year 2013).
DOI: http://dx.doi.org/10.15517/rgac.v0i48.12241
Publisher: Universidad de Costa Rica
Usage rights: Reconocimiento - NoComercial - CompartirIgual (by-nc-sa)
Categories: Physical/Engineering Sciences --> Geology
Physical/Engineering Sciences --> Paleontology
Physical/Engineering Sciences --> Geosciences, Multidisciplinary
Abstract: During the night of December 8th, 1994, along an unstable area at the northern flank of Irazú volcano, Costa Rica, a small volcanic debris avalanche and subsequent debris flows took place, followed by an “ashfall” that reached 30 km southwest from the volcano. At that time, these processes were interpreted as the result of a phreatic ex- plosion. However, a review of the descriptions given in the technical reports and other documents of the time, together with the analysis of the geomorphology obtained from aerial photographs taken on December 20, 1994, by the Costa Rica National Geographic Institute, lead to discuss the validity of this interpretation, which although not ruled out, it seems unlikely. This paper proposes, based on the facts and information available, a new hypothesis of the occurrence of a volcanic debris avalanche resulting from a major landslide near the northern summit area. Consequently, the fo- llowing “ash” fall could be interpreted as generated by the dust produced byauto-fragmentation of the debris avalanche, so subsequently called co-debris avalanche dust-cloud deposit.
Translated abstract: Durante la noche del 8 de diciembre de 1994, en un área inestable del flanco norte del volcán Irazú, ocurrió una avalancha de escombros volcánicos con lahares asociados, así como la caída de “ceniza” que alcanzó hasta unos 30 km al suroeste del volcán. En ese entonces, los procesos, fueron interpretados como el resultado de una explo- sión freática. Sin embargo, la revisión de las descripciones expuestas en los informes científicos y otros documentos de la época, en conjunto con el análisis de la geomorfología obtenida a partir de fotografías aéreas tomadas el 20 de diciembre de 1994, por el Instituto Geográfico Nacional, nos conduce a discutir sobre la validez de esa interpretación, que aunque no se descarta, parece improbable. En este trabajo se propone que los hechos e información disponible nos acercan más a la hipótesis de la ocurrencia de la avalancha de escombros volcánicos producto de un deslizamiento de un segmento septentrional importante cercano a la cúspide y, en consecuencia, la ̈ceniza ̈ podría ser interpretada como el polvo producto de la auto-fragmentación de los bloques rocosos de la avalancha.