Revista geológica de América central ISSN Impreso: 0256-7024 ISSN electrónico: 2215-261X

OAI: https://revistas.ucr.ac.cr/index.php/geologica/oai
The 1723 A.D. violent strombolian and phreatomagmatic eruption at Irazú volcano, Costa Rica
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Keywords

strombolian phreatomagmatic eruptions
proximal lithophacies
volcano tectonic seismicity
magma mingling
irazú volcano
costa rica
erupciones strombolianas y freatomagmáticas
litofacies proximales
sismicidad volcano tectónica
mezcla de magmas
volcán irazú
costa rica

How to Cite

Alvarado, G. E., & Schmincke, H. U. (2011). The 1723 A.D. violent strombolian and phreatomagmatic eruption at Irazú volcano, Costa Rica. Revista geológica De América Central, 48. https://doi.org/10.15517/rgac.v0i48.12212

Abstract

The largest of the recorded historic eruptions at Irazú volcano began on February 16, 1723 and lasted until at least December 11. We here critically examine deposits of this eruption exposed on the summit of Irazú. Our reconstruction of the eruption is based on the unique chronicle of the Spanish governor Diego de la Haya. The eruption began with a < 10 cm thick surge deposit of phreatic origin showing block sag structures. The deposit is overlain by 6 m-thick coarse-grained basaltic andesitic non-graded juvenile fallout tephra consisting of highly vesicular (22-59 vol.%) bombs and lapilli with minor hydrothermally altered lapilli (1-7 vol.%) and rare light colored andesitic vesicular lapilli (< 1%). These fallout deposits are interpreted as strombolian, possible generated during a short-lived scoria cone at the end of February 1723, dominate volumetrically in the proximal facies. Overlying <1.2 m thick phreatomagmatic deposits of finely laminated lapilli-bearing gray ash (fallout and surge deposits) some with contorted bedding and sag structures, are in turn overlain by a 1.2 m thick bed of ash matrix-rich bomb/block deposit. The 1723 eruption was accompanied by shallow volcano-tectonic earthquakes (Modified Mercalli scale Intensity MMI VI-VII, magnitude ML ~5.5) that possibly facilitated magma/water interaction. Phenocrysts in the basaltic andesite (~53-55 wt.% SiO2) bombs comprise plagioclase (6.1-21.6 vol.%, An52-35), clinopyroxene (2.5-10 vol.%), orthopyroxene (0.7-2 vol.%), olivine (0.1-2.2 vol.%; Fo76-88) and Fe/Ti-oxides (0.1-1%), in a groundmass (66.5-90.3 vol. %), dominated by plagioclase (An69-54), clinopyroxene and opaques in brown and black glass with the same range of chemical composition (SiO2= 57-64 wt.%). Rare white pumiceous lapilli in the scoria deposits are high-K, hornblende andesite (SiO2: 58-60 wt.%), geochemically unrelated to the scoria deposits. Thus, two different magmas co-existing in the magma chamber were mingled shortly before, and during, the eruption, suggesting that the eruption was triggered by magma mingling between hornblende andesite and basaltic andesite magma.
https://doi.org/10.15517/rgac.v0i48.12212
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