The 1723 AD violent Strombolian and phreatomagatic eruption at Irazu volcano (Costa Rica)

Alvarado, G. E. and Schmincke, Hans-Ulrich (2013) The 1723 AD violent Strombolian and phreatomagatic eruption at Irazu volcano (Costa Rica) Revista Geologica de America Central, 48 . pp. 41-61.

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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 fi nely 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 (Modifi ed 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.

Document Type: Article
Keywords: Strombolian/phreatomagmatic eruptions, proximal lithophacies, volcano-tectonic seismicity, magma mingling, Irazú volcano, Costa Rica
Research affiliation: OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-MUHS Magmatic and Hydrothermal Systems
Refereed: No
ISSN: 0256-7024
Date Deposited: 22 Oct 2013 10:25
Last Modified: 13 Sep 2016 10:36

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