Abstract
During the first week of the 2011 El Hierro submarine eruption, abundant light-coloured pumiceous, high-silica volcanic bombs coated in dark basanite were found floating on the sea. The composition of the light-coloured frothy material (‘xeno-pumice’) is akin to that of sedimentary rocks from the region, but the textures resemble felsic magmatic pumice, leaving their exact mode of formation unclear. To help decipher their origin, we investigated representative El Hierro xeno-pumice samples using X-ray computed microtomography for their internal vesicle shapes, volumes, and bulk porosity, as well as for the spatial arrangement and size distributions of vesicles in three dimensions (3D). We find a wide range of vesicle morphologies, which are especially variable around small fragments of rock contained in the xeno-pumice samples. Notably, these rock fragments are almost exclusively of sedimentary origin, and we therefore interpret them as relicts an the original sedimentary ocean crust protolith(s). The irregular vesiculation textures observed probably resulted from pulsatory release of volatiles from multiple sources during xeno-pumice formation, most likely by successive release of pore water and mineral water during incremental heating and decompression of the sedimentary protoliths.
Highlights
The El Hierro 2011–2012 eruptionEl Hierro is the westernmost and the youngest of the seven Canary Islands (1.2 Ma, Guillou et al 1996), which are widely attributed to an underlying mantle plume (e.g. Carracedo et al 2001; Geldmacher et al 2005; Zaczek et al 2015)
Xeno-pumice samples show total porosities from 63 to 75 vol.%, which were calculated from the segmented volume of voxels attributed to the ‘intra-vesicle’ phase relative to the total volume in the considered volume of interest (VOI) (Supplementary Table 2)
Textural observations of xeno-pumice in hand specimen and 3D reconstructions reveal that the isolated vesicles represent an estimated 1–2 % of the total volume, which may be more representative overall than the low values derived from Pore3D
Summary
The El Hierro 2011–2012 eruptionEl Hierro is the westernmost and the youngest of the seven Canary Islands (1.2 Ma, Guillou et al 1996), which are widely attributed to an underlying mantle plume (e.g. Carracedo et al 2001; Geldmacher et al 2005; Zaczek et al 2015). Carracedo et al 2001; Geldmacher et al 2005; Zaczek et al 2015). No historic volcanic activity was known on El Hierro before a submarine eruption commenced on October 10, 2011, off the southern. Volcanic activity continued for several months but remained submarine for the entire eruption period until activity ceased in March 2012 Carracedo et al 2012a, b, 2015; Gonzales et al 2013). Surface expressions of the eruption included green discolouration of seawater, together with intense bubbling and degassing, and the presence of two distinct types of floating lava bombs during separate stages of the eruption. During the first week of the eruption, abundant light-coloured pumiceous bombs, enclosed by a basanite coating were floating on the sea (‘xeno-pumice’), whereas entirely basanitic ‘lava balloons’, usually with hollow interiors, occurred throughout the entire eruption (Figs. 1 and 2, e.g. Kueppers et al 2012; Meletlidis et al 2012; Perez-Torrado et al 2012; Troll et al 2012)
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