Dynamics of a Basaltic Plinian Eruption: Microtextural Studies of Fontana Tephra (Masaya Volcano, Nicaragua)

Fontana Tephra, one of only 4 well-documented basaltic plinian eruptions, was erupted from Masaya in late Pleistocene. Basaltic explosive volcanism is typically characterized by an open-system magma ascent and degassing that produces lava effusion or weakly explosive eruptions. Fontana Tephra shows instead several plinian pulses involving basaltic-andesite ejecta (SiO2 ~ 53 wt%) and a pre-eruptive volatile content of approximately 2-3 wt%. All juvenile clasts are highly microvesicular and show a low and narrow unimodal density distribution (300-1300 kg/m3) relatively to the other basaltic plinian eruptions (e.g. 600-2400 kg/ m3 density range for the 122 BC Etna and 700-2400 kg/ m3 for the Tarawera 1886 basaltic plinian eruption phase). The density distribution does not change significantly with time during the entire eruption (mean densities between 640 and 840 kg/ m3). However, the groundmass exhibits heterogeneous textures with a variable abundance of microlites both among different samples and especially among clasts within individual samples. This heterogeneity is also reflected upon the vesicle textures, i.e. vesicle number density, size distribution and vesicles shape. Moreover, some clasts have internal gradient of vesicularity which increases from the rim to the center, suggesting that at least part of the clasts continued to expand after fragmentation. Groundmass and vesicle textures are very different from other silicic and basaltic plinian products, showing more similar characteristics to strombolian products, even if the vesicle number densities are prominently higher. We use these textural evidences to hypothesize that at least part of the degassing history of the Fontana Tephra melt was characterized by delayed, late bubble nucleation permitting development of high degrees of volatile supersaturation in a manner analogous to more silicic melts, even if its heterogeneous characters cannot be explained with a classical plinian eruption mechanism.