How to fragment peralkaline rhyolites: Observations on pumice using combined multi-scale 2D and 3D imaging

The Geomorphology, Structure, and Lava Flow Dynamics of Peralkaline Rift Volcanoes From High‐Resolution Digital Elevation Models

“…This may be particularly relevant for low yield stress lavas, including acrystalline lavas similar to those found in the MER. Experimental studies on the rheology of peralkaline rhyolites, similar to those at Fentale (Gibson, ; Webster et al, ) and elsewhere in the rift (Gibson, ; F. Giordano et al, ; Peccerillo et al, ), suggest that their viscosities are relatively low—~10 5.5 Pa s for a volatile‐free peralkaline rhyolite magma at 1223 °C compared to ~10 8 Pa s for a calc‐alkaline rhyolite (Di Genova et al, ; D. Giordano et al, ; Hughes et al, ). Viscosities of lava at time of emplacement are higher due to lower temperatures as well as crystal and bubble content, while high water content (present at Fentale; Webster et al, ) reduces viscosity especially for peralkaline melts.…”

“…Giordano et al, 2014;Peccerillo et al, 2007), suggest that their viscosities are relatively low-~10 5.5 Pa s for a volatile-free peralkaline rhyolite magma at 1223°C compared to~10 8 Pa s for a calc-alkaline rhyolite (Di Genova et al, 2013;D. Giordano et al, 2008;Hughes et al, 2017). Viscosities of lava at time of emplacement are higher due to lower temperatures as well as crystal and bubble content, while high water content (present at Fentale; Webster et al, 1993) reduces viscosity especially for peralkaline melts.…”

Silicic, high- to extremely high-grade ignimbrites and associated deposits from the Paraná Magmatic Province, southern Brazil

“…Magma fragmentation (e.g. Sparks, 1978;Alidibirov and Dingwell, 1996;Dingwell, 1996;Mangan and Cashman, 1996;Mader, 1998;Papale, 1999;Namiki and Manga, 2008;Gonnermann and Manga, 2012;Gonnermann, 2015;Hughes et al, 2017) may involve brittle fragmentation, inertia-driven fragmentation, water-magma interaction, or fragmentation by shear, and such processes predominantly depend on the magma rheology. Thus, brittle failure can occur in viscosities N 10 6 Pa s (Papale, 1999;Namiki and Manga, 2008) or in a fragmentation threshold of 10 8 to 10 9 Pa·s (Papale, 1999), either because: (1) it hinders bubble growth, leading to large tensile stress within the melt surrounding bubbles (Sparks et al, 1994;Toramaru, 1995;Koyaguchi and Mitani, 2005); or (2) because the deformation rate exceeds the inverse relaxation time of the melt (Webb and Dingwell, 1990;Papale, 1999;Gonnermann and Manga, 2003).…”

“…Nevertheless, low-viscosity magmas also can explosively fragment in a brittle manner, such as the Chaitén rhyolite (10 6-8 Pa s; Castro and Dingwell, 2009) and magmas with even lower viscosities, such as peralkaline (10 4-6 Pa s; Di Genova et al, 2013;Campagnola et al, 2016;Hughes et al, 2017, in press) and mafic (10 2-3 Pa s; Campagnola et al, 2016) magmas. This process would take place by rapid decompression following edifice collapse (Castro and Dingwell, 2009), rapid decompression associated with rapid ascent and extensive microlite crystallization (Campagnola et al, 2016), or high decompression rates coupled with strain localization and high bubble overpressures (Hughes et al, 2017). Indeed, there is a complex interplay between decompression, exsolution, and crystallization affecting magma rheology in the conduit (Gardner et al, 1996;Gonnermann and Manga, 2012;Campagnola et al, 2016).…”

“…Indeed, there is a complex interplay between decompression, exsolution, and crystallization affecting magma rheology in the conduit (Gardner et al, 1996;Gonnermann and Manga, 2012;Campagnola et al, 2016). Furthermore, high decompression rates are also associated with both low viscosity silicic and mafic magmas involving different fragmentation mechanisms, brittle and hydrodynamic (Namiki and Manga, 2008;Gonnermann, 2015;Mangan et al, 2014;Ferguson et al, 2016;Hughes et al, 2017).…”