Silicic magma in the shallow crust has the potential to violently erupt, depending on its ability to release overpressures caused by magmatic volatiles (outgassing). Deformation-induced outgassing is prevalent along volcanic conduit margins, where ascending magma is sheared at high rates. However, this mechanism limits outgassing to the contact with the host rock, leaving the bulk of the magma untouched and full of volatiles. This thesis presents a different mechanism of silicic outgassing that affects the interior volume of a magma body as well as the margins. Here, we present a case study of deformation features within the Miocene Sandfell laccolith, Eastern Iceland: a 0.57 km³ dome-shaped rhyolitic magma body with ~5 vol% phenocrysts and a microcrystalline groundmass. Similar textures have been reported in lava domes and intrusions with various compositions and crystallinities.

The range of deformation features are 1. porous flow bands, 2. elongated pores within flow bands, 3. 1–5 cm long tensile fractures aligned in bands, 4. 5–20 cm fractures within bands (often multiple fracture sets), and 5. breccia (densely spaced bands that are no longer distinguishable). The bands in each category range in length from ‘lenses’ (~15 cm) to laterally expansive (several meters), and usually taper at the tips. The bands are interlayered with coherent, undeformed rhyolite, and their morphology varies between planar, undulating, and anastomosing. The chapters within this thesis characterize the spatial distribution of each stage of ‘fracture banding’ and interpret their role in magma emplacement (Paper I); analyze the textures of each deformation stage on a micro-scale to interpret the rheology of the magma during formation (Paper II); investigate the mineral assemblage of fracture fillings and apply results to metal separation from parent magma in early ore systems (Paper III); and attempt to experimentally recreate fracture bands in a laboratory setting (Paper IV).

The results of these chapters suggest the deformation features formed from a rheological contrast between flow bands with different crystallinity. Emplacement-related stress localized along the weaker, more melt-rich flow bands, driving the ductile magma to deform through viscous and brittle processes. The fractures arrested against the stiffer rhyolite in the more crystalline flow bands, while drawing in surrounding melt and fluids. This, plus the interconnectedness of the fracture bands, implies an efficient outgassing system.

Here we show that fracture banding is an outgassing mechanism taking place in silicic magma undergoing deformation.