Giant gas bubbles in a rheomorphic vent fill at the Las Cañadas caldera, Tenerife (Canary Islands)

Abstract

During rheomorphism subsequent to fallout deposition, a portion of the densely welded fallout of the La Grieta Member flowed back into the vent from where it was erupted, while the rest of it flowed down the outer slopes of the Las Cañadas caldera in Tenerife. The welded fallout and conduit-vent structure are physically connected and constitute a rare example of this type of deposits rooted to its feeder conduit and exposed in the caldera wall. The lower part of the vent-filling rheomorphic rocks shows gas bubbles and cavities that increase in size (up to 4 m) down vent. Bubbles are deformed against other bubbles, against the steep vent walls, flattened parallel to the flow foliation planes, and elongated parallel to the flow lineation and flow fold axes. The preservation of such giant bubbles, rather than their formation, seems to be a pretty unique feature of the phonolitic products investigated here and it is likely the result of the combination of factors that acted to preserve, in the surrounding of the glass transition interval, the sealing and the late stage cooling of a pressurized system. In addition, strain drop at the base of the vent-filling rheomorphic flow caused by flow stopping against vertical vent walls may have promoted rapid gas exsolution and the formation of large bubbles.

Energetics of glass fragmentation: Experiments on synthetic and natural glasses

“…Natural glasses form in diverse environments and under a wide range of physical conditions including under elevated confining pressures and differential stresses. Examples of glass formation at elevated confining pressures include pseudotachylites, or frictional melts, resulting from high strain rate faulting of rocks [ Kendrick et al ., ; Lavallée et al ., ; Riller et al ., ; Sibson , ], volcanic dykes [ Noguchi et al ., ], and volcanic conduits [ Soriano et al ., ]. Glass formation in the above scenarios is linked to cooling of the involved melts at rates faster than crystallization and the confining pressure is elevated as a result of the overburden of the host rock (pseudotachylites and volcanic dykes) or the weight of the deposit itself (volcanic conduits).…”

Post-fragmentation vesiculation timescales in hydrous rhyolitic bombs from Chaitén volcano

“…1. Significant postfragmentation pyroclast vesiculation may occur if melt viscosity is sufficiently low and clast size sufficiently large such that cooling rates can remain low in pyroclast interiors (Thomas et al 1994;Soriano et al, 2009;Clarke et al, 2019). Post-fragmentation bubble nucleation, growth and coalescence is likely, hence, to occur in bombs erupted in Vulcanian explosions (e.g.…”