Additionally, contemporaneous explosive eruptions from the advancing lava accompanied effusion (“hybrid activity” Schipper et al., 2013 Castro et al., 2014 Castro and Walter, 2021). It is important because (1) it featured a wide range of emplacement processes, including endogenous and exogenous ( Farquharson et al., 2015 Magnall et al., 2017) and advance-limiting modes ( Castruccio et al., 2013) acting contemporaneously in different parts of the lava flow, and (2) the advance continued for ~8 mo after new magma ceased being erupted from the vent ( Tuffen et al., 2013). The 2011–2012 eruption of Cordón Caulle, Chile, was the first large-volume, crystal-poor (i.e., obsidian), rhyolite lava emplacement observed throughout the entire eruptive event. Observations at Active Silicic Lava Flows In so doing, we documented evidence of synchronous localized explosive processes intimately linked to the deformation. Specifically, this study examined how different fracture types and sizes relate to one another, and whether they formed through a single continuous deformation process, or if they represent different processes operating at different times. We attempted to constrain the relative timings, mechanisms, and conditions under which deformation occurred through a combination of textural and structural observations. 1), to investigate the emplacement of silicic lavas through the lens of brittle deformation of the upper surface and margins. This paper documents a field-based study at Obsidian Dome, California, USA ( Fig. Unlike intermediate-composition lava domes, where there are several active worldwide at any one time, most of what is known about the dynamics of rhyolitic and rhyodacitic lava flows must be inferred from prehistoric examples. They also form on low-relief caldera floors where they can spread radially (e.g., Deadman Dome, California, USA Sampson and Cameron, 1987). Lava flows occur on the flanks of volcanoes (e.g., Medicine Lake volcano, California, USA) where the lava is not restricted to a deep crater, and when sufficient magma supply is available, the flow develops into a unidirectional lava flow (a “coulee” e.g., the Chao dacite, Chile Big Obsidian Flow, Newberry volcano, Oregon, USA ). Helens, Washington, USA, 1980–1986, 2004–2008) and have been the subject of numerous studies (e.g., Fink et al., 1990 Sparks et al., 2000 Pallister et al., 2013), direct recorded observations of active rhyolitic lava flows did not occur until two Chilean eruptions, Chaitén and Cordón Caulle, in 20–2012, respectively ( Lara, 2009 Schipper et al., 2013). Whereas lava domes are ubiquitous at intermediate and silicic volcanoes (e.g., Mount St. Field-based cataloguing of the complexities of fracture surfaces provides qualitative constraints for the future mechanical modeling of effusive lavas.Įxtrusive silicic rocks are common throughout the geologic record from a range of tectonic environments and are important hosts of epithermal mineralization. Ornamentations preserved on the fractured surfaces record degassing and explosive fragmentation away from the vent throughout the lava’s emplacement, suggesting explosive activity was occurring during the effusive emplacement. The largest fractures developed during single, large fracture events in the final stages of the lava’s emplacement. We recorded ornamentations on these fracture surfaces that allow snapshot views into the rheological and outgassing conditions during the lava’s effusion. We found that the upper surface is characterized by small (<1 m) mode 1 tensile fractures that grew and initiated new cracks, which linked together to form larger tensile fractures (1–5 m), which in turn penetrated deeper into the lava. This study focused on mapping the textural-structural relationships of the upper surface of the lava onto high-resolution (<10 cm 2/pixel) orthorectified color base maps. In this study, we documented evidence of a continuum of brittle and brittle-ductile deformation and fracture-induced outgassing during the emplacement of the ~600-yr-old silicic lava from Obsidian Dome, California, USA. The first ever witnessed silicic lava eruptive events, Chaitén (2008) and Cordón Caulle (2011–2012) in Chile, were illuminating to the volcanology community because they featured a range of emplacement processes (endogenous versus exogenous), movement limiting modes, and eruptive behaviors (explosive versus effusive) that were often regarded as acting independently throughout an eruptive event. The scarcity of observed active extrusive rhyolitic lava flows has skewed research to extensively focus on prehistoric lavas for information about their eruptive and emplacement dynamics.
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