A reappraisal of explosive-effusive silicic eruption dynamics: syn-eruptive assembly of lava from the products of cryptic fragmentation

Silicic volcanic eruptions range in style from gently effusive to highly explosive, and may switch style unpredictably during a single eruption. Direct observations of subaerial rhyolitic eruptions (Chaiten 2008, Cord ' on Caulle 2011-2012, Chile) challenged long-standing paradigms of explosive and effusive eruptive styles and led to the formulation of new models of hybrid activity. However, the processes that govern such hybrid explosive-effusive activity remain poorly understood. Here, we bring together observations of the well-studied 2011-2012 Cord ' on Caulle eruption with new textural and petrologic data on erupted products, and video and still imagery of the eruption. We infer that all of the activity - explosive, effusive, and hybrid - was fed by explosive fragmentation at depth, and that effusive behaviour arose from sticking and sintering, in the shallow vent region, of the clastic products of deeper, cryptic fragmentation. We use a scaling approach to determine that there is sufficient time available, during emplacement, for diffusive pyroclast degassing and sintering to produce a degassed plug that occludes the shallow conduit, feeding clastogenic, apparently effusive, lava-like deposits. Based on evidence from Cord ' on Caulle, and from other similar eruptions, we further argue that hybrid explosive-effusive activity is driven by episodic gas-fracking of the occluding lava plug, fed by the underlying pressurized ash- and pyroclast-laden region. The presence of a pressurized pocket of ash-laden gas within the conduit provides a mechanism for generation of harmonic tremor, and for syn-eruptive laccolith intrusion, both of which were features of the Cord ' on Caulle eruption. We conclude that the cryptic fragmentation models is more consistent with available evidence than the prevailing model for effusion of silicic lava that assume coherent nonfragmental rise of magma from depth to the surface without wholesale explosive fragmentation.

Automated summary

Silicic volcanic eruptions can range from effusive to explosive, and may switch styles unpredictably. The processes behind hybrid explosive-effusive activity are poorly understood. Through observations of the 2011-2012 Cord ' on Caulle eruption, it was inferred that all activity was fed by explosive fragmentation at depth, and effusive behavior resulted from sticking and sintering of clastic products in the shallow vent region. The eruption also demonstrated episodic gas-fracking and pressurized pockets of ash-laden gas as driving factors for hybrid activity.