Breadcrust bubbles are a previously undescribed pyroclast providing unique insights into post-fragmentation conduit dynamics. We describe these particles from the ca. 7 Ma Rattlesnake Tuff deposits (Oregon, USA) and ca. 15 ka deposits at Laguna del Maule (Chile). The clasts comprise discrete, ∼1-mm-diameter, fully intact hollow spheroids composed of high-silica rhyolite glass with a series of cross-cutting cracks on the shell exterior akin to breadcrust texture on volcanic bombs. We interpret the spheroids as resulting from fragmentation of a thick-walled magmatic foam. As the pyroclasts accelerated up the conduit, breadcrust textures recorded cycles of brittle and ductile deformation to relieve overpressure. We exploit this texture to quantify parameters inaccessible through direct observation including strain accommodation and the time scale necessary for it to accrue. Breadcrust bubbles have additional potential to better describe and quantify the nature of fragmentation and transport conditions in high-silica rhyolite eruptions.

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