Abstract

Textures in an exceptionally preserved effusive rhyolite conduit at Torfajökull, Iceland, indicate that rising magma repeatedly fractured and healed at shallow levels in the conduit (RFH process). Anastomosing tuffisite veins filled by fine-grained juvenile clasts were generated by shear fracture of highly viscous magma in the glass transition interval. Welding of the particulate material during subsequent deformation led to thorough healing of veins, allowing repeated fracture of the same body of magma. We propose that the RFH process is a rechargeable trigger mechanism for hybrid seismicity and show that the time scale of the process and the fractures formed by it are consistent with the repeat time and magnitude of hybrid earthquakes during silicic eruptions. The RFH process may also form the flow banding that is nearly ubiquitous in obsidian.

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