At French Butte, Hopi Buttes Volcanic Field (Arizona, USA), an unusual in-vent folded dike is exposed with peperite entrained between the folds. Analogue experiments of collapsing silicon sheets and threads coated with sand indicate that the folded intrusion may have resulted from gravity-driven buckling of a planar feeder dike. Gravity-driven buckling is explained by magma drain-back and provides a new explanation for why phreatomagmatic activity can occur at the end of a volcanic episode marked by magmatic eruption styles. Late-stage folding of intrusions provides a previously unknown mechanism for mixing of magma with water-loaded sediment as a trigger for phreatomagmatic activity.

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