Pantelleria is dominated by a caldera 6 km in diameter that formed 55,000 yr ago during eruption of a rheomorphic fall deposit that is zoned from pantellerite to trachyte. Trachytic lavas then built a shield that nearly filled the caldera. Magma subsequently reinflated the shallow reservoir and raised the northwest half of the trachyte shield 275 m, creating Montagna Grande by trapdoor uplift. The lid eventually failed, and pantellerite magma erupted as domes, flows, and pumice cones along the trapdoor faults and their hingeline. We have identified a second caldera ∼93,000 yr old, within which the younger caldera is nested; a still-older structure may encircle them both. Replenishment of magma reservoirs beneath calderas can promote dilatant reactivation of older subsidence structures, resulting in multicyclic collapses that reuse these structures and produce nested calderas.

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