Magma chambers feeding hazardous Plinian eruptions of Somma-Vesuvius have been present throughout most of the volcano’s lifetime.

<p>Vesuvius is one of the most iconic active volcanoes on Earth. Historic and archaeological records document numerous hazardous eruptions with thousands of fatalities. Today, more than one-million people live around Vesuvius and are threatened by future volcanic activity. Petrologic and geochemical studies of eruptive products provide important insights into the evolution of the eruption-feeding magma reservoir prior to eruption. Here we quantify the duration of shallow crustal storage and track the evolution of phonolitic magmas prior to major explosive eruptions of Vesuvius employing in-situ uranium-thorium dating of garnet phenocrysts in tandem with detailed geochemical and textural characterization. Garnet uranium-thorium dates provide evidence for progressively shorter pre-eruption storage times throughout the lifetime of the volcano, decreasing from ~5,000 years for the pre-historic Mercato and Avellino eruptions to approximately 1,000 years for the historic AD 79 Pompeii and AD 472 Pollena eruptions. These decreasing residence times mirror the progressively shorter repose intervals between eruptions implying that distinct phonolite magma batches were present throughout most of the volcano’s evolution thereby controlling the eruption dynamics by preventing the ascent of mafic magmas from longerlived and deeper reservoirs. Frequent lower-energy eruptions during the recent history sample this deeper reservoir and suggest that future Plinian eruptions are unlikely without centuries of volcanic quiescence. Crystal residence times from other volcanoes reveal that discrete long-lived deep-seated reservoirs and transient upper-crustal magma chambers are common features of sub-volcanic plumbing systems.</p>