High magma decompression rates at the peak of a violent caldera-forming eruption (Lower Pumice 1 eruption, Santorini, Greece)

We use the deposit sequence resulting from the first catastrophic caldera collapse event recorded at Santorini (associated with 184 ka Lower Pumice 1 eruption), to study the shallow conduit dynamics at the peak of caldera collapse. The main phase of the Lower Pumice 1 eruption commenced with the development of a sustained buoyant eruption column, producing a clast-supported framework of rhyodacitic white pumice (LP1-A). The clasts have densities of 310–740 kg m⁻³, large coalesced vesicles that define unimodal size distributions and moderate to high vesicle number densities (1.2 × 10⁹–1.7 × 10⁹ cm⁻³). Eruption column collapse, possibly associated with incipient caldera collapse, resulted in the development of pyroclastic flows (LP1-B). The resulting ignimbrite is characterised by rhyodacitic white pumice with a narrow density range (250–620 kg m⁻³) and moderate to high vesicle number densities (1.3 × 10⁹–2.1 × 10⁹ cm⁻³), comparable to clasts from LP1-A. An absence of deep, basement-derived lithic clast assemblages, together with the occurrence of large vesicles and relatively high vesicle number densities in pumice from the fallout and pyroclastic flow phases, suggests shallow fragmentation depths, a prolonged period of bubble nucleation and growth, and moderate rates of decompression prior to fragmentation (7–11 MPa s⁻¹). Evacuation of magma during the pyroclastic flow phase led to under-pressurisation of the magma reservoir, the propagation of faults (associated with the main phase of caldera collapse) and the formation of 20 m thick lithic lag breccias (LP1-C). Rhyodacitic pumices from the base of the proximal lithic lag breccias show a broader range of density (330–990 kg m⁻³), higher vesicle number densities (4.5 × 10⁹–1.1 × 10¹⁰ cm⁻³) and higher calculated magma decompression rates of 15–28 MPa s⁻¹ than pyroclasts from the pre-collapse eruptive phases. In addition, the abundance of lithic clasts, including deeper, basement-derived lithic assemblages, records the opening of new vents and a deepening of the fragmentation surface. These data support numerical simulations which predict rapid increases in magma decompression and mass discharge rates at the onset of caldera collapse.