The 18,500 yr. b.p. Cape Riva (CR) eruption of Santorini vented several km3 or more of magma, generating four eruption units: a basal Plinian fall deposit (CR-A) and three pyroclastic flow deposits (CR-B to CR-D upwards). CR-B and CR-D are welded ignimbrites; CR-C consists predominantly of up to 25 m thick coarse, lithic-rich co-ignimbrite lag breccias resulting from a climactic phase of the eruption. The initial Plinian phase occurred from a localized vent in N Santorini, and subsequent column collapse resulted in emplacement of CR-B. Towards the end of CR-B, new conduits were activated and pyroclastic flows discharged from multiple vents to generate the lag breccias (CR-C). CR-D probably records a return to a localized vent as the eruption waned. The eruption sampled a zoned magma chamber containing rhyodacite overlying andesite, and leaks of these magmas were manifested as the Skaros-Therasia lavas preceding the CR eruption. Plinian and initial ignimbrite stages occurred while the magma chamber was overpressured; subsequent underpressuring, due to magma discharge, caused fracturing of the chamber roof, caldera collapse, and eruption of pyroclastic flows from multiple vents. Activation and widening of new conduits during collapse resulted in the rapid escalation of discharge rate favoring the formation of lag breccias by: (i) promoting erosion of lithic debris at the surface vent; and (ii) raising surface exit pressures, thereby resulting in a dramatic increase in the grain size of the ejecta.