Description: Highlights • Temporally close-spaced double eruption within a couple of hundreds of years. • Magmas are variably tapped from zoned magma chambers during eruptions due to changing magma discharge rates and/or vent migration. • Eruptions started with a series of fallouts featuring stable eruption columns followed by fluctuating and partially collapsing eruption columns. • Eruptive volumes sum up to a total of 25.6 km3 and 40.5 km3 tephra volume, eruption column heights have been between 20–33 km. • Potential hazards from similar sized eruptions around Coatepeque Caldera are indicated even in the distal regions around San Salvador. Abstract The Coatepeque volcanic complex in El Salvador produced at least four Plinian eruptions within the last 80 kyr. The eruption of the 72 ka old Arce Tephra formed the Coatepeque Caldera and was one of the most powerful explosive eruptions in El Salvador. Hitherto it was thought that the Arce tephra had been emplaced only by one, mostly Plinian, eruptive event that ended with the deposition of a thick ignimbrite. However, our stratigraphic, geochemical, and zircon data reveal a temporally closely- spaced double eruption separated by a gap of only a couple of hundred years, and we therefore distinguish Lower and Upper Arce Tephras. Both eruptions produced in the beginning a series of fallout units generated from fluctuating eruption columns and turning wind directions. The final phase of the Upper Arce eruption produced surge deposits by several eruption column collapses before the terminal phase of catastrophic ignimbrite eruption and caldera collapse. Mapping of the individual tephra units including the occurrences of distal marine and lacustrine ash layers in the Pacific Ocean, the Guatemalan lowlands and the Caribbean Sea, result in 25.6 km3 tephra volume, areal distribution of 4 × 105 km2 and eruption column heights between 20–33 km for the Lower Arce eruption, and 40.5 km3 tephra volume, including 10 km3 for the ignimbrite, distributed across 6 × 105 km2 and eruption column heights of 23–28 km for the Upper Arce eruption. These values and the detailed eruptive sequence emphasize the great hazard potential of possible future highly explosive eruptions at Coatepeque Caldera, especially for this kind of double eruption.

Description: Highlights • Masaya caldera is an unusual basaltic caldera in that it formed by voluminous magma extraction during explosive eruptions. We identify the nature, age and volume of these three eruptions of which the first, emplacing the San Antonio tephra, was by far most voluminous. • The by far largest fraction of the 9 km3 DRE erupted volume of this tephra was discharged during a Plinian eruption phase, which was bracketed by phreatomagmatic eruptions. We demonstrate that water contents measured in melt inclusions equilibrated during residence at shallow level shortly before eruption strongly underestimate original water contents during differentiation at higher pressure. We argue that the large fraction of exsolved H2O together with buoyancy pressure from connection to the deeper reservoir drove the eruptive high mass flux needed for the Plinian eruption phase. Masaya is unusual for a basaltic caldera because it formed by piston-subsidence in response to large-volume magma withdrawal by highly explosive eruptions, i.e. in a fashion typical of silicic calderas. The first and most voluminous of the three explosive eruptions formed the 6 ka old basaltic San Antonio Tephra (SAT). This eruption is also unusual in that most of the 9 km3 DRE basaltic magma was discharged by a plinian eruption. The subsequent eruptions of the basaltic Masaya Triple Layer (MTL, 2.1 ka) and the Masaya Tuff/Ticuantepe Lapilli (MT-TIL, 1.9 ka) each discharged 2 km3 DRE magma and enlarged the Masaya caldera. The SAT consists of a lower sequence of alternating scoria lapilli and ash layers, interpreted as an alternation between more or less phreatomagmatically influenced fallout events. These are followed by two prominent well-sorted lapilli layers: the first one formed by a climactic plinian eruption whose column height reached 21–29 km and discharged most of the total erupted mass including about 35 Mt. SO2. The second, lithic-rich lapilli layer probably formed by a phreatoplinian event when partial collapse of the magma chamber roof initiated increasing magma-water interaction which ultimately formed the upper sequence of phreatomagmatic cross-bedded surge deposits, accretionary lapilli-rich tuffs and a final fallout of dense lapilli. Phreatomagmatic activity may have been related to disruption of a hydrothermal system reflected in hydrothermally altered lithics, and/or by the caldera floor subsiding closer to the groundwater table. The bulk-rock chemical composition of the SAT is basaltic but the bimodal glass compositions demonstrate mixing of a basaltic with an andesitic melt probably in the conduit during eruption. The SAT basalt differentiated in a reservoir near the MOHO at 20 km depth by fractional crystallization of olivine, plagioclase, and minor clinopyroxene forming a tholeiitic fractionation trend. Minor intermediate-An plagioclase crystallized from the basaltic melt at H2O concentrations of about 2 wt% as measured by FTIR in melt inclusions. However, a key observation is that the melt inclusions are not in equilibrium with the high-An plagioclases hosting them. Re-equilibration of the inclusions requires initially higher water contents (about 5–6 wt%) which also fits the high Ba/La ~ 80 indicating input from the strongly hydrated subducting slab. Therefore, while the SAT magma evolved under hydrous conditions at depth, it was then stored at shallow level long enough to adjust to the low saturation pressure and to precipitate some intermediate-An plagioclase but still preserving its high temperature (around 1100 °C) and phenocryst-poor composition. Large overpressure due to connection to the deep-seated reservoir and water degassing during ascent limited the storage time at shallow level and drove the unusually intense and voluminous plinian-style eruption that facilitated piston-type collapse of the chamber roof.