Subaqueous intracaldera volcanism, Ilopango Caldera, El Salvador, Central America

Abstract

The Ilopango Caldera, located 10 km east of San Salvador, has erupted voluminous silicic pyroclastics four times in the last 57,000 years. The present caldera has a quasi-rectangular shape and is filled by Lake Ilopango. This paper provides a detailed description of a segment of the intracaldera stratigraphy at Ilopango caldera, with emphasis on the San Agustín Block Unit. Physical volcanology, petrology, and geochemistry establish the depositional environment and eruptive conditions of the intracaldera sequence and help to model the emplacement of the San Agustín Block Unit. The intracaldera stratigraphy comprises a sequence of pyroclastic density currents, unconformably overlain by lacustrine sediments and conformably overlain by the San Agustín Block Unit. A new radiocarbon age on wood near the top of the Lacustrine Unit indicates that a lake was present ≥43,670 years ago. The intracaldera sequence displays abundant evidence of emplacement in a subaqueous environment. The San Agustín Block Unit comprises a basal Fine Ash facies and an overlying Pumice Breccia facies. The basal Fine Ash facies is a hydromagmatic layer containing pumiceous and blocky angular glass shards, aggregates of fine ash and phenocryst fragments, and phenocrysts with a fine ash coating. The overlying Pumice Breccia facies is composed of pumice clasts up to three meters in length. The pumice clasts display a series of jointing textures indicative of hot emplacement and rapid cooling. These two facies suggest an initial subaqueous explosive eruption in which a vesiculated silicic melt fragmented upon contact with the water. When the magma had degassed sufficiently, the eruption style evolved to subaqueous dome growth that spalled quenched pumice clasts from a moderately vesiculated carapace.