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.

Pumice

Silicic

Caldera

Phenocryst

Breccia

10.1130/0-8137-2375-2.159

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Citations (20)

Role of mafic enclaves at the Soufriere Hills volcano, Montserrat

EGUGA (2010)

Crystal P. Mann John Stix T. Christopher

Source

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Citations (0)

Nature of Magma Recharge at Soufrière Hills volcano, Montserrat, West Indies

AGUFM (2006)

T. Barton Crystal P. Mann V. Hards G. Ryan John Stix

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Mafic magma injection triggers eruption at Ilopango Caldera, El Salvador, Central America

Geological Society of America eBooks (2004)

Mathieu Richer Crystal P. Mann John Stix

The most recent activity at Ilopango Caldera occurred in 1880 when a dacitic lava dome (68.4 wt% SiO2) emerged from the center of the caldera lake. Dark inclusions of basaltic andesite (53.9 wt% SiO2) represent 2 to 3 vol% of the dacite dome. Petrographic textures and geochemistry indicate that the mafic inclusions were solidified after injection into partially crystallized dacite magma. During and before solidification, plagioclase crystals observed in the dacite actively transferred into the mafic magma, entraining dacitic melt and producing a reaction rim on crystals. Plagioclase crystals with sodic cores (An40–50) and more calcic overgrowth rims (∼An70) derived from the dacitic melt are common inside mafic inclusions. Intimately mingled dacitic glass exhibits small but significant differences in chemistry relative to the host dacite, most notably a potassium enrichment, which we attribute to diffusion processes between the two magmas. We propose that an injection of mafic magma at the base of partially crystallized dacite magma triggered the 1880 eruption. The overpressure necessary for eruption was generated by the addition of mafic magma, which triggered vesiculation of the rising dacite. The presence of mafic magma in both the A.D. 429 and 1880 eruptions may indicate that this process is of general importance in the eruptive history of Ilopango.

Dacite

Magma chamber

Caldera

Lava dome

10.1130/0-8137-2375-2.175

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Citations (25)

Multi-university Internet video-conferencing course provides novel approach to student-directed learning

The Leading Edge (2007)

A. J. Durant William I. Rose Crystal P. Mann John Stix

Connecting graduate students with experts in a given discipline is vital to their success but has often been challenging because of logistics, costs, and scheduling. However, advances in Internet video conferencing now allow multiple parties to meet simultaneously and engage in real-time discussion.

Course (navigation)

Videoconferencing

Teleconference

10.1190/1.2794397

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Phenocryst-hosted melt inclusions record stalling of magma during ascent in the conduit and upper magma reservoir prior to vulcanian explosions, Soufrière Hills volcano, Montserrat, West Indies

Bulletin of Volcanology (2013)

Crystal P. Mann Paul Wallace John Stix

Melt inclusions

Pumice

Phenocryst