Trigger Mechanisms of Compositionally Zoned Explosive Eruptions: Insights From Cosiguina Volcano, Nicaragua

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Understanding the trigger mechanisms of explosive volcanic eruptions is an important goal of modern volcanology. An eruption may be triggered externally, e.g., by an earthquake, or internally, by processes occurring within the magmatic system underlying the volcano. Commonly invoked as an internal trigger is the recharge of the magma reservoir with a new batch of magma, direct evidence for which is often preserved in eruption deposits. Alternatively, progressive pressure buildup due to the formation and expansion of bubbles in the magma may eventually lead to fracturing of the magma reservoir roof, resulting in an eruption. This trigger mechanism proposed on theoretical grounds has recently received experimental and numerical modeling support, but has rarely been fingerprinted post hoc in eruption deposits. Several volcanoes, such as Cosiguina volcano, Nicaragua, produce compositionally stratified eruption deposits that are typically interpreted to reflect the progressive evacuation of a layered magma reservoir, in which a cooler magma was protractedly contiguous with and derived from a hotter magma. These stratified systems generally bear evidence of vapor saturation prior to eruption and often lack the diagnostic clues for a magma recharge trigger, although cryptic recharge cannot usually be ruled out. Here we propose to use a systematic approach, based on high-resolution sampling and geochemical and textural analysis of volcanic rocks, that will allow identifying the eruption trigger mechanism at Cosiguina. This research will advance our understanding of the origin of layered magma reservoirs and the direct causes of volcanic eruptions. The amount and distribution of volatile elements, dissolved in the melt and exsolved into bubbles, in magma reservoirs are key controls on the rheological behavior of magma and, ultimately, on eruption trigger mechanisms and eruptive styles. We plan to measure major, trace and volatile element concentrations in melt inclusions from pumice and scoria samples collected at strategic stratigraphic horizons within compositionally zoned eruption deposits at Cosiguina, allowing the reconstruction of the dissolved and exsolved volatile gradients within the pre-eruptive magmatic system. This geochemical and textural experiment is designed to differentiate the expected fingerprints left by a volatile-buildup trigger from those of a mafic recharge trigger, for which there is scarce evidence in many compositionally stratified eruption deposits. Cosiguina volcano produced multiple zoned explosive eruptions in the recent geologic past, including a VEI 5 eruption in AD 1835 that impacted global climate. The Cosiguina deposits share several similarities with those of classic zoned eruptions, such as the climactic Mount Mazama and the 1912 Katmai eruptions, and we anticipate our results to have broad implications for volatile gradients and eruption trigger mechanisms at compositionally zoned magmatic systems.