RHYOLITIC GLASS IN VOLCANIC ASH FROM THE 2015 ERUPTION OF TELICA VOLCANO, NICARAGUA

The Telica volcano in northwestern Nicaragua is considered a persistently restless volcano (PRV). The restlessness is characterized by a shallow magmatic-hydrothermal system that drives high-temperature fumaroles, low frequency seismicity and intermittent low-VEI eruptive activity. The most recent low explosivity eruptions occurred in 2011 and 2015. The 2011 single-episode eruption of Telica was characterized as a phreatic eruptive event because of lack of deformation in the edifice, small net changes in SO₂ flux, and the lack of juvenile material in the accretionary lapilli and ash. In contrast, the 2015 three-episode eruption consisted of larger ballistic blocks, and higher eruptive columns compared to that of 2011, while exhibiting similar fluctuations in low-frequency (LF) and high-frequency seismic events (HF) as the 2011 preceding eruptive activity, and possible deformation of the edifice. While the 2011 explosive activity at Telica volcano was largely amagmatic, the 2015 phreatomagmatic eruption in its later phases involved the mobilization of magma in the shallow volcanic plumbing system, shown by the presence of fresh glass (≤10%) in ash samples. Major element composition data was collected with Electron Probe Microanalysis (EPMA) for a suite of phenocrysts and glass fragments sampled from the 2011 and 2015 Telica ash. X-Ray Diffraction was used for further identification of mineral phases. Preliminary results confirm the presence of glass with high SiO₂ content (70-80%) in the 2015 ash. The observation of high-silica glass suggests the presence of evolved magma in the conduit, which could explain the decrease in gas emissions (2015) and seismicity (2011) immediately prior to the eruptions. A plug of rhyolitic magma could temporarily seal the conduit, allowing gas pressure to build up, ultimately fueling the ejection of large blocks and higher ash columns. This interpretation is supported by the observation of a small dome extruded onto the crater floor in 2017. By fully characterizing the juvenile glass composition, crystallinity and crystal composition in ash samples from both eruptions, we aim to assess the role of evolved, high viscosity magma in controlling eruptive behavior at Telica and other PRVs.