Collaborative research: How variable is magma decompression rate during a single eruption?

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Understanding the processes that govern transitions in volcanic eruptive style is one of the major outstanding questions in volcanology, with important implications for hazard mitigation efforts. Syneruptive magma decompression rate is thought to be a dominant control on volcanic eruptive style, due in part to its role in modulating the dynamics of bubble formation and migration. In recent years, techniques have been developed that use chemical gradients in rapidly quenched volcanic tephra to constrain magma decompression rates; however, the rarity of suitable samples for the application of these techniques has severely limited the acquisition of statistically robust datasets that could be used to explore temporal variations in magma decompression rate over the course of a single eruption. A newly developed magma decompression “clock” that exploits water concentration gradients in prevalent olivine crystals can now be used to overcome this limitation. We propose to apply this new magma decompression clock to study temporal variations in the decompression rate of magma erupted at Cinder Cone, Lassen Volcanic National Park, CA. The exceptionally well-preserved tephra sequence from Cinder Cone captures a transition in eruptive style from Hawaiian/Strombolian to Strombolian/Violent Strombolian, providing an excellent natural laboratory for studying the role of decompression rate as a control on eruptive style. The Lassen region is one of the most active volcanic regions in the US and poses a significant hazard to Californians, National Park visitors, and flights to and from the busy San Francisco Bay Area. The results of the proposed work will be shared with the California Volcano Observatory, the Cascade Volcano Observatory, and Lassen Volcanic National Park, which manage volcano-monitoring infrastructure and volcanic hazard mitigation efforts in California.In the proposed work, variations in magma decompression rate will be studied in the context of syneruptive temperature changes and textural variations in the 1666 C.E. Cinder Cone eruptive deposit. We will apply three independent techniques to estimate magma decompression rates: (1) water-in-olivine diffusion chronometry; (2) bubble number density; and (3) microlite number density. Additionally, we will constrain magma cooling rates using MgO zonation in olivine-hosted melt inclusions. We will build a statistically robust dataset of decompression rate and cooling rate estimates that will be used to assess (for the first time in a basaltic system) temporally resolved variations in decompression rate and cooling rate across a transition in eruptive style. This approach will allow the evolution of pressure-temperature- time conditions in the conduit just prior to eruption to be studied in unprecedented detail, thereby increasing our understanding of the controls on the explosivity of basaltic volcanic eruptions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.