INVESTIGATING THE EVOLUTION OF MAGMA PROPERTIES DURING DIKE PROPAGATION AT SUMMER COON VOLCANO, CO

Diking is a primary mode of magma transport at arc stratovolcanoes. Dike pathways through stratocones can lead to eruptions at the summit or along the flanks. This variability in intrusion trajectories is potentially controlled by internal factors, such as magma density and viscosity, as well as external factors, such as volcano loading. Furthermore, magmatic processes during intrusion, such as crystallization and volatile exsolution, lead to dynamic changes in magma properties which are rarely considered in studies of dike propagation. The depths of intrusion make direct observations of magma flow in dikes and magma characteristics challenging at modern arc volcanoes. Ancient, eroded volcanoes, where the magmatic plumbing system is exposed, present an opportunity to study how flow directions through dikes are related to magma density and other properties that influence driving forces for propagation.

The Oligocene age Summer Coon volcano, on the eastern margin of the San Juan volcanic field in Colorado, is an ideal natural laboratory to explore how magmatic processes affect dike propagation. Over 20 silicic and hundreds of basaltic-andesite dikes are currently exposed at variable depths of up to 1700 m within the original volcanic edifice. Here we focus on two of the well-exposed intermediate-composition dikes that have been previously analyzed for magma flow directions, and we track how magma properties are related to flow directions. We present preliminary work on changes in magma density, crystal fraction, and vesicularity across the length and width of the dikes. By comparing these new data to previously obtained magma flow indicators, we test whether intrusion pathways are influenced by changes in magma buoyancy during propagation. This work is a first step in using field data to understand how internal magma properties and processes influence dike trajectories and eruption locations at stratovolcanoes.