73-12 The Eruptive History and Feeder Dike Propagation of Two Adjacent Silicic Flank Vents at Summer Coon Volcano, Colorado

Session: Using Volcanic Deposits to Help Us Understand Volcanic and Magmatic Processes (Posters)

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Monogenetic flank eruptions commonly occur at stratovolcanoes and represent a significant hazard due to uncertainties linked with new vent locations and their proximity to, or even within, inhabited areas. Vent geometries and eruptive deposits have been widely documented for mafic flank eruptions, and monitoring techniques even allow estimates of the propagation direction of their feeder dikes. In contrast, silicic-composition flank eruptions are less common, with few occurring in the historical record and none at well-monitored volcanoes. Thus, examining extinct silicic vents is key to understanding their dynamics and magma feeding systems.

Summer Coon is an Oligocene-age stratovolcano in southern Colorado, where erosion has removed much of the edifice, exposing the central intrusive complex and a peripheral radial dike sequence. Earlier workers divided its eruptive history into three phases: early basaltic-andesite, middle rhyodacite and rhyolite, and late dacite and andesite.

On Rattlesnake Ridge along Summer Coon’s western margin, erosion has exhumed two flank vent complexes within 100 m of each other: a middle-phase rhyolite vent and a late-phase dacite vent. The extrusive portion of the rhyolite vent complex is composed of flow-banded lava extending >3 km west, while the intrusive portions include a funnel-shaped structure of finely jointed and variably brecciated rhyolite exposed to a paleodepth of 125 m. The lower portion of the structure transitions into a 50 m wide and 800 m long segment of the feeder dike, which continues ~3.5 km east to the central intrusive complex. The dike’s long length is similar to other silicic radial dikes at Summer Coon that propagated through the level of exposure along sub-horizontal or shallowly inclined directions.

On the northwest slope of Rattlesnake Ridge, extrusive late-phase dacite lava and breccia cover much of the older rhyolite lavas. The dacite erupted from a 350 m long, 2-3 m wide vent that likely matches the geometry of the feeder dike located just below the outcrop. The relatively short length for the dacite dike is consistent with dikes at Summer Coon that propagated through the level of exposure along a sub-vertical or steeply inclined direction. The contrasting propagation directions of feeder dikes from adjacent, different-phase flank vents highlight the complexity and temporal variability of magma sources and dike propagation directions for flank eruptions at the same volcano.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11194

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