IDENTIFYING VENT LOCATION AND ERUPTIVE BEHAVIOR AT THE SPROUL CRATER, SAN FRANCISCO VOLCANIC FIELD REU

The San Francisco Volcanic field (SFVF) is home to over 600 cinder cones and stratovolcanoes ranging in age from 6 Ma to 900 ybp. The Sproul Crater is the only recognized spatter cone in the SFVF. The shape of the Sproul Crater reflects a complex eruptive history involving changing vent location and eruptive behavior. The eruptive behavior can be described by fluxes in gas-to-magma content where more magma produces more cinder and more gas produces more lava fountaining and agglutination. Research on the Sproul Crater and its dynamic vent can be used to better understand other spatter cones. Stratigraphic columns and mapping reveal three distinct facies, which represent three eruptive events. The facies from the first event is distinguished by agglutinated spatter, the second event is characterized by “pancake” spatter, and the third by welded cinder, spatter, and lenses of agglutinate. The shape of each mapped facies is elongate and indicates that the vent was a fissure throughout the eruptive history. The correlation between stratigraphic columns suggests younger facies are present on the southeast end of the cone, which also suggests that successive eruptive pulses migrated towards the southeast. Lateral grading from larger to smaller clast size supports that the vent existed closer to the southwest rim of the crater. The eruptive behavior of the Sproul Crater involved fluctuations in gas-to-magma ratios. The first event consisted of more gas than magma, which is represented by the spatter-dominated facies. Nearly equal proportions of gas and magma were present for the second event because it is largely composed of partially agglutinated spatter. Gas and magma content fluctuated for the third event, which is shown by the alternating layers of welded cinder with spatter and rootless flow. The Sproul Crater’s eruptive history is similar to other, larger, spatter-dominated eruptions because it involves periods of lava fountaining with fluctuating levels of gas.