Role of Sills in the Development of Volcanic Fields: Insights from Lidar Mapping Surveys of the San Rafael Swell, Utah

Analysis of airborne and terrestrial lidar data demonstrates that > 0.4 km ³ of magma cooled in sills at shallow ( < 1 km) depth in the now-eroded Pliocene San Rafael Swell distributed volcanic field, Utah (USA). The volumes of each of seven sills are estimated from three-dimensional (3-D) models of the lidar data and range from 10 ⁻⁴ to 10 ⁻¹ km ³ . Directions of magma flow during emplacement are interpreted from precise sill thickness measurements and measurements of linear vertical offsets within the sills, helping to identify feeder conduits and dikes; 3-D map relationships derived from lidar data demonstrate that magma flowed into and out of sills from these active dikes and eruptive conduits. Mapped sill volumes account for > 92% of intrusive material within the 50 km ² study area. We conclude that sills played a significant role in modifying eruption dynamics during activity in San Rafael, and suggest that monitoring of sill inflation and deflation in active distributed volcanic fields may provide key information about unrest and potential eruption dynamics.