TOPOGRAPHIC ANALYSIS OF LOW SHIELD VOLCANOES IN SOUTHWESTERN ARIZONA

The Sentinel-Arlington Volcanic Field (SAVF) is located in southwestern Arizona. The SAVF is a Pliocene-Pleistocene basaltic volcanic field covering ~600 km². It is composed of 18 low shield volcanoes, three highly-eroded cinder cones, and a phreatomagmatic tuff ring. The low shield volcanoes range from 4-6 km in diameter and 30-200 m in height. Topographic analysis of SAVF low shields was completed after Bleacher and Greeley, 2003. Slopes of distal regions had values <5º and were affected by pre-existing topography. Low shield summit region morphology appeared to be independent of pre-existing topography and varied based on explosivity and groundwater interaction. Three main low shield summit types were identified, including: Type 1- capped by effusive lava flows; Type 2- capped by tephra deposits; and Type 3- exhibiting phreatomagmatic interaction, typically capping tephra deposits. Type 1 summits are characterized by lava flows capping the shield vent area with minimal to no tephra deposits. Type 2 summits are characterized by exposed cone and radial dikes encased in remnants of small cinder cones cemented by pedogenic calcium carbonate, with occasional spatter ramparts. Type 3 summits are characterized by presence of tuff deposits and/or palagonitized tephra near the vent of a low shield, interpreted to represent explosive interaction with groundwater. Type 3 summits can occasionally be capped by lava flows or found in conjunction with exposed radial and cone dikes. However, Type 3 summits have the lowest aspect ratios in SAVF. Field observations, including dike denudation, cinder deposit extents, flow margin erosion, and landscape development were used to approximate original edifice shape. Erosion rates for each summit type were calculated based on SAVF radiogenic ages. Slope analysis and field work can be used to distinguish different types of low shield volcanoes. Topographic expression can be used to determine relative ages of shields with the same vent type within a volcanic field. Aspect ratios could be used to identify potential phreatomagmatic interactions preserved by low shield volcanoes.