A GIS Based Model Used to Estimate the Number of Subsurface Small Vents in the Eastern Snake River Plain

An in-house GIS-based model, the Butte Volume Calculator Model (BVCM), is used to quantify volumes of monogenetic basaltic shields constructed on irregular or sloping topography on the eastern Snake River Plain (ESRP), Idaho. BVCM estimates are compared to field estimates for eight Holocene basaltic shields (e.g., Kuntz et al., 1992).

Volumes obtained for 132 digitized basaltic shields concentrated in and around the Axial Volcanic Zone range from 0.0002 to 11.6 km³ with an average of 0.76 km³. The majority of the 132 shields (> 75%) have volumes less than 1 km³. While shield size may be a function of eruptive stage or variables unaccounted for, the large proportion of low-volume shields likely attests to concealment by post-eruptive eolian deposition and erosion, thereby yielding smaller topographic exposures and volumes than what might actually exist in the subsurface.

Using published estimates for total ESRP basalt volume, ranging from 12,300 km³ to 41,000 km³, and the BVCM volumes for 132 shields an estimate for the total number of shields in the ESRP's subsurface ranges from 12,000 to 78,000. A more reasonable estimate, however, is obtained using 17 of the youngest shields (< 400 ka) having mappable boundaries and known areas with an average volume of 4 km³. This assumed average volume yields a range of 3,000 to 10,000 small vent eruptions in the ESRP subsurface.

This range is consistent with a previous estimate of 8,000 small vent eruptions in the ESRP subsurface and attests to the validity of using a GIS-based dimensional analysis to quantify small vent shields.