AN EMPIRICAL POWER LAW RELATION BETWEEN AREAS AND VOLUMES OF SMALL MAFIC SHIELDS ON THE EASTERN SNAKE RIVER PLAIN, IDAHO, USA

A newly created in-house Geographic Information System (GIS) based tool is used to quantify volumes and areas of individual basaltic shields constructed on irregular or sloping topography on the eastern Snake River Plain (ESRP), Idaho. Plains-style volcanoes are typically coalescent low-profile shields (<2° slopes) having subdued topography and shallow depositional slopes. However, ascertaining the sizes and shapes of partially exposed shields in a plains-style sequence is difficult. For a given type of volcano, the relation between aerial extent and volume may reveal certain fundamental characteristics related to its morphology and magnitude. A particular shield's volume is obtained by first mapping the exposed perimeter of a basaltic lava flow using GIS software (ESRI, ArcMap 9.1) on a digital elevation model (DEM) database. A “Butte Volume Calculator Tool” uses the marked perimeter to create an interpolated variable elevation base, subtracts DEM elevations from this base to find butte heights, and sums columnar volumes for each grid cell to arrive at a total volume. An empirical plot of area (km²) versus volume (km³) for 88 monogenetic basaltic shields reveals a power function relation given by the equation: Volume = 0.025 x Area^1.3. Areas and volumes for three silicic domes and two polygenetic eruptive centers on the ESRP reveal two additional power law relationships that differ distinctly from that of the basaltic shields. Relatively fresh basaltic shields such as Wapi (~2 ka) and Hells Half Acre (~5 ka) have ~100 percent topographic exposure, and volumes of 53 and 70 km³ respectively. Two older shields, Taber Butte (~100 ka) and Table Legs Butte (~300 ka), have topographically exposed volumes of 5 and 8 km³, respectively. The lower volumes attest to concealment by post-eruptive eolian deposition and erosion, which yields smaller topographic exposures than what might actually exist beneath the surface. Although further analyses are necessary, the empirical volume vs. area power law relationship suggests that if true height could be determined for a particular shield, it could be used to compute volume accordingly. This systematic and unique relationship between area and volume shows promise for calculating the volume of shields with minimal surface exposure on the ESRP and other regions with low profile shields.