AGE CONSTRAINTS ON CINDER CONES FROM SLOPE DIFFUSION AND DIGITALLY-ACQUIRED MORPHOMETRIC PARAMETERS – EXAMPLES FROM SPENCER HIGH POINT, SNAKE RIVER PLAIN, IDAHO

An analytical solution to a diffusion equation for cinder cones and a new digital method for collecting and comparing morphometric data on cinder cones are developed and used to constrain ages of undated cinder cones from the Spencer High Point (SHP) basalt plateau, southeastern Idaho. Selected cones have a mostly-symmetrical shape, are not steeper than 33 degrees, and may or may not be breached. Cinder cones of the SHP are difficult to date; phenocrysts in the erupted tephra that are suitable for radiometric dating typically formed during an earlier magma cooling stage than that represented by the cone eruption. Likewise, vents required for paleomagnetic dates are often mantled by tephra. In the absence of other techniques, relative dating methods based on morphometry and diffusion are constructive. Cinder cones consist primarily of unconsolidated fallout tephra that degrade over time (e.g., slopes flatten, edges smooth, and cone heights and bases respectively diffuse downward and outward). We assume that cinder cones diffuse at a steady state and that the geomorphic indices derived in other geographic regions are applicable in SE Idaho. Historically, morphometric data of cinder cones are gathered from topographic maps; however, we develop methods that use a Geographic Information System (GIS) to efficiently collect and calculate morphometric parameters from readily available 30-m resolution digital elevation models (DEMs). The methods are compared with gathering field data and with using higher resolution DEMs. Morphometric parameters gathered from DEMs of the cinder cones define boundary conditions for the diffusion model; the slope inflection point is used to determine original cone shape. The diffusion equation h_t = Kh_rr (h is the height, and r is the radius) models how a single profile degrades through time and depends on a diffusion constant K (m²ky^-1) that describes the erosion rate. We evaluate a range of K-values derived for the Intermountain west (3.9-10 m²ky^-1).