CINDER CONE MORPHOMETRY AND VOLUME DISTRIBUTION AT NEWBERRY VOLCANO, OREGON: IMPLICATIONS FOR AGE RELATIONS AND STRUCTURAL CONTROL ON ERUPTIVE PROCESS

Newberry Volcano of central Oregon covers greater than 1300 km² and is associated with over 350 basaltic cinder cones (Holocene-Late Pleistocene). Digital geologic maps and 10-m USGS DEMs were compiled with 182 single cones selected for morphometric and volume analyses using GIS. This robust data set provides a framework from which to evaluate cone volume distributions and relative ages in the context of erosional degradation models.

Based on visual inspection of DEM-derived shaded relief maps, each cone was qualitatively ranked with a morphology classification ranging from 1 (well defined cone-crater morphology) to 7 (very poorly defined cone-crater morphology). Morphometric measurements include cone height (H_c), average cone slope (S_c), long-axis diameter (D_l), short axis diameter (D_s), and height:width ratio (H_c/W_c where W_c=(D_l+D_s)/2). Individual cone DEMs were extracted and volumes (V_c) calculated using a kriging-based algorithm. Average slopes were derived from 10-m elevation nodes contained within cone polygons. Results according to qualitative morphology rank are summarized as follows: (A) Frequency (no.) 1=11, 2=21, 3=10, 4=35, 5=11, 6=35, 7=59; (B) Average V_c (m³) 1=1.46 x 10⁷, 2=1.53 x 10⁷, 3=1.25 x 10⁷, 4=4.88 x 10⁶, 5=4.65 x 10⁶, 6=3.07 x 10⁶, 7=1.10 x 10⁶; (C) Average S_c (deg) 1=19.9, 2=18.2, 3=18.1, 4=14.9, 5=14.4, 6=11.9, 7=10.2; (D) Average H_c (m) 1=132, 2=124, 3=126, 4=76, 5=78, 6=59, 7=50; (E) Average H_c/W_c 1=0.18, 2=0.20, 3=0.19, 4=0.15, 5=0.14, 6=0.13, 7=0.13. Existing cone degradation models demonstrate that with increasing cone age, S_c, H_c, and H_c/W_c decrease, respectively. Systematic t-tests (a=0.05) of these parameters between morphology classes statistically separates cones into two relative age groups: (1) "Younger"=ranks 1-3, and (2) "Older"=ranks 4-7, suggesting that there are two distinct age populations of cinder cones at Newberry. Spatial analysis of cone-volume distributions shows maxima oriented NW-SE, parallel to regional fault trends (Tumalo Fault and Northwest Rift zones), implying that these structures may have an important control on eruptive processes in the region. This study provides a framework to guide future geomorphic analysis and radiometric age dating of cinder cones at Newberry Volcano.