A GIS APPROACH TO RECONSTRUCTING A BREACHED CINDER CONE, CRATERS OF THE MOON LAVA FIELD, IDAHO

Cinder cones are commonly breached by lavas erupting from their vents, a process that occasionally results in the near total destruction of the cone edifice. North Crater is a breached basaltic cinder cone in the Pleistocene to Holocene Craters of the Moon lava field on the eastern Snake River Plain of southern Idaho. We utilized a Geographic Information Systems (GIS) approach to document the volume of cone material rafted from the North Crater area, compare this volume to the volume of the breach in the cone, and interpret the results.

The Highway, Devil's Orchard, and Serrate lava flows erupted as viscous block/a'a flows from vents within, or near, North Crater. These flows rafted blocks of cone material (bedded and oxidized spatter, agglutinated cinders, and ash) up to 13 km away from North Crater. Rafted blocks are often coherent and tens of meters in dimension but many were disaggregated and became incorporated into the transporting flow. Using GIS, we summed the area of the domains of disaggregated blocks and large discrete blocks and then calculated a volume based on representative block heights. We estimated the volume of the sub-lava flow roots of the blocks based on simple isostatic principles and the densities of the rafted blocks and lava flows. These techniques allowed us to estimate the total volume of rafted material in the lavas. We also used GIS analysis to determine the relationship between the area of a cone's footprint, its volume, and its height for eight fairly symmetrical un-breached cinder cones within Craters of the Moon lava field.

The breach in the northwest side of North Crater has a volume of only 1.23 x 10⁶ m³. Putting the 46 x 10⁶ m³ of rafted material back onto the current North Crater edifice would create a paleo-North Crater with a volume of 88 x 10⁶ m³, requiring a footprint area of 1.52 x 10⁶ m², a radius of 700 m, and a height of 175 m. However, it is unlikely that one large, monogenetic paleo-North Crater of such a size ever existed. An alternative model is that a series of smaller paleo-North Craters were built at this location and destroyed by eruptions causing block rafting. Recent paleomagnetic data may support this model. Our approach enabled us to quantify an otherwise speculative volume of rafted blocks in terrain not easily visited on foot and hypothesize about the eruption history of a complex vent area.