High Resolution Magnetic and Gravity Surveys to Constrain Maar Geometry and Eruption Mechanisms, Rattlesnake Crater, Arizona

Located 25 kilometers east of Flagstaff, Arizona, Rattlesnake Crater is an oblong phreatomagmatic feature in the San Francisco Volcanic Field. The shallow crater is approximately 1.4 kilometers at its widest point, and surrounded by an uneven tuff ring which is overlapped by a scoria cone volcano on the southeastern side. Improved understanding of its formation and evolution requires geophysical study because there are very few outcrops, and no digging is permitted on site. Geologic features related to the crater are further obscured by deposits from the overlapping scoria cone, as well as tephra from eruptions at nearby Sunset Crater. We present the results of a detailed magnetic and gravity survey in and around Rattlesnake Crater. A substantial NW-SE trending elongate magnetic anomaly (1400 nT) and a smaller similarly trending anomaly are observed inside the crater, as well as a longer wavelength positive gravitational anomaly (+1.0-1.5 mGal) across the crater. The magnetic survey was completed on foot with a 50 meter line spacing inside the crater, and 100 meter line spacing across a portion of the surrounding area outside the crater. The gravity survey was done on two intersecting survey lines - one running west to east, and another roughly north to south, with recordings every 100 meters extending at least 1000 meters outside the crater in all four directions. 2D models of the magnetic and gravity data are presented illustrating the possible geometry of the diatreme, and the approximate size and shape of the major intrusive features. Eruption estimates based on the models are calculated, and the models are favorably compared to the size and depth estimates given in a recent publication (Valentine 2012) that used xenolith content to estimate the size and depth of the diatreme.