USE OF GEOPHYSICAL TECHNIQUES TO ENHANCE GEOLOGIC MAPPING OF RATTLESNAKE CRATER IN THE SAN FRANCISCO VOLCANIC FIELD

The San Francisco Volcanic Field (SFVF) is located in the southern part of the Colorado Plateau and consists of more than 600 volcanoes. Previous geologic investigations established that the youngest rocks are located in its eastern part and northeastward migration of volcanism in this area of the field has been recognized. Rattlesnake Crater, estimated to be less than 0.7 Ma, is located in the southeastern part of the SFVF and is characterized as a hybrid volcano having both cinder and phreatomagmatic eruptions. Geophysical analysis is extremely beneficial for improving our understanding of Rattlesnake Crater and, as a result, various geophysical techniques are utilized to enhance the geological mapping of the area by tracing the continuation of the phreatomagmatic structure and the morphology of the lava flow, structures that are not evident at the surface due to the high levels of erosion and soil formation in the area.

Three methods, refraction seismic, magnetics, and ground penetrating radar (GPR) were used to gather data from the shallow subsurface of selected areas, providing an image of the structural settings of the underlying rock. The data gathered from the subsurface using refraction seismology and GPR agree, supporting the validity of these results. Both show evidence of the phreatomagmatic structure underlying the cinder cone, strongly suggesting that the phreatomagmatic eruption took place before the strombolian eruption that formed the cinder cone. In the magnetic surveys we locate anomalies within the Earth’s magnetic fi­eld that result from the magnetic properties of the underlying rocks. The results from this technique are congruent with the results from the other two methods. Magnetic surveys conducted on the lava flow to the south of the cinder cone show that magnetic perturbations that correlate with the depth to the underlying basalt, with high-amplitude anomalies observed where basalt is exposed at the surface.

These shallow geophysical techniques prove to be helpful in the acquisition, processing, and interpretation of data that subsequently results in the successful mapping of the subsurface of Rattlesnake Crater, giving a better understanding of the geological structure and evolution of the volcano.