Ground Penetrating Radar and Magnetic Investigations of Phreatomagmatic Tephra Rings in the San Francisco Volcanic Field, Northern Arizona

Abstract:

Ground Penetrating Radar (GPR) can be a valuable geophysical tool for studying near-surface volcanic stratigraphy in areas where outcrops do not exist. Likewise, high resolution ground-based magnetic surveys have the potential to reveal significant features not exposed at the surface, especially in the case of small-volume basaltic volcanoes. Here we present the results of geophysical studies to investigate the eruptive history of deposits surrounding phreatomagmatic eruption sites, and why some may become magnetized.

Magnetic surveys undertaken at basaltic phreatomagmatic sites suggest that some tuff rings carry no discernable magnetic signature, while others reveal slight to significant magnetic anomalies. Material deposited in the tephra ring could become magnetized through Thermal Remanent Magnetization – emplacement of magnetically susceptible material above 560° C. In this case tephra layers would need to be deposited in sufficient thickness to retain high temperatures long enough for the magnetic material to orient itself to the magnetic field.

To test this hypothesis we examine GPR data collected at Rattlesnake Maar in the San Francisco Volcanic Field, Arizona, and we will collect GPR data at two other tephra rings in the same volcanic field. The first site, Sugarloaf Mountain, is an active quarry with excellent exposures of tephra ring stratigraphy. Although this site is rhyolitic in composition and not suitable for magnetic study, it is an excellent site to determine how well GPR reflectors correlate with actual stratigraphy. The second site, an un-named phreatomagmatic ring nearby, will then be studied by GPR and walking magnetic survey.

GPR reflectors will be compared to depositional patterns defined in previous studies and correlated with magnetic survey results to determine if a correlation can be made – little to no magnetization where only thin units are recorded by GPR, and positive magnetization where thick units are recorded.