SP Crater in the San Francisco volcanic field of northern Arizona

SP Crater (right foreground), like many volcanoes in Arizona’s San Francisco volcanic field, erupted mafic lava that lacks sanidine crystals. Scientists dated this eruption at approximately 70,000 years old by using groundmass for argon dating, as well as another technique called cosmogenic nuclide surface exposure dating (Fenton et al., 2013; Fenton and Niedermann, 2014). Photo by Kellie Wall, 2022.