ABSTRACT The Grand Canyon is perhaps our planet’s most widely recognized and single most important geologic landform. The goals for this trip are to give participants an understanding of the canyon’s formation and its dynamic hydrologic system. While our destination is clear, the journey will also provide opportunities to discuss Arizona’s larger geologic setting within the Basin and Range, Transition Zone, and Colorado Plateau physiographic provinces. Stops and discussions will include: (1) geologic setting and groundwater environment of the Phoenix basin; (2) Cenozoic landscape development of the Transition Zone; (3) Montezuma Well, a unique arid-land spring contained within a travertine mound; (4) ascent of the Mogollon Rim, the state’s second largest landform and entryway to the Colorado Plateau; (5) the San Francisco Volcanic Field and surrounding volcanic features, including Sunset Crater, a late Holocene scoria cone; and (6) multiple stops in Grand Canyon National Park to discuss its varied geology. The principal focus here will be on evolving concepts of the canyon’s formation since the time of John Wesley Powell, including the flurry of research results proffered in the past 20 years. Participants will walk the Trail of Time, Earth’s largest man-made geologic exhibit at over 2 km. Another equally important discussion will cover the modern hydrologic system of the canyon, which yields a tenuous supply of potable water from a single inner-canyon spring for over six million annual visitors and 2,500 full-time residents. The National Park Service has prioritized the replacement of the Trans-Canyon Waterline due to climate change concerns.

ABSTRACT The San Francisco volcanic field stretches from Williams, Arizona, in the west, to northeast of Flagstaff, Arizona, on the east. Within the ~5000 km 2 area, more than 600 volcanoes are primarily monogenetic and basaltic, but silicic stratovolcanoes and domes are present as well. This field guide focuses on five broadly basaltic cones (Government Prairie vent, Red Mountain, SP Crater, Colton Crater, and Sunset Crater) and two silicic volcanoes (Kendrick Peak and San Francisco Mountain) in the field, with an emphasis on the different kinds of volcanic activity represented and the petrological variations. Hazards assessment indicates that is it possible for future eruptions to affect Flagstaff, but the probability is low. As information in this guide indicates, hazard assessments need to be improved to encompass a wide range of eruption types, and additional data are needed to improve models of the rate of volcanic activity and how the locus of activity has shifted over time.