Flagstaff is home to a variety of geologic hazards including volcanism, earthquakes, sinkholes, flooding, and debris flows; these last two especially in the wake of a wildfire. Although these hazards have the potential to affect lives and property, they are for the most part natural processes that have operated on the landscape since long before humans came to inhabit it.
Volcanism and fault movement (which generates earthquakes) throughout the past few million years created most of the iconic Flagstaff landscape. A lot of great geologic studies have investigated the volcanoes and associated hazard in the San Francisco volcanic field. The upcoming Geologic Hazards of Flagstaff guided tour at Flagstaff Festival of Science will instead focus on some of our other hazards: faults/earthquakes, post-fire flooding, and sinkholes. You must pre-register for the tour, and enrollment is capped at 25 participants.
The tour will begin at Bottomless Pit in east Flagstaff. You may have guessed by its name that this is a sinkhole! Bottomless Pit sits on the floodplain of Rio de Flag, and during high river flows water drains into the pit. The pit used to have a direct connection to cave systems in the Permian Kaibab Formation, but due to safety concerns it was partially filled with debris in 1987. On the way to our next stop, we will pass by Lake Elaine, an empty manmade reservoir in Country Club Estates. The land beneath the lake was riddled with sinkholes, which swallowed up all the water intended for the so-called lake!
The forces of faulting uplifted both McMillan Mesa (formerly called Switzer Mesa) and Anderson Mesa. Both mesas are bounded on their western side by a fault and have smaller faults that slice through their interior. On our tour, we will drive up and over McMillan Mesa, then examine a fault exposure on its west side. Although the age of the fault system of McMillan Mesa is only loosely constrained to less than 6 million years ago (the age of basalt cut by the fault), the potentially correlative Lake Mary fault system to the south, which uplifted Anderson Mesa, cuts through a 1-million-year-old (young!) lava flow and is considered active.
The flood hazard in Flagstaff has been exacerbated in places by human activity, such as the rerouting of Rio de Flag from its original channel into an inadequately small ditch through Wheeler Park and Southside. In the wake of a wildfire, flooding is especially severe, since the ground loses its ability to absorb water. The 2022 Pipeline Fire badly burned the upper Schultz Creek watershed, and the following Monsoon storms formed sediment- and debris-rich floodwaters that raced down its channel. A poorly designed culvert at Fort Valley Road plugged up, and the floodwaters rose quickly on the upstream side, inundating the floodplain. A vent partway through the culvert erupted pressurized floodwater that inundated the neighborhood along Stevanna Way. Residents and volunteers dug a trench through an empty lot to allow the water to pass through into Rio de Flag. A larger culvert system has since been installed, in addition to other flood hazard mitigation structures upstream on Schultz Creek.
Geologic hazards in Flagstaff have the potential to affect the lives and property of residents and visitors. However, these same natural processes are largely responsible for creating the beloved Flagstaff landscape. Understanding the natural geologic processes and the associated geologic hazard is the first step to most safely and effectively living among them!
Board member, Adam Marsh, and lead paleontologist at Petrified National Forest, works with a team of scientists to uncover an approximately 300 million-year-old fossil skeleton discovered at Canyonlands National Park in Utah (June 2024)