by Wayne Ranney

Introduction

Flagstaff lies in one of the most beautiful parts of the Colorado Plateau. Numerous volcanic rocks — extruded as cones or lava flows within the past 6 million years — cover parts of the landscape. These igneous rocks erupted onto colorful sedimentary rocks subtly exposed in places beneath the volcanic veneer. The sedimentary rocks contain precious reserves of groundwater. They are more readily seen nearby in Oak Creek Canyon and Grand Canyon, where they are a scenic draw for millions of visitors a year.

At first, local rocks may appear to have only historical importance as in the construction of some early city buildings. Yet they also show what natural forces shape our environment. As citizens begin to develop a more sustainable relationship to the landscape, an understanding of Flagstaff’s geologic history provides valuable lessons that can better inform citizens in the future.

Geologic History

Flagstaff has not always been a mountain paradise. Beginning about 525 million years ago, the area was near sea level. For the next 450 million years a thick stack of sedimentary rock accumulated in nearshore, tidal, and sandy desert environments. Each successive change in environment resulted in a different sediment type, yielding layers such as Redwall Limestone, Kaibab Limestone, and the Moenkopi Formation. The Redwall beneath Flagstaff is a groundwater aquifer that provides domestic water from the Woody Mountain well field. Several layers above it is the white Kaibab Limestone, a durable rock that caps the scenic Mogollon Rim and Grand Canyon. Flagstaff’s quarries of red Moenkopi Formation yielded a stone called “Arizona Red” that was used in the construction of many beautiful buildings here and elsewhere in the west. To view what the Flagstaff area used to look like see: http://cpgeosystems.com/ColoPlatPalgeog.html.

Rocks of dinosaur age were removed from the Flagstaff area as the land was gradually uplifted beginning about 80 million years ago. By 6 million years ago, volcanic activity commenced when a lava flow from Woody Ridge spread northeast towards Buffalo Park. More intense activity began about 2 million years ago, forming the San Francisco strato-volcano through the next 1.5 million years. Mt. Elden erupted as a dacite dome volcano about 500,000 years ago. Sunset Crater is the youngest volcanic feature in the area being slightly less than 1,000 years old. People were living in the area when this cinder cone erupted and witnessed the power of our planet’s interior heat engine. To learn more about Flagstaff’s volcanic heritage see: http://geopubs.wr.usgs.gov/fact-sheet/fs017-01/

Natural Hazards – Volcanism, Seismicity, Flooding

With so much evidence for past volcanism in and around Flagstaff, people may wonder if this is a hazard. Although the possibility cannot be ruled out, the recurrence interval for eruptions here is on the order of a few thousand years – the short-term risk is considered low with future eruptions likely to occur east of the city limits. Associated with the movement of subsurface magma (but not limited to it) Flagstaff also experiences occasional earthquakes as evidenced from a series of strong quakes on San Francisco Mountain in the early 20th century. The Arizona Earthquake Information Center keeps records of this potential hazard. Flagstaff has seen its share of floods and the Rio de Flag, one of the main surface drainages off of San Francisco Mountain, snakes its way through the heart of the city. The Rio’s channel has been modified repeatedly to minimize its effects on property but future flooding cannot be ruled out.

Summary

Flagstaff is situated in a beautiful setting that may appear tranquil and idyllic, yet evidence observed in its foundations suggest that it is subject to numerous earth-making processes. Human time is measured in years or decades but Earth measures time in millennia and eons. With many respected scientific institutions located here, Flagstaff is in the enviable position for a city its size to seize this geologic awareness and embed it into the decision making process.

Glossary

• Colorado Plateau – One of 26 geographic provinces in the United States, it is characterized by colorful, widespread, flat-lying sedimentary rocks.
• aquifer – A rock layer capable of holding groundwater in its pores and fractures.
• strato-volcano – A type of volcano containing alternating layers of lava flows and cinder deposits. Strato-volcanos are among the earth’s largest volcanic features.
• dacite – A volcanic rock containing about 65% silica, making the lava extremely viscous (not runny).
• dome volcano – A type of volcano that extrudes very viscous lava, creating a large mound of hardened lava on the landscape
• cinder cone – A type of volcano that erupts droplets of lava that cool into pea-size grains called cinders or scoria.
• recurrence interval – The average period in years between any given geologic event.