An Ancient Landslide in the McDowell Mountains, Arizona
The McDowell Mountains, 20 miles northeast of the Phoenix metro area, are home to one of Arizona’s largest landslides. About 500,000 years ago, a portion of the east-central summit of the McDowell Mountains suddenly collapsed into a catastrophic rock avalanche.
The chaotic mix of 5.5 million cubic meters of granite rock, vegetation and soil flowed eastward for 1.5-kilometers (about 1 mile) before coming to rest. The resulting debris-flow deposit – the Marcus Landslide -- is 500-meters wide (1,650 feet), 1,200-m long (nearly 4,000 ft), and stands 30 meters (100 ft) above the valley floor. The Marcus Landslide is a stark reminder that landslide hazards are present in the mountains of the American Southwest.
Figure 1. Digital elevation location map (DEM) of McDowell Mountains and geography.
Anatomy of the Marcus Landslide
The Marcus landslide originated in weathered granite bedrock at 1,334 meters (4,067 feet) above sea level -- near the second highest ridgeline in the McDowell Mountains; known locally as East End.
The landslide entrained nearly 5.5 million cubic meters (194.2 million cubic feet – or enough Earth material to fill six ASU Sun Devil Stadiums!) of loose granite bedrock and weathered granite (coarse sand-size particles known as grus). The estimated total weight was about 11.7 billion kilograms (25.8 billion pounds), equivalent to 1.5 times the weight of water in Tempe Town Lake!
Figure 2. Annotated birdseye view of the Marcus Landslide.
The event probably started as a mass of sliding rock before quickly transforming into a debris flow. The Marcus Landslide probably reached speeds of 16 to 44 miles per hour before reaching the valley floor 400 m (1,200 ft) below. The resulting gaping scar, referred to as the “pocket”, below the East End is stark evidence of the violent nature of this event.
Steep levee walls – 15 to 30 meters high (45 to 90 feet) -- define the sides and toe of the slidemass. The interior of the slidemass has crude, large-scale flow features, or pressure ridges that formed as the slidemass buckled. The debris flow transported several hundred boulders, each weighing between 100 and 1,000,000 kg (170 and 2,400,000 pounds respectively), to the valley floor below.
The energy released during this single landslide event was more than 46 Tera-Joules; the equivalent of energy released in one atomic bomb, or that of driving a small car round-trip from Los Angeles to New York over 1,850 times!
Granitic boulders, many of which are several meters in diameter, help distinguish the slidemass from the surrounding plain. One particularly large boulder, “Clubhouse Rock” (See Figure 3), is 10 meters wide (30 feet) and weighs about 1,090,090 kilograms (2,400,000 pounds) -- equivalent to the weight of ~130 mammoths! Close examination shows that each boulder is fixed in a matrix of clay- to sand-size sediments. And each boulder hosts a dynamic micro-ecosystem of burrowing animals, plants, fungus, and bacteria.
So what triggered the Marcus Landslide?
Tens-of-thousands of years of exposure to water, ice, and rain, combined with root and ice heaving along pervasive fractures (geologists call these joints) probably weakened the granitic rock below the East End to the point of collapse. Steep slopes combined with water saturation in the “pocket” primed the hill slope for failure. Poised for collapse, a heavy rain, a bolt of lightning, or an earthquake could have spontaneously triggered release of nearly 12 billion kilograms of rock.
The Last 500,000 Years
In the intervening 500,000 years, streams slowly knifed back into the landslide, eroding away the clay and grus matrix (See Figure 4) while leaving the granite boulders standing in their original position. Erosion of matrix from underneath and in-between boulders created small cavities to large caverns or grottos. At the same time, the combined action of wind, water, and microbial activity produced thin coatings of a black manganese stain, or desert varnish. The desert varnish coatings provide scientists with a tool for calculating how many years the boulders have been exposed; in the case of the Marcus Landslide, the data indicate approximately 500,000 years of exposure.
Life and Climate 500,000 Year Before the Present
Collapse along the east-central McDowell Mountains coincided with Earth’s most recent Ice Age during the Pleistocene epoch (1.8 million years to 10,000 years ago). Central Arizona’s climate then was wetter and cooler than today. And the McDowell Mountains would have been home to indigenous dwarf conifer woodlands rising out of a grass-covered plain that supported mammoths, giant sloths, saber-tooth cats, camels, and horses. It is likely that traces of former life are still preserved in the Marcus Landslide, just waiting to be discovered.
Landslide Hazards in the Valley of the Sun
The Marcus Landslide shows that large landslides are a real geologic hazard in the mountains of the Central Arizona. The proximity of the landslide to Scottsdale argues for a more careful analysis of landslide hazards in the McDowell Mountains, and all the ranges surrounding the Valley of the Sun (See Figures 5 and 6).
By examining the characteristics that led to the development of the Marcus Landslide, we can learn and apply them to other areas throughout the Southwest, a necessary task for assessing the risks of landslide hazards. Geoscientists of the Arizona Geological Survey are dedicated to identifying geologic hazards and to working with Arizonan’s to minimize the risk to life and property.
Much remains unknown and uncertain about the Marcus Landslide. Are there other massive landslide deposits as yet unidentified? Where and when will the next major landslide occur? What do the biologic and climatic signatures in the landslide tell us about its history? Can we get a better age-estimate on when the landslide occurred?
Preserved in the McDowell Mountain Regional Park and the McDowell Sonoran Preserve, the Marcus Landslide provides an excellent site for future research, recreation, and education for generations to come. And it’s right in our own back yard. For more information on how to visit or visualize the Marcus Landslide see the links below.
Acknowledgements
Co-discoverer Dr. John Douglass and Dr. Ronald Dorn have been a continuous source of motivation and determination to study the Marcus landslide. Without their efforts little would be known about it. Since its discovery in 2002, the following persons and institutions have made ongoing research and education possible to study the landslide: Rand Hubbell with McDowell Mountain Regional Park, BJ Heggli and Dan Gruber with McDowell Sonoran Conservancy, Scott Hamilton, Bob Cafarella, and Claire Miller with the City of Scottsdale, Todd Luther and Donna Benson with Mesa Community College, John Douglass with Paradise Valley Community College, Ron Dorn with Arizona State University, Joe of Luke Air Force Base, and Michael Conway and Ryan Clark with the Arizona Geological Survey.
Figure 7. Boulders mark the toe of Landslide with Four Peaks on horizon.
References
Christenson, G.E., Welsh, D.G., Péwé, T.L., 1978–1979. Folio of the McDowell Mountains Area, Arizona. Arizona Bureau of Geology and Minerals Technology Folio Series No. 1, Maps GI-1-A, B, G, scale 1:48,000. Arizona Bureau of Geology and Minerals Technology, Tucson.
Douglass, J., Dorn, R.I., Gootee, B.F., 2004. A large landslide on the urban fringe of metropolitan Phoenix, Arizona. Journal of Geomorphology, v.65, pp. 321-336.
Péwé, T.L., Bales, J., Montz, M., 1983. Environmental Geology of the Northern McDowell Mountains. Arizona Geological Survey Contributed Report, CM94-E: Plate 2, Arizona Geological Survey, Tucson.
Skotnicki, S.J., 1996. Geologic Map of Portions of the Fort McDowell and McDowell Peak Quadrangles, Maricopa County, Arizona. Arizona Geological Survey, Tucson.
Welsch, D.G. and Péwé, T.L., 1979. Environmental Geology of the McDowell Mountains Area, Maricopa County, Arizona (Geologic Hazards Map). Arizona Geological Survey Folio Series, 1: Map GI-1-G. Arizona Geological Survey, Tucson.
Photo Credits
Banner photo taken by Joe Viera . All other photos taken by Brian F. Gootee.