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Environmental Geology and Geologic Hazards in Arizona

Environmental Geology

Environmental Geology applies geologic concepts and principals to addressing geologic hazards and environmental issues. As a multidisciplinary science involving human interaction with, and impact on, the environment – earth, air and water – it shares close ties with engineering geology, which applies geologic knowledge to engineering practice. This page is dedicated to the memory of two geologists, Cathy Schulten Wellendorf and Dr. Troy L. Péwé of Arizona State University, who contributed greatly to environmental geology studies here in Arizona.

Environmental geology is a broad field that draws upon a number of geologic sub-disciplines: hydrology (surface water and groundwater resources and management), geomorphology and the study of processes that shape the earth’s surface, waste containment and pollution of air, water, and soil, and oil and gas resource management. Perhaps the best known application of environmental geology is to manage and mitigate geologic hazards that threaten people, their property and infrastructure. This includes hazards such as floods – both flash floods and regional floods, earthquakes, landslides (avalanche, rock fall, slump and creep), karst features (sinkholes and fissures), debris flows, volcanic activity (pyroclastic flows, lava flows and lahars), earth fissures, and radon gas among other geologic hazards.

For a cogent description of geologic hazards in Arizona, and some advice on how to avoid or minimize their impacts, see AZGS’s A Home Buyers Guide to Geologic Hazards in Arizona, a volume in our Down-to-Earth popular geology series that was dedicated to Dr. Troy L. Péwé. Geologic hazards encountered in Arizona are briefly described below.

Geologic Hazards of Arizona

Earth fissures

Earth fissures are associated with basin subsidence that accompanies extensive ground water withdrawal. In Arizona, fissures were first noted near Eloy in 1929. The physical appearance of fissures varies greatly, but they may be more than a mile in length, up to 15 feet wide, and 100s of feet deep. Fissures erode rapidly during torrential rains to form extensive gullies that present a substantial hazard to people and infrastructure. Importantly, fissures potentially provide a ready conduit to deliver runoff and contaminated waters to basin aquifers. Rapid population growth in southern Arizona has increasingly juxtaposed suburban development and fissures.

Online Resources

Earthquakes Back to list

The Earth’s crust in the Western U.S. is riddled with faults. When crustal rocks move abruptly along the fault, earthquakes occur. Deaths and injuries from earthquakes usually stem from collapse of buildings, roads and bridges, or due to cascading events such as fires, landslides, or tsunami. Arizona has experienced historical earthquakes, and has many faults that have generated large prehistoric earthquakes.

In October 2012, Arizona joined California, Utah and states in the Western U.S. to host a shakeout earthquake preparedness drill – in our case, the Great Arizona Shakeout. Some important Arizona earthquake resources are available at our earthquake page.

Online Resources

Floods & debris flowsBack to list

Floods constitute the most commonly encountered geologic hazard both nationally and regionally. Arizona’s monsoon season, with its predilection for sudden, torrential and localized rainfall, coupled with a landscape incised by thousands of washes and gullies, is ripe for dangerous flash floods. Of particular concern from a floodplain-management perspective is flooding on alluvial fans, where channels diverge downstream and broad areas outside of channels may be inundated in large floods. Arizona’s major rivers and streams are more likely to swell and flood during the sustained regional storms that occur in the fall and winter seasons.

Debris flows are mixtures of water and debris (typically 80% solids and 20% water) that move downslope under the influence of gravity. They entrain and transport particles ranging from clays, silt, and sand to boulders tens of feet in maximum dimension. They resemble slurry of wet concrete as they rush down gradient in channels or stream valleys. In Arizona, they are particularly prevalent during monsoon storms in areas recently scorched by wildfires, e.g., debris flows on the east flank of the San Francisco Peaks following the Schultz wildfire of summer 2010.

Online Resources - Floods

Online Resources - Debris flows

Environmental Geologists

Troy Lewis Péwé (1918 – 1999) and
Cathy Schulten Wellendorf (1956 – 1988)

Péwé, Wellendorf, Bales
In the field with (left to right) Dr. Troy Péwé,
Cathy Schulten Wellendorf, and Jim Bales, ca. 1980.

Troy Péwé

When Troy Péwé arrived at Arizona State University (ASU) in 1965, the field of environmental geology was in its infancy. As a geoscientist specializing in Quaternary studies, Péwé was well situated to become a leader in this new field. Before arriving at ASU, Péwé was a renowned researcher of Arctic permafrost. To this day, the International Conference on Permafrost honors Péwé’s memory with the Troy L. Péwé award, bestowed every four years to a young researcher who shows extraordinary talent in the field.

Péwé’s environmental geology research in Arizona ranged broadly from the impact of desert dust on man, to the origin and distribution of earth fissures, to geologic hazards of the Phoenix Basin. Péwé and his students constructed environmental geology maps of 7.5-minute quadrangles for Phoenix and environs – the first of their kind in Arizona. These maps remain useful today and are being published as part of AZGS’s Contributed Map series. (For a more comprehensive listing of Péwé’s accomplishment and research, see A.L. Washburn’s “Memorial to Troy Lewis Péwé, 1918-1999.”)

Cathy Schulten Wellendorf (bibliography)

Besides his pioneering efforts in environmental geology, Péwé mentored dozens of graduate students. Cathy Schulten Wellendorf was one such student. Cathy’s thesis project, “Environmental Geology of the Tempe Quadrangle, Maricopa County, Arizona” led to an AZGS publication, “Geologic Investigation Folio GI-2”, which she co-authored with Troy Péwé and James T. Bales.

In 1980, Cathy joined the Bureau of Reclamation at their Arizona Projects Office. In 1984, she was appointed lead geologist on the Stewart Mountain Dam Modification project where she directed preconstruction geologic investigations and monitored seepage conditions and rock-slope stability of the auxiliary spillway. She described that project in “Stewart Mountain Dam: Current Geologic Investigations” in the Winter 1986 issue of Arizona Geology. Cathy was a gifted and highly respected geologist.

Following a short illness, Cathy passed away in December 1988. In recognition of her contribution to the Stewart Mountain Dam project, the Salt River Project posted a bronze memorial plaque at the dam. Her family established the Cathy Wellendorf Memorial Fund with the Arizona Geological Survey to support projects and activities in environmental and engineering geology. That fund provides financial support for some AZGS environmental geology projects:

  • partial funding of an early report on earth fissures in Arizona (Land Subsidence and Earth Fissures in Arizona);
  • digitizing and publishing 64 environmental geology maps of Phoenix and environs co-authored by Troy Péwé and graduate students at Arizona State University;
  • preparing environmental geology web resources.


Upcoming map sets:

  • Reconnaissance Environmental Geology of the Tonto Foothills, Scottsdale, Maricopa County, Arizona (4 map sheets)
  • Environmental Geology Guadalupe Quadrangle, Maricopa County, Arizona (4 map sheets)
  • Environmental Geology Rio Salado Development District – Eastern Part, Maricopa County, Arizona (9 map sheets)
  • Geology of the Carefree Basin, Maricopa County, Arizona (10 map sheets)
  • Environmental Geology & Material Resources of the Phoenix Mountains, Maricopa County, Arizona (8 map sheets)
  • Environmental Geology of the Paradise Valley Quadrangle, Maricopa County, Arizona (14 map sheets)

Radon gasBack to list

From our webpage on radon, “Radon (Rn) is a noble gas formed by radiogenic decay of uranium. It is a colorless and odorless radiogenic gas. Radon’s most common radiogenic isotope, 222Rn, has a half-life of 3.8 days; one half-life is the period of time during which one-half of the parent isotope decays to form daughter isotopes.” Radon is the second-leading cause of lung cancer in the U.S. Fortunately, indoor radon in Arizona is among the lowest in the nation.

Online Resources

Problem soilsBack to list

Swelling and shrinking soils – soils that alternately expand or contract due to wetting or drying conditions - cause more damage to homes and buildings than floods, tornadoes and hurricanes combined. Why? Because about 50 percent of all American homes are built on problems soils. And 50 percent of those homes suffer some damage – cracked foundations, damaged wallboard, or broken pipes, among other damages – from swelling and shrinking soils.

In Arizona, some clay-rich soils, notably those of the Willcox and Red Lake playas, host damaging giant desiccation cracks (GDC). These features resemble common mud cracks, but are 10s or 100s of feet in length and 3 to 9 feet deep, in contrast to mud cracks, which are measured in inches.

Online Resources (source: Natural Resource Conservation Service)

Landslides Back to list

Landslides occur when large masses of earth material – soil and rock – move downslope. There are a variety of landslide phenomena, such as rock fall (very fast), avalanche (fast), slump (slow), and creep (very slow). Triggers for landslide events include heavy or prolonged rain, ground shaking from earthquakes, forest fire, and construction-related slope modification, among other things. In March 2008, a landslide near Payson, Arizona, buckled Highway 87 and cost millions of dollars to repair.

Online Resources

VolcanismBack to list

Arizona boasts seven young – by geologic standards – volcanic fields. The two youngest are the San Francisco and Uinkaret volcanic fields on the Colorado Plateau of northern Arizona. The San Francisco field is situated near Flagstaff and Williams and was active as recently as 1,000 years ago with the eruption that formed Sunset Crater. The Uinkaret field, perched on the north rim of Grand Canyon, also evinces volcanic activity as recently as about 1,000 years ago.

There is good reason to anticipate additional eruptions in these two volcanic fields, which will probably result in the formation of a small cinder cone and related lava flows. The chief hazard is from cinder and ash fall downwind of the vent and from lava flows proximal to the vent. Larger volcanic vents, such as volcanic domes, stratovolcanoes (e.g., San Francisco Peaks), and calderas represent a substantially greater geologic hazard, but the likelihood of such eruptions in Arizona is very small.

Online Resources

Karst formationBack to list

“Karst is the name applied to topography that develops on land underlain by soluble rocks such as limestone, gypsum, and salt. Karst terrain is characterized by solution features such as caves, sinkholes, depressions, enlarged joints and fractures, and internal drainage,” from Ray Harris’ description of karst features of the Colorado Plateau of northern Arizona. Much of the Colorado Plateau of Arizona is underlain by limestone and gypsum, giving rise to numerous karst features – chiefly sinkholes and open fractures.

Historic sinkhole formation has occurred near Sedona. Recent subsidence studies at the Holbrook Sinks, Navajo County, by the Arizona Division of Water Resources shows that dissolution of subsurface salt deposits continues there and is leading to localized collapse at the surface..

Online Resources

Arsenic Back to list

Arsenic occurs naturally in rock and soil. It is commonly present in trace amounts in groundwater. Arsenic is a known carcinogen at concentrations above 300 ppb (parts per billion). In Arizona groundwater, the arsenic concentration ranges broadly from about 1 ppb to nearly 1,000 ppb.

Online Resources

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