History of the Ajo Mining District, Pima County, Arizona by David Briggs

Geologist David Briggs has written another interesting paper on the history of mining in Arizona. This 18-page paper, History of the Ajo Mining District, Pima County, Arizona, was just published by the Arizona Geological Survey and is available as a free download:

I was particularly interesting in the Ajo paper because as a geologist, I conducted exploration at the mine and in the district. Although the mine is now inactive, there is remaining mineralization that can be mined given the right economic conditions. The Ajo orebody is particularly interesting to geologists because paleomagnetic and geologic evidence indicates that the Ajo ore deposit has been tilted to the south a total of approximately 120 degrees in two separate tectonic events. (Source) There is also speculation that a detached piece of the original orebody lies hidden nearby.

Briggs begins his story as follows: “The hostile environment of southwestern Arizona’s low desert presented many challenges to those who sought to discover and exploit the mineral wealth

of the region. Ajo’s remote location combined with hot summer days and scarce water created a number of obstacles that needed to be overcome. Despite these impediments, the district’s wealth was mined by Native Americans long before the arrival of first Spanish explorers, who recognized its potential soon after establishing outposts in this region.”

The Ajo area has a long history. Prior to the arrival of the first Spanish explorers in the 1530’s, the native Tohono O’odham Indians and their ancestors mined hematite, an iron oxide, which they used as body paint. Establishment of Spanish missions in Southern Arizona provided bases from which prospectors combed the country.

With the signing of the Treaty of Guadalupe Hidalgo at the end of the Mexican American War on February 2, 1848, and the subsequent Gadsden Purchase in June 1854, many prospectors tried their luck at Ajo.

Briggs provides great detail as he recounts the many lives of mining ventures in Ajo. Following is a very brief sketch of major events.

The first formal mining began in 1855 and a wagon road was constructed to the railroad at Gila Bend. Ore was also sent by wagon to San Diego and shipped to Swansea, Wales for smelting. High transportation costs eventually made the venture uneconomic.

Briggs recounts the era between 1898 and 1908 when the Ajo deposit saw many promotions and fraudulent mining schemes.

In 1911, the Calumet and Arizona Mining Company, which was operating mines in Bisbee, became interested in the Ajo properties and acquired the New Cornelia Copper Company which owned Ajo at the time. Calumet began an extensive drilling program which confirmed the presence of a large sulfide body of mineralization. They began open pit mining in 1915.

In 1931, Phelps Dodge merged with Calumet and Arizona Mining Company and continued to operate the mine which they did until 1985 when a combination of low copper prices and stricter regulations for smelter air quality caused the company to close the mine.

The Ajo property is now owned by Freeport-McMoRan, Inc. through its merger with Phelps Dodge. According to Briggs, “Freeport continues to periodically assess the economic feasibility of returning the Ajo project to production. As of December 31, 2015, this project is estimated to contain a sulfide resource of 482 million short tons, averaging 0.40% copper, 0.010% molybdenum, 0.002 oz. of gold/ton and 0.023 oz. of silver/ton.”

Other papers by David Briggs, published by the Arizona Geological Survey:

History of the Warren (Bisbee) Mining District

History of the San Manuel-Kalamazoo Mine, Pinal County, Arizona

Recovery of Copper by Solution Mining Techniques

Superior, Arizona – An Old Mining Camp with Many Lives

History of the Copper Mountain (Morenci) Mining District

History of Helvetia-Rosemont Mining District, Pima County, Arizona


The Pirate Fault of Canada del Oro


The Pirate fault forms the western boundary of the Santa Catalina Mountains near Tucson and separates the mountains from the Cañada del Oro basin to the west. The fault occurs just east of the communities of Saddlebrooke, Catalina, and Oro Valley. Remnants of this fault, exposed for about 15 miles along the mountain front, are described in a paper from the Arizona Geological Survey (see reference below). The paper describes geological features of 10 sites along the fault trace.


The AZGS says that this fault represents an expression of Basin & Range faulting which was active between 12 million and 6 million years ago. Vertical displacement on the fault is estimated to be about 2.5 miles with the west side down relative to the Santa Catalina Mountains uplift on the east. The fault dips from 50° to 55° west along its entire trace. The Basin & Range era was a time of crustal extension which formed much of the topography in Southern Arizona.

According to AZGS: “ Following cessation of active uplift, the fault was buried under detritus eroded from the uplifted Santa Catalina block and, currently, is being exhumed by the down-cutting Cañada del Oro and its tributaries. This field examination reveals the fault to have left a sparse but diverse collection of remains implying a varied history of fault development and evolution.”

“Deposition of basin-fill material in the Cañada del Oro basin culminated in Pleistocene time (1-2 Ma) following cessation of active uplift on the Pirate fault. Alluvium deposited during this latter time forms the high-stand surface of coalescent alluvial fans composed mostly of detritus eroded from the Santa Catalina Mountains.” That material contains placer gold deposits. The gold was derived from gold-bearing quartz veins in the Santa Catalina Mountains.

The Pirate fault disappears beneath alluvium to both the south and north. If one projects the northern trace, the Pirate fault could intersect the southeast-to-northwest trending Mogul fault. Indeed, near the projected intersection is a decorative stone quarry whose source rock is highly fractured, deformed, and altered bedrock that may be evidence of the projected fault intersection.

Parts of the exposed Pirate fault are stained red by hematite, an iron oxide, suggesting that mineralizing hydrothermal solutions were present during the development of the fault. The exact nature of this mineralization is enigmatic and according to the AZGS, “would seem to defy ready explanation.” “The picture that emerges is that of the Pirate fault as a geologic entity whose tenure as an active participant in the extensional Basin-Range tectonic event has left behind a somewhat sparse and locally enigmatic set of remains from which to infer, caveat emptor, its past.”

Hoxie, D.T., Exhuming the Remains of the Inactive Mountain-Front Pirate Fault, Santa Catalina Mountains, Southeastern Arizona. Arizona Geological Survey, Contributed Report CR-12-F, 18p.

Free download:

See also: The Gold of Cañada del Oro

The Basin & Range Province of North America

American Geosciences Institute’s Critical Issues program


The Arizona Geological Survey’s winter e-magazine features an article about the American Geosciences Institute’s Critical Issues program (

The aim of this AGI program is to pioneer a new approach to sharing societally-relevant science with state and local decision makers. “Here in Arizona, we are sharing this with state and local decision-makers to help them wrap their heads around the complex issues involving groundwater, geologic hazards, and sustainable natural resource management.”

The program aims to support connections and communication between the geoscience community and decision makers. Although the program caters to decision makers at all levels, it particularly focuses on state and local decision makers because these stakeholders are commonly underserved by geoscience policy efforts.

The program convenes meetings, such as the AGI Critical Issues Forum, but its main interface is a web-based platform of resources that bring the expertise of the geoscience community to decision makers by offering a curated selection of information products from sources that include state geological surveys, federal and state agencies, and AGI’s member societies.

The Critical Issues program offers the following freely accessible information services:

Research database: Over 4,000 publications primarily from state geological surveys and the U.S. Geological Survey.

Webinars: Free webinars on a variety of topics that bring geoscientists and decision makers together to discuss potential solutions to challenges at the interface of geoscience and society.

Maps & Visualizations: 144 interactive maps and visualizations covering all 50 states and the District of Columbia.

Case studies: A new product that is coming online in Spring 2017. Specific applications of geoscience to societal problems.

Fact Sheets: A new product that is coming online in Spring 2017. Provide more in-depth information on the big issues.

Frequently Asked Questions: 105 questions on topics including: climate, energy, hazards, mineral resources, and water.

Read more at:

This AZGS e-Magazine also includes an article about groundwater use in the United States.


American mineral production for 2016

The U.S. Geological Survey has just released their annual summary of non-fuel mineral production in the U.S. for 2016. They estimate that the value of all non-fuel minerals produced from U.S. mines was $74.6 billion, a slight increase over production in 2015. “ Domestic raw materials and domestically recycled materials were used to process mineral materials worth $675 billion. These mineral materials were, in turn, consumed by downstream industries with an estimated value of $2.78 trillion in 2016.”

Principal contributors to the total value of metal mine production in 2016 were gold (37%), copper (29%), iron ore (15%), and zinc (7%). The estimated value of U.S. industrial minerals production in 2016 was $51.6 billion which was dominated by crushed stone (31%), cement (18%), and construction sand and gravel (17%).

Nevada was ranked first with a total mineral production value of $7.65 billion, mainly from gold. Arizona came in second in total production with a value of $5.56 billion and first in U.S. copper production. Texas, California, Minnesota, Florida, Alaska, Michigan, Wyoming, Missouri, and Utah, in that order, were next in value of production.

“In 2016, U.S. production of 13 mineral commodities was valued at more than $1 billion each. These were, in decreasing order of value, crushed stone, cement, construction sand and gravel, gold, copper, industrial sand and gravel, iron ore (shipped), lime, phosphate rock, salt, soda ash, zinc, and clays (all types).” Does that order surprise you?

Most of the material mined (stone, sand, lime, clay) is used in construction of our infrastructure.

Gold is used as coinage and to manufacture jewelry. Because gold does not corrode, it is used in solid state electronic devices that use very low voltages and currents which are easily interrupted by corrosion or tarnish at the contact points.

Copper is used mainly to generate and transmit electricity and it occurs in all our electronic devices.

Zinc is used for galvanizing to prevent corrosion and, combined with copper to make brass. Zinc is also combined with other metals to form materials that are used in automobiles, electrical components, and household fixtures. Zinc oxide is used in the manufacture of rubber and as a skin ointment.

Iron is used mainly to make steel.

Phosphate rock is used mainly as a fertilizer and also as a nutritional supplement for animals and humans.

Soda ash (sodium carbonate) is an essential raw material used in the manufacturing of glass, detergents chemicals, softening water, making baking soda, and used in many industrial products.

“U.S. mine production of copper in 2016 increased slightly, to about 1.41 million tons, and was valued at about $6.8 billion. Arizona, New Mexico, Utah, Nevada, Montana, and Michigan, in descending order of production, accounted for more than 99% of domestic mine production; copper also was recovered in Missouri. Twenty-four mines recovered copper, 17 of which accounted for about 99% of production.”

A note on reserves and resources:

Reserves data are dynamic. They may be reduced as ore is mined and (or) the feasibility of extraction diminishes, or more commonly, they may continue to increase as additional deposits (known or recently discovered) are developed, or currently exploited deposits are more thoroughly explored and (or) new technology or economic variables improve their economic feasibility. Reserves may be considered a working inventory of mining companies’ supplies of an economically extractable mineral commodity. As such, the magnitude of that inventory is necessarily limited by many considerations, including cost of drilling, taxes, price of the mineral commodity being mined, and the demand for it. Reserves will be developed to the point of business needs and geologic limitations of economic ore grade and tonnage. For example, in 1970, identified and undiscovered world copper resources were estimated to contain 1.6 billion metric tons of copper, with reserves of about 280 million tons of copper. Since then, more than 500 million tons of copper have been produced worldwide, but world copper reserves in 2016 were estimated to be 720 million tons of copper, more than double those of 1970, despite the depletion by mining of almost double the original estimated reserves.


As can be seen in the table above, there was a decline in the production of coal, probably due to the rise in natural gas production. Metal production also decreased. According to the USGS, “Several U.S. metal mines and processing facilities were idled or closed permanently in 2016, including iron ore mines in Michigan and Minnesota; three primary aluminum smelters in Indiana, Missouri, and Washington; one secondary zinc smelter in North Carolina; a titanium sponge facility in Utah, the only such facility in the United States; and titanium mineral operations in Virginia.” In 2016, imports made up more than one-half of the U.S. apparent consumption of 50 non-fuel mineral commodities, and the United States was 100% import reliant for 20 of those.

The 200-page report gives detailed information for each commodity.

The full report is available online here:

A Guide to the Geology of Sabino Canyon and the Catalina Highway

The Arizona Geological Survey has recently released a 56-page booklet which points out areas of geologic interest in Sabino Canyon and along the Catalina Highway to Mount Lemmon. The booklet is available for free download here.

The citation is:

Bezy, J.V., 2004, A Guide to the Geology of Sabinho Canyon and the Catalina Highway. Arizona Geological Survey Down to Earth, DTE #17, 56 p.

AZGS introduces the booklet:

“ Upper Sabino Canyon Road, also known as the 1 Sabino Canyon Shuttle Route, and the Catalina Highway to Mount Lemmon offer a variety of spectacular geologic features. Because of the relatively sparse vegetation in the lower part of the range, most of these features are easy to recognize and photograph. Some of these features are common throughout this southern part of the Santa Catalina Mountains. Others occur in many other parts of the American Southwest. This booklet is your field guide to the geology of this spectacular mountain landscape. All of the geologic features described in the text can be reached by short walks from the Sabino Canyon Shuttle Route or the Catalina Highway. This book is written for the visitor who has an interest in geology, but who may not have had formal training in the subject. It may also help assure that the visiting geologist does not overlook some of the features described.”

The booklet provides short geologic descriptions of Sabino Canyon and the Catalina Mountains, and describes 11 features in Sabino Canyon and 14 features along the Catalina Highway, all of which are illustrated by photographs, maps, and diagrams. This booklet can make your visit to these areas more interesting and informative.

Below are maps of Sabino Canyon and the Catalina Highway showing the location of geologic features described.



More articles on Tucson area geology:

Beneath the Tucson Valley

Gold of Cañada del Oro and rumors of treasure

Old mines of the Tucson Mountains

History of the Copper Mountain (Morenci) Mining District, Greenlee County, Arizona


The Arizona Geological Survey has just published a well-written history of the Morenci, Arizona, mining district. The report was written by geologist David F. Briggs and was published as AGS Contributed report Cr-16-C. The 79-page report is available for free download:

The Copper Mountain (Morenci) mining district is located approximately 115 miles northeast of Tucson, Arizona.

Mining began in 1873. This district has produced more than 36 billion pounds of copper from 1873 to 2015. Since 1985 is has been America’s largest domestic copper producer.

The discovery of copper at Morenci during the turbulent years of the American Civil War brought new opportunities for many, but foreshadowed the end of a way of life for Native Americans, who had lived in the region for millennia. A diverse cast of characters has played a role in Morenci’s history, including veterans who ventured west after the war, as well as immigrants eager to make a new life in America.

Briggs provides an interesting narrative of the development of the district as different companies gradually consolidated the mines. Briggs breaks the history into five phases of development as the owner(s) dealt with different types of ore, changing technology, new discoveries, and the sometimes volatile copper market.

Phelps Dodge Corporation operated the district beginning in 1917. Freeport-McMoRan Copper and Gold, Inc. (renamed Freeport-McMoRan, Inc. in July 2014) acquired an 85% interest in the Morenci project through its merger with the Phelps Dodge Corporation in March 2007, and has been operating the mine since then.

The report contains many maps and both current and historical photographs. This report is an interesting read and its story is one that was similar to that of many mines in the West.



 More reports from AZGS:

AZGS field guides to Arizona Geology

A guide to the geology of the Sedona & Oak Creek Canyon area of Arizona

A Guide to the Geology of the Santa Catalina Mountains

A Guide to the Geology of Organ Pipe Cactus National Monument and the Pinacate Biosphere Reserve

A Guide to the Geology of the Flagstaff Area

A Guide to Geology of Petrified Forest National Park

A Guide to Oak Creek-Mormon Lake Graben

AZGS Guides to Northern Arizona Geology

History of the Copper Mountain (Morenci) Mining District, Greenlee County, Arizona

Another Greenland melting scare

From the “it’s worse than we thought department”:

A new paper published in Science Advances claims that the amount of melting of coastal glaciers in eastern Greenland has been underestimated by about 20 gigatonnes per year. (Link to full paper titled “Geodetic measurements reveal similarities between post–Last Glacial Maximum and present-day mass loss from the Greenland ice sheet”) The paper does not mention global warming or climate change. The melting is due entirely to geologic processes. But the press manages to sound an alarm.

The New York Post translates the 20 gigatonnes figure to pounds to make it scarier sounding: “The new study, published in Science Advances, discovered that the island is losing 550 trillion pounds of ice a year — 40 trillion, and about 7.6 percent, more than scientists previously thought.”

The Post quotes a professor:

“It is pretty scary,” Michael Bevis, a professor at Ohio State University and co-author of the study, told the AP. “If you look at the last 15 years since we’ve been having these measurements, it’s clearly getting worse.” According to Bevis, the extra ice will add approximately 1/50th of an inch a decade to global sea level. So 1/50th of an inch per decade is scary?

An article from Climate Central (an alarmist site) begins with these paragraphs:

Rising temperatures are melting ice and sending it to the ocean, a process that is pushing sea levels higher and altering the landscape at both poles. The latest news comes from Greenland, where researchers have used high-tech satellite and GPS measurements to see how much mass the ice sheet is losing.

Their results, published this week in Science Advances, indicate that it’s melting faster than previous estimates, particularly in areas where the ice sheet comes in direct contact with the ocean. It’s a troubling finding for the future of coastal areas around the world.

Greenland hot spotThe claim that melting is due to rising temperatures is debunked by the Science Advances study itself. In the study, they show that isostatic rebound following the last glacial maximum is tilting the continent and causing east coast glaciers to flow faster into the sea. They also note that “The onset of increasing flow of the northeast Greenland ice stream (the largest flow feature of the ice sheet), for example, has been linked to a geothermal hot spot.”

As I note in my article Greenland surprises:

Ice-penetrating radar and drilling have led to some surprises in Greenland during the last few years. The continent is bowl-shaped, it has a massive canyon running down its middle, and it contains a large aquifer of liquid water beneath the ice. That means that the continental ice sheet is in no danger of slipping into the ocean as some have claimed.

Regardless of the cause of melting, is it “a troubling finding for the future of coastal areas around the world” as claimed by Climate Central?

According to calculations at the Watts Up With That blog, melting of 550 trillion pounds of ice would cause a sea level rise of 0.689 millimeters or 0.0271 inches per year. That additional 40 trillion pounds actually added 0.045 mm/yr to global sea levels. The total melt contributes to sea level rise of less than the thickness of a penny. Do you find that scary?

To put things in further perspective, consider this report:

“A considerable change of climate inexplicable at present to us must have taken place in the Circumpolar Regions, by which the severity of the cold that has for centuries past enclosed the seas in the high northern latitudes in an impenetrable barrier of ice has been, during the last two years, greatly abated. 2000 square leagues [approximately 14,000 square miles] of ice with which the Greenland Seas between the latitudes of 74N and 80N have been hitherto covered, has in the last two years entirely disappeared.”

That report is an extract from a letter by the President of the Royal Society addressed to the British Admiralty, written in 1817 (Royal Society, London. Nov. 20, 1817. Minutes of Council, Vol. 8. pp.149-153).

Sea also:

The Sea Level Scam

New Zinc-lead-silver mineral deposit discovered in SE Arizona




Arizona Mining Inc. (formerly Wildcat Silver) has just announced a major new discovery of zinc-lead-silver mineralization on their Hermosa Taylor project in Santa Cruz County near Patagonia, Arizona.

I have previously reported on what is now called the Hermosa Central deposit which lies to the southeast of the new deposit: see Manganese may be mined in Arizona. According to a prefeasibility study completed in December, 2013, that deposit contains reserves of 145 million ounces of silver and 7.2 billion pounds of manganese, with inferred additional resources of 235 million ounces of silver and 10.3 billion pounds of manganese. The mineralization occurs mainly as a oxide manto (blanket).

The new discovery, called the Hermosa Taylor deposit, discovered in 2015, lies about 2,000 feet to the northwest of Hermosa Central. This mineralization is a stratabound carbonate replacement deposit. This deposit contains sulfides of zinc, lead (with silver) and copper. A resource estimate as of Feb., 2016, claims 39.4 million ton of 11.04% zinc equivalent. (An “equivalent” grade adds in prorated values for copper, lead and silver, see chart here.) There is also an overlying oxide zone according to the cross-section (see here).

Arizona Mining reports that the best hole to date at Taylor Deposit HDS-361 intersects 8 mineralized intervals including 105 feet grading 13.65% Zinc, 10.33% Lead and 3.36 opt Silver and 73 feet grading 15.20% Zinc, 11.20% Lead and 3.89 opt Silver within a larger interval of 504.5 feet grading 6.51% Zinc, 4.88% Lead and 1.72 opt Silver. (Opt = ounces per ton)

The new Taylor deposit remains open to the north, west, and south, so the actual resource could be much bigger than currently estimated. See exploration potentialmap here.

Update, September 13: Arizona Mining reports that a step-out hole sited 1,300 feet northwest of the existing resource encountered intense alteration and recrystallization of the carbonate host and five distinct mineralized intervals including a 24.5 foot interval which assayed 23.1% zinc, 13.5% lead, 0.10% copper and 7.3 ounces per ton silver within a 49.5 foot thick broader zone of mineralization which assayed 13.6% zinc, 8.04% lead, 0.11% copper and 4.79 opt silver. This result bodes well for the possibility of greatly expanding the resource.


Both the Central and Taylor deposits are hosted by Paleozoic sandstones and limestones and by Cretaceous and Tertiary volcanic rocks. See here for more detail.

Arizona Mining’s current interpretation of the geology:

The major lithologic host for the Hermosa deposit is an epiclastic sandstone that is locally interbedded with very fine-grained tuff. High-angle faults, trending predominantly north-south and east-west, bound the horst blocks in the area, and may have served as conduits for mineralizing fluids.

Sections through the deposit oriented both east-west and north-south suggest that the strata are deformed into a doubly-plunging anticline the apex of which is coincident with the highest grade-thickness of silver accumulation, which suggests that deformation pre-dated the mineralizing event, that mineralizing fluids migrated to the topographically highest permeable area, and that post-mineral tilting of the deposit has occurred.

The company still has more drilling and metallurgical testing to do, but that’s the easy part. The hard part is navigating the regulatory quagmire for permitting.

The explosive geology of the Chiricahua Mountains


The Chiricahua Mountains of southeastern Arizona where once the site of very explosive volcanic eruptions. They now host some very interesting rock formations including hoodoos, rock spires, and balanced rocks. (All are erosional forms which develop into fantastic pinnacles, towers and grotesques shapes. Hoodoos have a cap rock.)

Chiricahua location

Like the Tucson Mountains, the Chricahuas have had several periods of volcanism. The principal and most explosive episode began about 27 million years ago.

As described by the U.S. Geological Survey (see reference below), “a large mass of magma accumulated within a few miles of the surface, forming a magma chamber” just to the south of where Chiricahua National Monument occurs in the northern part of the range.

“Eventually, the overlying rock ruptured and the resulting decrease in confining pressure allowed volatiles (mostly water vapor and carbon dioxide) to separate from the magma and form gas bubbles. Foaming magma formed pumice and expanded as much as 50 times in volume, causing a series of large explosive eruptions. The eruptions blew more than 100 cubic miles of magma out of the volcano and buried a region of at least 1,200 square miles in a thick blanket of hot ash and pumice. For comparison, the 1980 eruption of Mount St. Helens produced only one tenth of a cubic mile of magma and the 1991 eruption of Mount Pinatubo in the Philippines, which was one of the largest eruptions of the century, produced only about one cubic mile of magma.”

The USGS report notes, “The eruption produced boiling clouds of very hot (greater than 1,000°F) ash, pumice, rock fragments, and gas that were propelled into the atmosphere and across the land surface at speeds of 50 to more than 100 miles an hour, scouring everything in their paths like superheated jets from a sandblaster. As the clouds lost gas and deflated, they became more dense and flowed downslope from the volcano as pyroclastic flows that ponded in valleys to form thick deposits of steaming ash and pumice.” As the still-hot ash and pumice came to rest is compacted and fuse into a rock geologists call welded tuff.

Eruption of that great volume of magma caused the magma chamber to collapse in on itself to form a caldera (now called the Turkey Creek caldera) that was about 12 miles in diameter and at least 5,000 feet deep. The caldera was partially filled by some of the just-erupted ash and by subsequent eruptions. See geologic map below.

Following the explosive ash eruption, an eruption of less-volatile lava covered most of the caldera and prevented much of the tuff from being eroded away.

During the 20 million years following eruptions, large faults cut the volcanic edifice and dropped part of it down to form the San Simon Valley on the east and the Sulphur Springs Valley on the west.

HoodoosAlso during that time, water and wind eroded the welded tuff along fractures to produce the weird shapes we see today. The USGS opines, “Recent study has shown that contrary to previous ideas, the joints were not produced as the tuff was squeezed and fractured between faults, as if in a vise. If that were the case, one would expect to see a systematic pattern in the orientation of the joints. Instead, the joint directions vary widely and they curve, features that indicate they resulted mainly from contraction brought about by the original cooling of the tuff. Cooling joints form at right angles to surfaces where heat is removed from a lava flow or tuff layer; these are mainly the top and bottom surfaces and the resulting joints are usually vertical planes. The intersection of joint planes form rock columns.”

The USGS calculates that the rock spires and columns are about 2.4 million years old based on the local erosion rate of the tuff. And, they say, “Surprisingly, the columns are quite strong, and even the ‘balanced rocks’ are not as fragile as they appear. Engineering analysis shows that the columns are well within their mechanical failure limits for static load; they are not about to fail under their own weight. In fact, the 187-foot-high Totem Pole could be suspended upside-down without breaking. Dynamic failure is much more likely, in which columns would be ‘knocked over’ by a lateral force. Lateral forces occur during earthquakes, but surprisingly, few of the columns appear to have been destroyed by the nearby magnitude 7.2Pitaicachi earthquake of 1887, despite widespread damage to buildings in the region. Perhaps, a ‘tuned’ frequency of earthquake ground waves is required to get the columns swaying enough to fall.”

geo map



geo map index



Pallister, J.S., du Bray, E.A., and Hall, D.B., 1997, Guide to the Volcanic Geology of Chiricahua National Monument and Vicinity, Cochise County, Arizona, USGS MAP I-2541



See more photos here.

Tucson Mountains geology – an update


The Tucson Mountains form the rampart on the west side of the city. If you drive over Gates Pass, take a look at the road cuts, especially near the top, you will see a chaotic jumble of different rocks, mainly volcanics. According to the Arizona Geological Survey: “Tucson Mountain Chaos is a formal geologic name, describing one of the more confusing, complex, and controversial areas in southern Arizona.”

Like many mountain ranges in Southern Arizona, the Tucson Mountains have experienced several episodes of volcanic eruption. Major eruptions occurred during Triassic-Jurassic time (~190-200 Ma), early Laramide (74 Ma), later Laramide (62 Ma), and one late Tertiary (~20 Ma). (Ma means million years ago.) There were also several interspersed minor eruptions.

The early Laramide (74 Ma) eruption was very explosive and produced great volumes of rhyolite tuff (Cat Mountain tuff). The rapid eruption caused the volcano to collapse in on itself to form a caldera. That collapse produced megabreccia called the Tucson Mountain Chaos. (Breccia is simply a bunch of angular fragments cemented together.) Within that breccia are small to very large fragments of other rocks including house-sized blocks of limestone. The breccia could have formed in three ways (and there are proponents of each way): moat in-filling of the caldera, landslides, or fluidized material brought up from below.

TM geo map3The Tucson Mountain caldera is not a typical caldera with equal subsidence all around. Rather, it is a “trap door” caldera with the “hinge” area on the southeast and major subsidence on the west. The western ring fault (called the Museum Fault) parallels Kinney Road from about Old Tucson to just past the Arizona-Sonora Desert Museum, then swings east around a granite pluton. This structure was first proposed by Peter Lipman of the United States Geological Survey in 1994 and later supported by geophysical investigations.

Beginning about 25 to 30 million years ago, Arizona and the West experienced crustal stretching which began to tear things apart. It was proposed, about 10 years ago, that the Tucson Mountain volcano and caldera formed over where the Santa Catalina Mountains now stand on the east side of Tucson. It was posited that crustal stretching slid the caldera to its present location. (You can see an explanation and cross-sections of that story in a 2009 article from my Wryheat blog.) That was such a neat story that the Arizona-Sonora Desert Museum constructed a mechanical model which Docents (including me) used to interpret the story. That particular “kit” has been retired because subsequent evidence shows that the story is probably in error. The main evidence against the sliding story is that the chemistry of the volcanics in the Tucson Mountains is incompatible with the proposed generating pluton in the Santa Catalina Mountains. There are also some structural inconsistencies.

TM section

One other thing: there was a Tucson Mountain dinosaur. Dinosaur bones were found within one of the blocks of megabreccia about 1800 feet NNW of Gates Pass. This dinosaur is classified as a large Hadrosaur (duck-billed dinosaur). This dinosaur lived in Tucson some time between 72 and 83 million years ago.


Kring, D.A., 2002, Desert Heat – Volcanic Fire, The Geologic History of the Tucson Mountains and Southern Arizona, Arizona Geological Society Digest 21

Lipman, Peter, 1993, Geologic map of the Tucson Mountains Caldera, southern Arizona, U.S.G.S. IMAP 2205. (link)

Lipman Peter W., 1994: Tucson Mountains caldera; a Cretaceous ash-flow caldera in southern Arizona. U S (link)

Marshall, L. and Stokes, P., 2012, the Tucson Mountains Caldera: Using Gravity and Magnetic Anomalies to Test Trapdoor Subsidence and Locate Subsurface Plutonic Bodies. (link)

Spencer, G.L. et al., 2005, The late Cretaceous Tucson Mountains dinosaur, New Mexico Museum of Natural History and Science Bulletin, 29 (link)

See also:

Arizona Geologic History: Chapter 1, Precambrian Time When Arizona was at the South Pole

Arizona Geological History: Chapter 2, Cambrian and Ordovician Time

Arizona Geological History: Chapter 3: Devonian to Permian Time

Arizona Geological History Chapter 4: Triassic Period

Arizona Geological History Chapter 5: Jurassic Time

Arizona Geological History Chapter 6, The Cretaceous Period

Arizona Geological History Chapter 7: The Cenozoic Era

Old mines of the Tucson Mountains