Tucson Mountains Chaos

“Tucson Mountain Chaos is a formal geologic name, describing one of the more confusing, complex, and controversial areas in southern Arizona.” So says the newsletter of the Arizona Geological Society.

If you drive over Gates Pass and look closely at the road cuts, you will see a jumble of various-colored rocks. Within the beds of volcanic ash are big chunks of other volcanics, limestones, granites, and schists. The mountain range appears to be composed of a mega-breccia.

The origin of the Tucson Mountains is still subject to geologic debate. The following is what I think is the most probable chain of events. Like many stories in the very complex structural geology of the Western U.S., even the probable may seem fantastic.

[NOTE:  New evidence obtained since this article was written shows that the caldera did not form over the Catalina Mountains as postulated below. See:


for updated information.]


Our story begins during the Laramide Orogeny, when the Rocky Mountains were being built about 70 million years ago. The North American continent was speeding westward at 2 inches a year and it was crashing into oceanic crust under the Pacific Ocean. The heavier oceanic crustal rocks dove under (were subducted beneath) the lighter continental crust. This caused compression, mountain building, and volcanism.

As subduction of the ocean crust continued, it reached a depth that was hot enough to melt it. Great blobs of magma rose like balloons through the continental crust. Some of these blobs became the copper deposits we have in Arizona, others reached the surface and became volcanoes.

One such volcano was formed where the Catalina Mountains are now, east of Tucson. It was a large volcano that erupted in violent explosions which eventually caused collapse of the volcanic edifice to form a caldera about 10 miles across.

Portions of the wall rocks fell into the caldera. This probably accounts for the chaotic mixture of rocks in the Tucson Mountains. But that’s just half of the story.

If the volcano was east of where Tucson is now, how did the rocks wind up to the west of the city?

The North American continent was still moving westward. Sometime between 40- and 20 million years ago it overrode a spreading center called the East Pacific Rise. This area was similar to the spreading center of the Mid-Atlantic ridge that gradually separated Africa from South American, and Europe from North America. Today, this western spreading center runs up the Gulf of California and separates Baja from mainland Mexico. It is also the driver of the San Andreas fault in California.

The compression that built the Rocky Mountains was turned into extension. The western part of the American continent began to be torn apart.

By about 30 million years ago, crustal stretching and heating from below, arched-up the area beneath the “Tucson Mountains” volcano. It may have looked something like Figure A.


About 25 million years ago, stretching caused low-angle faulting to detach much of the volcanic edifice from underlying rocks. The volcano and its caldera began to slide to the west. (Figure B).


By the way, detachment faults crop out along the western base of the Catalina Mountains, amid all that nice expensive foothills property.  Sometime between 12- and 6 million years ago, the on-going crustal stretching reached a limit and things started to break. High-angle faults formed. This produced the Basin and Range topography we have now. (Figure C.)


As the valleys dropped, erosion filled them with debris from the mountains. The glacial epochs added water. (Figure D)



The Tucson Mountains represent just one example of the consequences of crustal stretching. Picacho Peak has a similar history. Near Green Valley, the old Twin Buttes mine, the Mission mine, and the Eisenhower pit are a few pieces of what once was one deposit that got sliced up and fanned out like a deck of cards.

Many geologists disagree that the Tucson Mountain volcano formed over the Catalina Mountains, rather, they think the volcano is where the rocks now sit. The main evidence they cite is that the chemistry of the volcanics in the Tucson Mountains is incompatible with the proposed generating pluton in the Catalina Mountains. They also cite structural inconsistencies. Notice in the last cross-section the Tucson Mountain volcanics dip to the east (left) as would be consistent with the story above. However, other rocks in the Tucson Mountains dip to the west and the volcanics of Tumamoc Hill are horizontal. That may negate the detachment theory. If this latter hypothesis is correct, then the volcanics of the Tucson Mountains represent the west half of a caldera. The rest of it is buried in the Tucson Valley to the east.

In either case, it remains the Tucson Mountain Chaos.


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

Scarborough, Robert, The Geologic Origin of the Sonoran Desert, Arizona-Sonora Desert Museum, http://www.desertmuseum.org/books/nhsd_geologic_origin.php

The graphics used in this article came from this paper.



  1. Mother Nature has worked long and hard to create the beautiful landscapes in and around Tucson. The citizens of Pima and Santa Cruz counties should consider long and hard the relatively short lived value of allowing a corporation like ASARCO or others to permanently scar the face of Mother Nature with open pit mining. What these corporations destroy will take how many *MILLIONS* of years to fix(if ever).

  2. Well Ado, that you think Ma Nature had to work long and hard is beyond me.
    After all, she put all that copper, moly and other valuable minerals in the ground so we could mine it, build you a car, a bike, a jetliner, a house, a highway, an airport, a gas station, a fuel pipeline, a water well, a water pipeline, a school in which to work, a capitol building in which your politicians can sit back and await ‘gifts’ from special interest groups (NEA), a big ol’ Stihl chain saw to cut down good ol’ Ma nature’s majestic Spruce trees so we could build you a house, mine some limestone so we could spray some stucco on your new wooden house, mine some clay so we could fire you up some nice ceramic tiles for your loo, your kitchen, your fireplace, dig up some more iron ore and build a great, huge floating oil drilling platform so you can have the petro-chemicals for your vitamin pills, your blood pressure pills, your BC pills, your estrogen patches, your breast cancer fighting drugs, your paints and canvasses for your artistic endeavors, the dashboard on your Subaru Wagon, the cute little plastic clogs you wear to work, the nice petro-chemical fibers used to make all that oh-so-cool outdoor wear from Patagonia, Columbia, North Face and others.
    You see, Ado, as Donne said so long ago, in his many ways…”No man is an island…” and that includes you and your damned backyard.
    If not a mine in Santa Cruz or Pima Counties, then why don’t you, personally, take a world map and decide just exactly which small, third world community will host the mines, oil wells and factories needed just to produce all the ‘stuff’ you will consume in your lifetime and then pass on to your descendants?
    Please be certain to include with your decision on where to place the mines, oil wells and factories of yours, just how much misery, sickness and death will be visited upon those places.
    Most likely, the residents ‘there’ will not have the benefits of the benevolent mine owners looking after their safety as will you, with a mine in your backyard.
    As far as Ma Nature taking “*MILLIONS* of years to weather away or “fix” any scars left from mining, the reality is a couple of hundred years if the mines did zero reclamation and probably a generation after the mine is reclaimed.
    If you don’t like mining and being around mines, why did you buy property in an old mining town? How do you think that property became mining property in the first place?
    It was made into a mine through the collective will of the People of the United States of America through their elected representatives at state, local and federal levels?
    It is really too bad our public schools are now so poorly run, our children so poorly educated, that I have to explain all of this to you.
    Had you been taught science and math and government by competent teachers in a well run and administered school system, you would already know all I have just told you.
    Do we also need to go into Global Warming/Climate Change as relates to Political Science vs. The Scientific Method? Or are you so ignorant of the world around you that you gobble up that stuff, hook, line and sinker as well?
    Follow the money on climate change, Ado, it leads straight to YOUR wallet!

    1. So open pit mining can create an eyesore that will be around for years and that doesn’t bother you?
      Oh that’s right, you are a *NEVADA* DEM who doesn’t give a rip about how Arizona looks.

    2. Oh, for Zeus’ sake…not you again.  I think you’ve used up your lifetime quota of words, ND.

      1. ND, very nicely put, to bad your a democrat, Why is that? I’m a Tusconian
        Looks quite beautiful all around Tucson so It seems to be a non issue.

  3. As we all know; Humans have only been around a short time compared to the time line of the earth. Personally I beleive the human footprint will be short for we are on a road to extinguish are existance. Thats okay; the earth and mother nature will repare herself after we are gone.

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