Your right foot smeared on a little nubbin of white quartz, your left high stepped on an exfoliating flake. Looking up, a crack slashes across the face of the boulder.  Jamming your hands in, you feel the toothy crystals find purchase in the soft flesh of the back of your hand. Feeling flustered and tired, the sloping lip of the boulder seems out of reach, but you muster the reserve to slap your hands up there and pull yourself over the lip. In view, you see rising mountains in multiple directions with flat, wide basins stretching across the space between. You feel good having finished a boulder problem, and you think about the moves and reflect on the effort you put into climbing the boulder. But what about the boulder itself, and the features that allowed you to ascend the boulder? In Tucson we are surrounded by geology - with the Catalina Mountains looming above the skyline of the city, and the Tucson Mountains silhouetted against the radiant Technicolor sky of our sunsets. The geology of Southern Arizona is as much a part of our climbing experience as the equipment and the techniques we use to climb, but we often overlook this interesting piece of our sport. With this in mind, I hope to provide a brief but useful look at the geologic history of the Catalina and Tucson Mountains.

Deciphering the complex series of events that created our landscape is a tricky task, using clues from the past to determine how our mountains came to be, and ended up with the unique features that afford us so many climbing options. Here in Southern Arizona it is an especially difficult task, because there doesn’t seem to be a solid consensus among geologists about how our mountains came to be. This is a roundabout way to escape blame for anything I say being completely wrong - but I will do my best to summarize and explain some of the most common and simple explanations about how our landscape came to be. Familiarity with some basic geologic terms and processes is assumed, but a good intro can be found here and here.

Tucson and the surrounding mountains are part of the Basin and Range region of the western US. This type of topography is formed from the uplifting of mountains along fault lines, which causes downfaulting of basins as the mountain ranges rise up. As you survey the landscape of Southern Arizona from high on a summit, you will notice sharply rising mountain ranges in many directions, with wide expanses of relatively flat basins between the various ranges. Tucson sits in such a basin, roughly surrounded on four sides by the Catalina, Rincon, Tucson and Santa Rita Mountains. When first formed, the ranges in our area were as much as 10,000 feet higher in elevation than the basins below - but millions of years of erosion of the mountains and deposition of sediments in the basins below made the elevation change much less drastic.

In the Catalinas - Mt. Lemmon areas, Sabino Canyon, Ventana Canyon, etc - the rocks we climb are granite and a metamorphic granite called gneiss (pronounced “nice“). Granite is typically formed from magma cooling  just beneath the surface of the earth, and exposed when the soil and rocks above erode away. In the Tucson Mountans, the boulders are mostly a rhyolite tuff, which is a type of rock formed from cooled volcanic ash following a volcanic explosion. The granite and gneiss are the result of three major granitic intrusions that occurred beginning almost 1.5 billion years ago. The volcanic rock of the Tucson Mountains is the result of a period of volcanic activity dating back to about 30 million years ago.

CATALINA MOUNTAINS

The first igneous intrusion occurred about 1.45 billion years ago, with the formation of the Oracle granite formation. The Oracle granite is one of the oldest exposed rocks found in the Southwest, with some still exposed near the top of Mt. Lemmon and more exposed on the backside of the Catalina’s, especially around the town of Oracle. The Oracle intrusion in SoAZ was part of a series of igneous intrusions all across North America that also helped form other notable climbing areas such as Rocky Mountain National Park and Vedauwoo, WY. 

Over the next billion years or so there was extensive erosion - and much of the evidence of what happened has been erased - but there was nothing major occurring as far as mountain building or rock formation. For some of this time, part of our region was even  covered by a shallow inland sea, as evidenced by the sedimentary deposits south of Saguaro National Park West, which include petrified wood and dinosaur bones.

The next major intrusion occurred about 50 million years ago, when the Wilderness granite intruded into the Oracle granite. The final intrusion took place about 26 million years ago, with the formation of the Catalina monzogranite - a form of granite less rich in quartz. While all the granite of the Catalinas originated in a similar way, there is a tremendous diversity in the way these rocks look and feel today. This is a result of both modern processes - such as wind and water erosion - as well much older processes that shaped the rocks. As granite cools and forms beneath the Earth’s crust, differences in the mineral composition of the granite create slightly different rocks (for example the Catalina monzogranite having less quartz minerals, and the Wilderness granite having much more garnet). The rate of cooling tends to effect the size of the crystals within the rocks - the slower magma cools and becomes rock, the larger the crystals will be. Rock that cools relatively quickly will have a much smaller crystal size. 

A major factor that created many of the differences in our local rocks is the metamorphosing of granite into gneiss. Through various periods of heat, pressure and movement, much of our granite deformed into gneiss. As a general rule of thumb for the Catalinas, the further you move away from the summit of Mt. Lemmon, the more deformed and gneissic the rocks become. Near the summit of Mt. Lemmon, we have examples of relatively undeformed Wilderness granite - Rap Rock, The Fortress, Wilderness of Rocks - and Catalina monzogranite - Reef of Rocks. Moving away from the summit, you have gneissic granite around Windy Point, and even lower - in areas such as Sabino and Bear Canyon - you have severely deformed granite that forms a banded gneiss. 

These three granite formations help form what is known as the Rincon-Catalina metamorphic core complex. A metamorphic core complex is a geologic structure composed of igneous and metamorphic rocks that have been uplifted as a result of the stretching and thinning of the Earth’s crust. As the crust of the Earth moves along a low-angle fault line, the crust thins, and the underlying rocks are pushed to the surface and exposed. As this happens, surface rocks slide around themselves and on top of the newly exposed formation, grinding and deforming the various granites. These interactions between the different granite layers helped create some of the gneiss rock formations we climb on. The Rincon-Catalina core complex is the largest such complex in the Basin and Range region of the Southwest US.
  
TUCSON MOUNTAINS

The same tectonic processes that were causing the surface of the Earth to stretch and create the core complex were also helping to create the conditions that allowed the Tucson Mountans to form. About 30 million years ago there was a period of extensive volcanic activity and mountain building in the western US. This period is known as the Laramide progeny. During this time Tucson was a hot bed of volcanic activity, with enormous amounts of lava and ash clouds being emitted. Eventually, so much material was pumped out from below the surface of the Earth that the surface collapsed onto itself,, forming a huge depression in the Earth. This caldera (depression) was over 15 miles in diameter, and eventually began to fill in with the rhyolite and tuff formed from volcanic emissions. This accumulation of volcanic rocks, along with existing lava tubes and remnant sedimentary rocks, form the complex aggregate known as the Tucson Mountain Chaos. While complicated to decipher exactly how this all happened, the boulders scattered throughout the Tucson Mountains are relics of this period of volcanism and mountain building. 

CLIMB ON

Millions of years after the formation of our boulders and cliffs, they loom on the horizon, beckoning us to climb them. Over this time erosion and exfoliation have subtly altered the rocks, creating climbable features for our hands and feet, but the clues about the original creation of these rocks are all around. The sharp crimps on the Matterhorn Boulder are a reminder of the granite slowly cooling below the surface of the Earth. The heavily banded and streaked boulders of Tanque Verde and Hairpin are there because of the various rock formations sliding over each other as the core complex formed. Topping out the tall boulders of Gates Pass is a little like climbing a volcano. 

The story of these events is pretty amazing (although a little confusing!), and the result is certainly stunning. Mountains full of boulders and cliffs. Adventures that take place in a billion year old setting. Rocks created in turbulent and chaotic conditions, now providing us peaceful and serene moments of thoughtful movement over stone. 






REFERENCES AND LINKS TO MORE INFORMATION


"Flakes, Jugs & Splitter - A Rock Climbers Guide To Geology" by Sarah Garlick
"Roadside Geology Of Arizona" by Halka Chronic
"Geology Of The Tucson Mountains" from the National Park Service