Geology of Saguaro National Park

Rocks on the Move

Scientists estimate that saguaro cactus have been growing in this area for anywhere from 4,000 to 8,000 years, but the rocks they grow on are more than a bit older!

Pinal Schist, the oldest rock in southern Arizona, was deposited at the bottom of a deep ocean 1.6 billion years ago. It crops up along the loop drive at the Rincon Mountain District (east) of the park. At the lime kilns site on the Cactus Forest Trail, the pioneers used Horquilla Limestone (over 300 million years old) to make lime for mortar. Nearby, Lime Falls is named for this rock where it outcrops above the Catalina Fault.

The Recreation Red Beds along Kinney Road at the Tucson Mountain District (west) are at least 159 million years old and have revealed the footprints of a carnivorous theropod dinosaur. Later, the Amole Arkose formed in a lake at the base of a mountain range more than 100 million years ago. That's when the "Tucson Mountain dinosaur" (a plant eating duck-bill) wandered here, and the dynamic earth was beginning to face major changes. Starting a few million years before the death of the dinosaurs, things got really interesting here, and the rocks you see today were soon on the move.

Take a virtual tour of the geology in the Rincon Mountains

 

Eruption and Collapse Event

70 - 75 million years ago

Geologists know that our seemingly solid landscape is the result of slow movement of large plates of earth's crust jostling each other atop flowing molten material underneath. About 75 million years ago, a Pacific ocean floor plate rammed into the larger and lighter North American plate. Heavier ocean crust was driven downward in a process called subduction. Friction of the two plates rubbing against each other melted the descending oceanic rocks charged with sea water. Bubbling to the surface, the molten rock and super-heated water blasted out in a series of enormous volcanic eruptions, which, within a few million years, created at least seven major volcanoes in what is now southern Arizona. Each of these explosive features would be marked by a large collapse feature called a caldera: a huge bowl rimmed by volcanic rock and ash.

Arizona then looked nothing like it does today with a landscape of steaming and smoking volcanic features. Some areas were covered with lava flows, and others were covered with deep layers of compacted ashfalls. The caldera would have dominated the horizon northeast of where Tucson is today. That collapsed depression was an elongated oval with walls up to 1,500 feet high on one end and about 300 feet high on the other. Some molten material rose up and cooled underground into a granite mass, which can be seen on Amole and Apache Peaks at the Tucson Mountain District (west). While still erupting, the high walls of the caldera further collapsed inward filling the 'bowl' with a jumble of broken and mixed up volcanic rocks. Geologists once called this the "Tucson Mountain Chaos." Now designated the Cat Mountain Tuff, this is the rugged bedrock found from Gates Pass to Wasson Peak

 

The Wide Stretch Event

20 - 30 million years ago

Another round of earth movement began 20 - 30 million years agoo when forces in Arizona stretched and thinned the earth's crust. Great cracks (faults) formed, and huge slabs of crust moved in relation to each other. Tilted, horizontal breaks (detachment faults) formed. In this area, slices of upper rocks were pulled south by west; underlying granites moved up into the spaces left behind. Enormous pressures along the fault were caused by the weight of the upper rocks and the earthquakes associated with fault movement. These pressures transformed the granite into new rocks, including cataclasite and mylonitic gneiss.

Domed mountain masses of altered granite, surrounded by unchanged sedimentary or volcanic rocks, are know as metamorphic core complexes. The Rincon Mountains in the east district of the park and the Catalina Mountains to the north are world-class examples of this kind of mountain building event. The rocks of the old volcano were on the top side of the detachment fault and were pulled 15 to 20 miles southwest by the stretching crust, coming to rest far from their original position. Granites once buried deep underground far to the west rose up to the east to become the Rincon and Catalina Mountains.

 

Geologically speaking, the Tucson Mountains were once in the east and the Rincon Mountains were in the west.

NPS photo

 

What to See Today

Rincon Mountain District

• The gentle hills viewed from the Rincon Mountain Visitor Center and on the first 2 miles of the Cactus Forest Loop Drive are composed of 1.5 billion-year-old Pinal Schist.
• The cactus forest sprawls across the nearby flat gravel bajada at the base of the Rincon Mountains, representing the most recent erosive stage of the geological story.
• An unmarked, paved pull-out at mile 5.5 of the loop drive shows shattered granite (now chlorite breccia) bedrock and a view across the trace of the Catalina detachment fault.
• At Javelina Rocks overlook, see a dramatic example of the stretching of granite into striped mylonitic gneiss.
• The Rincon Mountains metamorphic core complex rises impressively from Javelina picnic area.

Learn more about the geology of the Rincon Mountains

 

Tucson Mountain District

• Rocks from the dinosaur times are found in the red beds of the hills around the Red Hills Visitor Center and along the first mile of the King Canyon trail where you can find arkose deposits from the bottom of an ancient lake.
• Find igneous rocks that cooled underground along the Bajada Loop Scenic Drive. You can see granite in the narrow canyon at the Sus Picnic Area dam and granodiorite covered with dark desert varnish (and petroglyphs) at Signal Hill petroglyph site.
• Valley View trail overlooks the bajada, which is a modern erosion feature covered with thousands of saguaros in the fault-dropped Avra Valley.
• The tumbled and jumbled rocks of the volcanic caldera are best viewed in cliffs rising above Tucson Mountain Park and up-close at Gates Pass overlook south of Saguaro National Park.

Learn more about the geology of the Tucson Mountains