Day trails explore Saguaro NP's geology

Javelina Rocks

Saguaro National Park sits in the Sonoran Desert, which stretches 100,000 square miles across northern Mexico and the southwestern United States. It is centered on the Gulf of California, ensuring the desert enjoys subtropical warmth in winter. Rainfall is rare, with an average of 3.5 inches falling annually.

Most of the national park consists of basins and ranges. In the park’s western district, most of the terrain is flat dessert except for the southcentral area where the Tucson Mountains rise. In contrast, the Rincon Mountains cover most of the park’s eastern section with only the western and southwestern edges flat desert.

For most of Earth’s history, the area now making up Saguaro National Park was underwater, a great contrast to its aridity today. Around 1.7 billion years ago, it likely was near the boundary of two tectonic plates, where rocks heated into magma then cooled and solidified into Pinal Schist. The park’s oldest rocks, which can be seen off of Cactus Forest Loop Drive in the park’s eastern Rincon Mountain District, is this schist. Nearby on the Tanque Verde Ridge are 1.4-billion-year-old altered granites. The ridge can be hiked on the aptly named Tanque Verde Ridge Trail. 

Around 600 million years ago, shallow seas covered the area. As rivers carried sediment into the sea and ancient shelled sea life died and floated to the sea floor, sandstone, limestone, and shale formed.

About 80 million to 30 million years ago, western North America underwent an era of mountain building. Volcanoes formed the Tucson Mountains about 70 million years ago, during the Age of Dinosaurs. When one volcano collapsed, it formed a caldera 12 miles wide; debris and lava since has filled it. Granite from the magma chamber that fed these volcanoes can be seen today at the Sus Picnic Area.

For the past 30 million years, the mountains and basin that Tucson sits in have been undergoing great change. The Earth’s crust from northern Mexico to Oregon is thin and so pulling apart along fault lines. As the crust stretches, the areas along fault lines drop in elevation, pushing up the harder, erosion-resistant rock next to it. The Tucson and Rincon Mountain in the park, we well as the Santa Catalina Mountains north of Tucson, all rose to their current height in this way.

This also has exposed the crystallized granite, known as Santa Catalina gneiss, in the Rincon Mountain district, most notably at Javelina Rocks. The same type of rock sits below the Tucson Mountain but has yet to reach the surface.

Other great geology-oriented trails to hike at Saguaro include:
• Ernie's Falls Trail (waterfall)
• King Canyon Trail (heads to Wasson Peak's summit) 
• Turkey Creek Trail (heads to Mica Mountain's summit)