The Geology of Organ Pipe Cactus National Monument: Sonoran Desert Volcanoes and Basins

Organ Pipe Cactus National Monument lies on the U.S.–Mexico border in Pima County, Arizona, about 170 km (106 mi) southwest of Phoenix and 64 km (40 mi) south of Ajo. Proclaimed on April 13, 1937, it covers about 133,825 hectares (330,689 acres) of Sonoran Desert — mountains, bajadas, valleys and washes whose plant and animal communities include its namesake cactus.

Life here has adapted to extreme heat, intense sunlight and little rain. The monument holds 26 species of cactus, many rare plants, and endangered species such as the Quitobaquito pupfish, Sonoran pronghorn and lesser long-nosed bat. Its biology has been studied continuously since the early 1940s, and in 1976 UNESCO named it a Biosphere Reserve for resources representative of the natural Sonoran Desert ecosystem.

Organ pipe cactus. National Park Service.

Three chapters of geologic history

The monument's varied rocks and landforms record a complex past. Since the start of the Mesozoic, the region's history falls into three main episodes:

1. Jurassic and Cretaceous igneous intrusions, volcanism and deformation.
2. Late Cretaceous and early Cenozoic intrusions, with widespread metamorphism and compression.
3. Neogene (Miocene–Quaternary) volcanism and crustal stretching that lowered the landscape.

Widespread bedrock units include the Jurassic rocks of La Abra and the Miocene Childs Latite, Batamote Andesite complex and rhyolite of Montezuma's Head. But most of the monument's surface is Cenozoic alluvium and colluvium, forming the broad alluvial plains typical of the Basin and Range province.

The Ajo Range volcanoes

The monument includes part of the ancient Pinacate volcanic field, which covers more than 1,900 square miles (4,920 km²). Its rocks are on display along the steep front of the northern Ajo Range: dark, massive rhyolite flows separated by lighter layers of tuff, with veins of opal and silica in the rhyolite.

• The rhyolite came from short, thick, bulbous lava flows in quiet, non-explosive eruptions. Rich in silica and therefore viscous, the lava traveled only a few miles from its vents.
• The tuff is volcanic ash and small rock fragments thrown out in shorter, explosive eruptions.

The stack of flows is more than 2,000 feet (610 m) thick — a brief but intense burst of volcanic activity between 16 and 18 million years ago. The volcanic field last erupted in the 1600s, southwest of the monument, and a present-day eruption would drop ash or cinders on it.

Alamo Canyon. National Park Service.

Maps and reports

The NPS Geologic Resources Inventory has produced a geologic map and report for the monument, available through the NPS DataStore; A Guide to the Geology of Organ Pipe Cactus National Monument and Pinacate Biosphere Reserve describes its volcanic rocks in more detail.

Sources

• National Park Service: "NPS Geodiversity Atlas—Organ Pipe Cactus National Monument, Arizona," citing National Park Service (2016) and Bezy and others (2000). The atlas gives the distance to the most recent eruptions as "400 miles (250 km)," which do not agree, so no distance is given here; a photo caption's "Oregon Pipe Catus" is corrected.