Ninety-five miles southeast of Tucson, Arizona, and partway up the steep slope of Montezuma Peak, lies Coronado Cave (Figure 1). The cave is located in Coronado National Memorial, which pays tribute to its namesake Spanish conquistador who entered what is now the United States via the nearby San Pedro Valley in the 1540s. Evidence exists in the park for historic and prehistoric human presence, but none has been found in the cave to date (Graham, 2011). To access the cave, one must hike half a mile up a steep trail before climbing the limestone steps to the blocky entrance. The cave is mostly straight, heading west to east, with some undulations along the way. A short scramble down large blocks leads into the first and largest room of Coronado Cave. The room is up to 10 meters high with a diameter of about 30 meters. Large to small blocks litter the floor of this room (Figure 2). As you continue along, the passage tapers down, with a small, deeply incised and sinuous side channel on the right. Just past this feature, a hole marks the entrance to a crawl passage that can be followed on hands and knees for about 25 meters before it dead ends in a plug of sediment.
Back in the main passage, the other side of the cave has large hanging fins of dissolutional and erosional features. Continuing down the main passage, a section of “narrows” in the middle of the cave – a one meter wide section between steeply dipping bedding planes – leads to the other large room. Unlike the first room, there are few large blocks on its floor. The room gives the appearance of being a large underground sports arena, with a smooth oval-shaped floor and ceiling. This room also contains some speleothems; two large stalagmites (Figure 3) mark the entrance and two large columns (Figure 4) mark the east end of the cave. A steep climb to a gated back entrance and some short crawl passages are the last destinations in Coronado Cave.
Geology of Coronado Cave
The geologic story of Coronado Cave is a multifaceted one, spanning three main episodes: Paleozoic formation of the limestone host rock, Jurassic intrusion of granite displacing and altering the host rock, and Quaternary speleogenesis and cave passage evolution.
Coronado Cave is a limestone solution cave formed in the Paleozoic Naco Group limestone of southeastern Arizona. The cave’s host rock formed approximately 250 to 300 million years ago. Although the Jurassic (200 to 145 million years ago) Montezuma Caldera, in which Coronado National Memorial completely resides, produced collapse megabreccia with Paleozoic blocks up to 1 km in length, the block housing Coronado Cave is simply a roof pendant resting on top of the intruded Jurassic Huachuca Granite. The cave formed during the Quaternary period (2.6 million years ago to the present) through the process of acidic groundwater dissolution of the carbonate rock mass. Three factors contributed to the sizable rooms of Coronado Cave: carbonic acid from atmospheric and soil carbon dioxide; sulfuric acid from the iron sulfides in the contact skarn along the margins of the intruded granite (Figure 5); and an estimated prehistoric groundwater flow rate of 50,000 gallons per minute (Graham, 2011; Hon and Lipman 1994; Hon et al. 2007).