By far the most noticeable natural features in the park are the rhyolite rock pinnacles for which the monument was created to protect. Rising sometimes hundreds of feet into the air, many of these pinnacles are balancing on a small base, seemingly ready to topple over at any time. The Civilian Conservation Corps (CCC), during their occupation here in the 1930s, named many of the rock formations that can be seen today.
Other geologic features of the park include shallow caves, faults, mountain formations, ancient lava flows, and a giant volcanic caldera. The Turkey Creek Caldera is located just to the south of the park. A cataclysmic volcanic eruption, roughly 27 million years ago, spewed ash and molten debris at super-sonic speeds and formed the approximately 12 mile wide caldera. Ash and debris settled and compacted, forming a thick layer of rock called rhyolite tuff. This rock layer has fissured and eroded over time, forming the spectacular rock pillars of Chiricahua National Monument. The Chiricahua Mountains are one of many sky islands in southern Arizona, and are part of the larger Basin and Range Province.