Carlsbad Caverns National Park: Fantastic Realm

One of the most remarkable geological features of the Chihuahuan Desert actually lies far beneath the surface, the Carlsbad Caverns, a vast limestone cave decorated with a fantasy of natural stone shapes. The cavern is the most spectacular and accessible of many such caverns that have formed in the once-sea-bottom layers of limestone that underlie the region.

The story starts 250 million years ago with the drift to the bottom of a warm shallow sea of the calcium-rich remains of often-microscopic sea creatures. As the continent shifted and the sea bottom subsided, the skeletons of these creatures formed layers of mud thousands of feet thick. As the subsidence continued and erosion piled deep layers of calcium-rich mud and sediment on top, the heat and pressure eventually fused the mud layers into limestone.

The restless Earth shifted once again, and the now-fused limestone layers rose again toward the surface, cracked and fractured by the pressure of that fall and rise. Near the surface, rainwater made slightly acidic by its passage through the soil seeped into the cracks and fractures of the still-buried layers of limestone. Eventually, pressures from below pushed the limestone layers up toward the water table, the zone at which the rocks are saturated by water percolating down from above. Here, the acidic water seeping down from above ran through the cracks and gradually began to dissolve the calcium carbonate in the limestone along those fractures. Other chemical reactions may have hastened this process, including the creation of sulfuric acid from the mineral pyrite found in the lagoon deposits from which the limestone was formed. This acidic groundwater eventually chewed away at the rock to create the caverns. Some geologists think that perhaps the combination of limestone and sulfuric acid formed gypsum, which eventually decomposed in the sulfurenriched air of the cavern, causing the cavern to essentially hollow.

Eventually, continued uplift and perhaps climatic shifts caused the formation of the bizarre rock formations for which the caverns are famous. That process started when the water table fell below the level of the cavern, leaving it hollow. At that point, heated, pressurized mineralrich water seeping along fractures from the surface would drip by drip enter the relatively cool, unpressurized air of the cavern. The sudden change in temperature and pressure caused the minerals held in solution in the water to precipitate from liquid to solid form. This process has created a wild variety of rock formations in the cave, including almost transparent curtains of stone, drip-castle spires, iciclelike stalactites hanging from the ceiling, and spearlike stalagmites bristling up from the floor. Although the cave itself may have started forming some 12 million years ago, most of the breathtaking, colorful formations probably started forming more like 10,000 to 20,000 years ago.

The limestone layer in which the cavern formed was actually a massive reef some 250 million years ago, similar to Australia's Great Barrier Reef. At some point, the increasing saltiness of the shallow sea in which it grew apparently killed the reef-building animals. After the reef stopped growing, it was buried. The burial caused cracks to form, which became the fractures along which the cave would eventually develop.

The caves took shape when this ancient buried reef was uplifted some 12 million years ago beneath the Guadalupe Mountains. The caverns now have three major levels, some close to the surface and some about 3,500 feet (1,067 m) below the surface. Evidence suggests that other, higher levels of the caverns were eroded away as the Guadalupe Mountains rose. The cave's temperature remains at a constant 56°F (13°C). The caverns extend through an area 4,500 feet (1,500 m) long, 2,700 feet (900 m) wide, and 900 feet (300 m) from top to bottom.