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Geology Of Carlsbad Caverns National Park

Geology of Carlsbad Caverns National Park


Geologic History

The area where Carlsbad Caverns is located today was the coastline of an inland sea 250 million years ago. Due to the warm, shallow, tropical seas, the reef formed from mostly sponges and algae instead of coral, like many reefs today. Landward of the reef redbeds, evaporites, lagoonal mudstones, pisolitic grainstones, and back-reef grainstones were deposited during this time period. Today, the reef deposit makes up the Capitan Limestone formation, which is 750 feet thick, and most of the National Park is within this limestone formation. Eventually, by the end of the Permian, the sea dried up and the reef became buried by sediment for tens of millions of years. Fossils from this time period can be seen within Carlsbad Caverns, including ammonites, crinoids, snails, nautiloids, bivalves, brachiopods, and trilobites. 

Over the past 20 million years local faulting and stresses of the Earth’s crust uplifted the reef sediment by nearly 10,000 feet, creating the Guadalupe Mountains, and the sediment eroded, exposing the reef. Within the Guadalupe Mountains there are more than 300 caves, and 119 known caves within Carlsbad Caverns National Park. The Big Room in Carlsbad Caverns is the largest cave chamber in North America, with 8.2 acres of floor area. Additionally, Lechuguilla Cave is the deepest and third-longest cave in the United States.

The caves in Carlsbad Caverns are unusual, as they formed from sulfuric acid instead of carbonic acid. Most caves in the world are formed when water interacts with carbon dioxide and seeps downwards. The location of the Guadalupe Mountains played a key role in the creation of these caves. Carlsbad Caverns National Park is located near the border of New Mexico and Texas, and close to the Permian Basin, which hosts large oil fields. Hydrogen sulfide-rich water rose from the oil reservoirs as late as 12 million years ago and mixed with groundwater to form sulfuric acid. The limestone was dissolved at the water table, and therefore, Carlsbad Caverns was dissolved from below and not from above. Gypsum blocks, a byproduct of sulfuric acid dissolution, can be seen today on the floor of the Big Room of the Carlsbad Caverns.

Image: Natural Entrance to Carlsbad Caverns
An amphitheater at the natural entrance of Carlsbad Caverns.

Eventually, the roof of the cave collapsed, creating an above-ground entrance to the cave, and allowing air to circulate through the cave. While the cave did not form due to carbonic acid dissolution, after the roof collapsed, several of the structures within Carlsbad Caverns were formed by carbonic acid dissolution. Speleothems, including stalactites and stalagmites, are formed from dissolved limestone. The speleothems in Carlsbad Caverns are due to rain and snowmelt soaking through limestone, and the water absorbing gasses and minerals from the limestone. When this water evaporates and emits carbon dioxide, calcite remains and slowly builds up over time. Stalactites form on the ceiling, while water that falls to the floor deposits minerals, creating stalagmites. There are several other types of speleotherms found within Carlsbad Caverns National Park, including columns, soda straws, draperies, helictites, and popcorn.

The Guadalupe Mountains are situated within a desert, and therefore these caves are drier than typical cave systems. Therefore, few speleothems are growing today. Instead, most of the speleothems grew ~10,000 years ago during the last ice age, when the Guadalupe Mountains received more rainfall.

Other National Parks are also famous for their cave system, including Mammoth Cave National Park, which is one of the more famous karst topographies in the word.

This is a photo of Massive stalagmites.
Massive stalagmites in parts of the Big Room of Carlsbad Caverns.