The Carlsbad Caverns – November 24, 2020
A majority of the world’s limestone caves are created through carbonic acid in surface water flowing down through cracks in limestone rock, eroding and enlarging passageways. Carbonic acid is formed by rain and snowmelt combining with carbon dioxide in the air and soil as it seeps downward. How Carlsbad Caverns, a World Heritage Site and National Park in the Chihuahuan Desert of southern New Mexico, got its start is a bit different given the geologic setting.
The caverns reside within the “Permian Basin” which spans western Texas and southeastern New Mexico. The now desert landscape used to be an inland sea 250 million years ago, home to a large array of lifeforms making up a large reef. When the sea dried up, the reef was buried by layer upon layer of sediment for millions of years before tectonic uplift and erosion created the Guadalupe Mountains and revealed these limestone remnants of a lost world. The Carlsbad Caverns are found within the tomb of millions of marine organisms (more on these fossils next week).
The Permian Basin is well-known for another fact: it is the largest petroleum-producing basin in the US (with a cumulative 28.9 billion barrels of oil and 75 trillion cubic feet of gas). Millions of years ago, hydrogen sulfide began migrating upward from these petroleum reservoirs deep under the limestone. When the hydrogen sulfide rich water met groundwater, it combined with oxygen in the water to form sulfuric acid: H2S (hydrogen sulfide) + 2O2 (oxygen) = H2SO4 (sulfuric acid). This led to aggressive dissolving of the overlying limestone and creation of very large chambers. The famous “Big Room” of Carlsbad Caverns, in fact, is the largest chamber in North America.
Geologists also noted the presence of a soft white mineral called gypsum coating the walls of many parts of the caverns. This forms as a product of the reaction between sulfuric acid and limestone, solidifying the hypothesis that sulfuric acid played a big role in creation of these spectacular caverns. Here’s the chemical reaction: H2SO4 (sulfuric acid) + CaCO3 (limestone) + 2H2O (water) = (CaSO4 · 2H2O) (gypsum) + CO2 (carbon dioxide) + H2O (water).
Over time, the “acid bath” of sulfuric acid drained and a natural entrance to Carlsbad Caverns was formed through collapse of overlying ground, which led to an infiltration of air and water. As mentioned above, rainwater and snowmelt pick up carbon dioxide from the air and soil to form a mild carbonic acid, which can then dissolve any limestone rock encountered as it percolates down through the ground. The mineral calcite (CaCO3) gets dissolved from the limestone and the water “holds” the dissolved elements. When the element-laden water eventually contacts air in the cave, it releases carbon dioxide gas, like when a bottle of soda is opened. As the carbon dioxide gets released, calcite gets precipitated (redeposited) on cave walls, ceilings and floors. As the redeposited minerals build up after countless water drops, a stalactite is formed. If the water that drops to the floor of the cave still has some dissolved calcite in it, it can deposit more dissolved calcite there, forming a stalagmite.
Other cavern formations include draperies, or sheet-like structures formed by water dripping down a sloped ceiling, and “popcorn,” clusters of small bulbous protrusions, likely formed by a convection-like system of air movement in the caves. As warm humid air travels from floor to ceiling, it absorbs minerals from rocks it encounters. The air rises and loses heat to the rocks and its relative humidity decreases. As the air cools, it begins to sink. The moisture in the air evaporates, causing it to deposit the minerals on the walls and formations it meets. Popcorn is sometimes found only on one side of a formation, indicating the direction of air flow at the time of formation.
If you plan on visiting caves, please don’t touch formations. Natural oils on our skin prevent water from depositing minerals on a formation, thereby stopping new growth.