Carlsbad Cave Popcorn

Carlsbad Cave Popcorn

The problem with Carlsbad Caverns today is the same problem that plagued it from the very beginning- it is very isolated and is not even on the way to many other places of major interest (except El Paso, of course). For the first 20 years or so, the cavern was primarily a source of bat guano fertilizer for the California citrus orchards. Slowly, and with the persistance of a few key individuals, word of this wondrous underground cathedral spread.

Today, Carlsbad Caverns is visited by approximately 500,000 people per year. The cavern is in remarkably good shape considering the large number of people who walk the several miles of underground trails 364 days per year.  Curiously, one of the big pollution concerns is lint from the clothing of shuffling visitors which settles on the formations.

The above photo is a snapshot of a common evaporite formation referred to as cave popcorn. The box in the photo encloses an area about the size of the palm of your hand and if you look closely, you can see water droplets clinging to the small mineral protuberances.  The colors in the photo are a good representation of most of the cavern.

Entrance to Carlsbad Caverns

Entrance to Carlsbad Caverns

The decorated caverns are the result of several kinds of chemical processes. The internal spaces were dissolved out of the regional limestone formation. This formation is thought to be the remnant of an ancient reef. It is believed that aqueous hydrogen sulfide migrated up from the anaerobic permian formations below and was subsequently (air) oxidized to more corrosive species.

The park people point out that H2S was oxidized to sulfuric acid which is responsible for the chemical digestion of the limestone. Sounds reasonable to me, though the chemistry of sulfur oxidation is full of many kinds of intermediary species on the way from sulfide to sulfate. The presence of gypsum (calcium sulfate) inside the cavern supports the claim that sulfuric acid was the corrosive agent. What was not mentioned was whether or not sulfate is found in the surrounding formations. [Note: a commenter made a good point about the bio-oxidation of sulfide]

Stalactites festooned with cave popcorn, Kings Palace caves, Carlsbad Caverns

Stalactites festooned with cave popcorn, Kings Palace caves, Carlsbad Caverns

The decoration of the interior spaces left by digestion of the limestone happened by the action of seepage of rainwater and carbonic acid though the upper layers of sedimentary rock. The water dissolves many mineral components including calcium. The calcium carbonate rich liquors seeping in from the roof of the caverns wetted the surfaces below and deposited calcite and other mineral species by way of intermittant accretion.

If you examine the few smaller and broken mineralizations along the trail, you can clearly see that the accretion results in substantially crystalline material. So, while the formations are not large calcite or aragonite crystals, the small scale structure is crystalline.

There is much to see in the world if you just bother to look. Along I-25 in southern Colorado is a modest looking feature. It is called Huerfano Butte and sits along the highway near Walsenburg. This igneous intrusion is more resilient to weathering than the surrounding sedimentary formations.  Radiating from the nearby Spanish Peaks are an array of dikes indicating the presence of past magma intrusion.

Huerfano Butte- a weathered igneous intrusion.
Huerfano Butte- a weathered igneous intrusion.

An synthetic chemist is in a great position to absorb some geochemistry. Once one is keyed into a bit of geology, the mechanisms of mineral formation begin to become apparent, with a little study of course. For myself, this is a great motivation to kick around in the weeds and explore the world. Gotta watch out for rattle snakes, though.

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