August 25. Carlsbad Caverns

The Capitan Reef that we discussed yesterday is not all the prominent, high-standing Guadalupe Mountains. Parts of it are in the subsurface, and like any limestone, given the proper conditions of water and climate, limestone can be dissolved by water percolating and flowing through the rock. That’s the process that makes caves.

In addition to the limestones in the reef, as the Permian Basin became more and more restricted toward the end of the Permian Period, sea water evaporated and salts precipitated out. Halite, common table salt, was one common precipitate, as well as gypsum, calcium sulfate. The petroleum that formed from the source rocks in the forereef are also contained sulfur. When sulfur reacts with water, sulfuric acid forms. We’re not talking about huge hissing pools of acid – just enough to make the groundwater a bit on the acidic side. Enough to actively dissolve some of the limestone. This makes the caves in this area, in particular Carlsbad Caverns, different from most caves, which are dissolved by the weak carbonic acid that forms when rainwater reacts with carbon dioxide in the atmosphere.

The dissolution of limestone to produce Carlsbad Caverns took place a few million years ago – most estimates say 4 to 6 million years, but the process could have begun a few million years before then. That was a time when New Mexico and West Texas were much more humid and rainfall was more plentiful than today. When rainwater mixed with the sulfuric groundwater, it provided a great agent for dissolving the rock.

Carlsbad Caverns. Photo by Eric Guinther licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Eventually, probably within the past one million years or so, the country became more arid, and the water table began to fall, leaving the upper caves dry. Parts of the caves collapsed, allowing for surface water to get in – there’s still water around, just not as much, and the caves themselves are mostly empty of water today. That water, relatively modern surface and groundwater resulting from glacial climates, modified the cave by dissolving some limestone and redepositing it in the kinds of features we associate with caves today – stalagmites, stalactites, draperies, flowstones, and much more. Because of the presence of sulfur, many of the cave features in Carlsbad Caverns are composed of gypsum, calcium sulfate, as well as the more common calcite, calcium carbonate, the same as the limestone rock that was dissolved.

Much of the modern cave activity – the formation of stalactites and such – has more or less ended at Carlsbad Caverns today, because the climate today is so arid. But many of the formations were probably pretty actively growing as recently as 12,000 years ago, when the last ice age ended and the climate changed.

Carlsbad contains some of the largest caverns known on earth, including one that’s nearly 4,000 feet long and 225 feet high. And Carlsbad Caverns is just one of many caves in the Permian reef of southeastern New Mexico. Nearby Lechugilla Cave, explored in 1986, is the fifth longest cave in the world, with more than 136 miles of mapped passages and a total depth of more than 1600 feet. The gypsum cave formations in Lechugilla are even more spectacular than those of Carlsbad.

—Richard I. Gibson

Photo by Eric Guinther licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.   

June 24. Cave passages

We talked about Mammoth Cave the other day, but here’s a little more about its development. Cave passages tend to be linear, in part because moving water preferentially dissolves limestones along lines of weakness, such as linear joints or fractures and along bedding planes.  

Sometimes in places where joints and fractures are regularly spaced and relatively rectilinear, forming rectangles in map view, a cave system might approximate that pattern too. But water isn’t usually strictly limited to the joints, and collapsing caves can create new patterns as well. The map above of Mammoth Cave was compiled in 1897. There’s a certain degree of regularity, and several directions such as northeast-southwest that seem to predominate, but there’s actually quite a wide variety in the development of the cave passages dissolved in Mississippian limestones. Regular patterns do happen, but natural systems tend to be complex enough that regularity is usually the exception rather than the rule.

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Today’s birthday is David Dale Owen, born June 24, 1807, near New Lanark, Scotland. He was the son of Robert Owen, a reformer who established a social experiment at New Harmony, Indiana, in 1825. It was an attempt to establish a utopian society. It was an economic failure but New Harmony became a cultural and educational center, noted for scientific research. David Dale became a prominent geologist, doing some of the first work in the Midwestern United States. He was the State Geologist of Indiana, Kentucky, and Arkansas at various times between 1837 and his death in 1860.

—Richard I. Gibson