Caves are very different than most natural environments. In a cave, especially a large cave, there seems to be little daily change. Ceaseless darkness is the norm. Temperatures vary slightly through the year.
According to the National Park Service, temperature inside Mammoth Cave in Kentucky stays around 54°F year round. There is less seasonal change inside a cave than the environment above. Water levels in caves do sometimes fluctuate but not as much as surface environments.
Carlsbad Caverns lies beneath a section of the Northern Chihuahuan Desert but relative humidity within it’s ‘Big Room’ ranges from 87.5% in dry winter months to 100% in summer when there is less air exchange with the surface. This is in an area that, according to the National Park Service, receives only 14.9 inches of rain annually, most of that during seasonal monsoons. According to Weather Spark, Carlsbad New Mexico, the surface town, has an average maximum of 13% relative humidity in August, the rainiest time. Despite seeming static, caves do have wind, and the caver maxim when confronting a small passageway is ‘If it blows, it goes’. The maxim means that if wind is blowing from anything from a small crack to a large opening, there is a cave passage on the other side. The truth is though that wind in caves is always more of a sigh than a raging storm. Some refer to the air exchange between a cave and the outside environment as the cave ‘breathing’.
The most common cave, the classic cave with stalactites and stalagmites, is a solution cave made of limestone. A solution cave is one created by the reaction of rainwater with the surrounding rock, which then dissolves into solution. Limestone is reactive to even a little acid and rainwater is slightly acidic with an average pH of 5.6 as opposed to pure water with a pH of 7. There are some odd caves that aren’t solution caves though.
Lava tubes are one of the more interesting of caves. When lava cools on the outside, the inside often remains liquid. As the inner liquid of a lava flow moves, it sometimes leaves the hardened outer shell behind, creating a tube of rock. Kazumura Cave of Hawaiʻi is a perfect example of a lava tube cave. Most lava tubes are rather small but surveyed at 40.7 miles, Kazumura is the world’s longest known lava tube cave.
Other caves are formed by the slow erosion of ocean waves against cliffs, creating so-called sea caves. Sea caves are relatively short in length but can be quite wide and open.
Much like sea caves, other caves can be sometimes formed by normal, physical erosion. I’ve explored some caves like this, carved out by streams flowing through mudstone. Caves made out of mudstone can be especially fragile and I’ve felt them partially collapse on me, luckily not injuring me. Caves like this can also hold interesting trinkets imbedded in their walls, an example is a cave in western Colorado locally known as ‘Bone in the Wall Cave’… you can guess why.
Classic caves are limestone. The amazing thing is limestone is formed by oceans. Shallow, warm oceans and sometimes lakes are the most common places where limestone forms. Limestone is chemically Calcium Carbonate (CaCO2). In the warm, shallow seas where limestone forms, there are many organisms that form shells from Calcium Carbonate. The hard skeletons of coral polyps known as reefs are also Calcium Carbonate.
It’s hard to say for certain, but it’s thought that the majority of limestone is formed from the deposit of animal shells over long periods of time that become a limestone formation. Limestone can also form from Calcium Carbonate precipitating directly from water but this is thought to be rarer than limestone from organisms. Snails, clams and oysters all create shells mainly of Calcium Carbonate and no more than 2% protein which helps strengthen the overall shell.
According to geology.com, limestone is still forming mainly in shallow waters between 30 degrees north and 30 degrees south latitude. An example of limestone formation happening now is the Bahamas Platform where a reef system abundant with shelled animals is creating large limestone deposits. The platform is a protruding shallow seafloor over 100 miles wide.
Most caves start to form after the limestone deposits have been lifted above sea level. It’s amazing how much change geological movement can make. The top of Mount Everest, the highest mountain in the world at 29,029 feet above sea level in elevation, is a limestone containing seafloor fossils. Why should it be surprising that caves in Texas, New Mexico or even South Dakota are made of seafloor sediments? In fact once North America was cut in half by a shallow ocean perfect for the creation of limestone, allowing for the eventual formation of enormous caves such as Jewel Cave or Carlsbad Caverns.
Even after the painstakingly slow process of limestone deposition and geologic uprising or drop of sea level that allows limestone to be exposed, caves must be carved out of it. Small cracks and pores in limestone are the beginnings of caves. Rainwater trickles or seeps into these small flaws in a limestone deposit and slowly start to dissolve away the rock. This slow process is what also creates aquifers, which are porous limestone ‘sponges’ surrounded by less permeable rock which traps water within them.
Eventually the slow action of water can carve out large passages through limestone and sometimes water levels drop or drain away, leaving the caves largely dry. As water carves out caves, it also leaves small deposits of the dissolved limestone, sometimes in drips or runs, creating evocative cave formations. Stalactites and stalagmites are both created by dripping water. Flow stone is the smooth, limestone formation covering most surfaces in undisturbed and intact caves.
Caves also create strange life forms. In the darkness, many blind and albino organisms evolve. On a cave biology expedition I volunteered for in Arizona we used sweet potato to lure rare cave crickets out. The crickets quickly ate the sweet potato and their orange, sweet potato filled stomachs were clearly visible through clear bodies. Cave evolution is the evolution of economy. Without light, with less to eat, efficiency is of utmost importance.
It’s hard to imagine the lengths of time involved in the formation of a cave and it’s eerie life forms. According to the National Park Service, it took Carlsbad Caverns between 4 and 6 million years to form. The longer Jewel Cave of Jewel Cave National Monument started forming about 40 million years ago. These things seem ancient and by human standards, they are but it’s important to keep in mind that the caves were talking about started growing long after the dinosaurs became extinct. Compared to the estimated 4.6 billion years of the earth’s total age, 40 million is just a drop in the bucket. If you get a chance visit a cave. Stand in the dark and listen to near perfect silence and contemplate how old and vast your home really is.
By Zach Fitzner, Earth.com Contributing Writer