Geology Of Mammoth Cave pt. 2: Making the Cave
Geology of Mammoth Cave pt. 2
How fore art though formed, caves of mystery?
Limestone Dissolves in Acid. The end.
Oops! That’s not enough!
What about caves made from other rocks? What about not-caves made of limestone? What acid? HCl, H2SO4, HNO2, Cola? That’s it!
Limestone Dissolves after Dinosaur Spilled Dino-cola. The end.
Actually, it doesn’t work. While the previous blog mentioned that the rocks which make Mammoth Cave were still being formed 320 million years ago, the formation of Mammoth Cave didn’t start until about 305 million years after that. (Mammoth Caves started about 15-20 Million Years Ago. MYA)
What happened in between? Well, there were probably other ecologies of detritus deposited atop the layers which make Mammoth Cave. These would have caused the pressure and protection necessary for the sediments to lithify (become rock,) The shallow seas and intertidal zones which predominated the area now known as Kentucky in the Mississippian, probably became the swamplands, estuaries, and lowlands which made up the area during the next period: The Pennsylvanian.
More about that in the future. Above the Pennsylvanian would have come the Mesozoic, and it’s very possible Dino’s once roamed the land which would eventually become Kentucky. Or Marine reptiles could have been swimming through a sea which overtook the rocks preserving an older ecosystem. Or both. There was enough time for both. But we don’t know for certain what. (Educated guesses can be made, but I’m not making them here.)
Rocks probably weren’t being made all of the time, and sometimes they were actively being destroyed. Eroded by water, wind, life, and time.
The point is we’ve spent a long time together, me writing and you reading, pondering the environment and condition leading to the rocks that enabled Mammoth Cave. But as you walk about the park, you can imagine yourself hiking through missing potential, a world of life and ecology we may never know. Built before we-could-ponder, and destroyed before we could ever know.
Erosion, that’s what makes some caves. First there is stuff. Then something makes a hole in that stuff. (See Geology pt. 1: Most of the Stuff in Mammoth Cave is/was Limestone.) The holes get bigger. Eventually, if nothing is there to stop it, that hole will be the only thing left. Your presence on the surface of Mammoth Cave is set there in the absence of everything which had been there before.
This is the essence of Karst Topography. Which make up a pretty decent % of the US. (Check out the map linked at the bottom.
What is Karst Topography? It’s lands shaped by the dissolution of soluble rocks, like limestone, gypsum, and marble. So something dissolves the rock, and shapes the land. (probably not Dino Cola though.)
Late-Stage Karst topography looks like this:
Xingping, Guangxi, China
Super Late-Stage Karst Topography looks like this:
Rocks gone.
Mammoth Cave hasn’t gotten that far. Mammoth Cave is still relatively young: (15 million years young.) It’s more rock than empty space, and we don’t yet have the photogenic karst towers in Kentucky. Instead, what we have is mostly rock. But inside that rock: A solution cave.
What is a solution cave? A cave where mysteries are solved easily because you are given the solution.
Actually, its referring to the process from the start of this blog: Dissolution.
How is a solution cave made?
For the formation of cave system like Mammoth Cave, and the other caves in the area, you need a 4 things: Dissolvable Rock. – Check. The layers mentioned in Geology part 1 are mostly made of limestone. Something to dissolve that rock. – Check #2. This is easy. (chemistry warning: Chemistry incoming.) Acid + Limestone à Mineral Salt + CO2. (usually in the presence of water. A caprock which protects most of the dissolvable rock. – Check #3. The top layer of Mammoth Cave is a sandstone, which keeps most of the acid out, and Time – Check #4. We are working with 15 million years.
So how does it work?
The caprock protects most of the rock beneath, but eventually something gives. Earthquakes open cracks. Tree roots burrow through soil and rock in search of water. A deeper layer of rock collapses. Or other.
Once there is a way for things to get into the limestone, things get into the limestone. Namely something acidic. If you have ever taken a field geology class of any length, you might have been issued a small bottle of dilute HCl. (Hydrochloric Acid.) You can drop this on any rock to learn if they have something dissolvable inside. If your rock happens to be limestone, you will start to see little bubbles. That’s the CO2 leaving the Limestone (which is Calcium Carbonate.)
In nature there is not a lot of HCl just raining down from the skies. (Fortunately.) Instead, there is another acid. I’ve already mentioned it Rain. Rainwater will naturally form Carbonic Acid. This will dissolve limestone. Slowly. In addition, rain may also contain nitrous dioxide or sulfur dioxide which would help dissolve things faster. Whatever acids are in the water, we know one thing about it. Water likes to go down, downstream, down into the soil, and down until it reaches the water table or a caprock. But, if there are cracks in that caprock, it will go down into them, until it reaches the ground water table.
From there it is just a matter of time. The water and acids, (and acids in water,) will make their way through the cracks in the caprock into the limestone. The limestone will absorb that water or be slowly dissolved to make small pores. Those small pores in that limestone will collect it. In these pores, the water becomes more acidic, dissolving more of the limestone. Eventually you get underground holes. (AKA: Caves.)
While limestone likes to absorb water, caves like to collect it. So, water typically collects in these holes. That water becomes more acidic, and the holes get bigger. Eventually these holes may collect and make a network. (A Cave Network!) And sometimes, even, an underground river. Underground rivers, like above ground rivers, are even better at breaking up rock. They can make the cave even bigger.
Then, if there is even more limestone below the cave network, the water might continue to make it’s way down, and the process might repeat. In Mammoth Cave’s case, there are three levels cave passages. Rivers still run through the lowest layer.
There are more than 400 miles of passages linked through time simply because the right conditions were set up. All through both the slow methodical chemical weathering of limestone rock, and the faster mechanical weathering which comes with the flow of rivers. These have formed the hollow formations carved throughout the layers of rock in Mammoth Cave.
When those collapse, they become sinkholes. (Another popular feature in the area) but we will go into those more in the future.
Inside Cedar Sink in Mammoth Cave (see the layers of rock, elsewhere these make up the walls of Mammoth Cave.)
* There has also been some research to suggest that microscopic organisms might contribute to the chemical weather of limestone. I couldn’t find a good enough source to go into that now. Maybe later, once JRIC gets to Carlsbad Caverns.
Thanks for indulging me by reading my dumb article explaining how this grand cave came into being. I hope that the goofy illustrations, at least, entertained.
I’ve collected a few other sources if you want to learn more. Surprisingly (sarcastic, it’s not surprising at all) you can learn a lot more from Articles written by the rangers of Mammoth Cave themselves. At least with me, you get dumb dino cola.
Sources
Obvious article source from Mammoth Cave National Park
How Mammoth Cave Formed - Mammoth Cave National Park (U.S. National Park Service) (nps.gov)
https://www.nps.gov/maca/learn/nature/how-mammoth-cave-formed.htm
About Karst in the US (mapping)
https://digitalcommons.usf.edu/sinkhole_2020/ProceedingswithProgram/GIS_Mapping_and_management/1/
I mostly wrote about the dissolution of limestone from memory, but I included a source going deeper into the chemistry.
https://www.researchgate.net/publication/313171146_Dissolution_Carbonate_rocks
Wolfgang, Dreybrodt. (2004). Dissolution: Carbonate rocks. Encyclopedia of Caves and Karst Science. 295-298.
About Solution Caves: With better illustrations than mine.
Solution Caves - Caves and Karst (U.S. National Park Service) (nps.gov)