Can the act of breathing in a cave cause it to grow larger? Limestone caves are usually formed by dissolution of limestone as the result of the interaction of carbon dioxide dissolved in water seeping through cracks in bedrock. Typically this begins deep underground and as voids expand and reach the surface animals may enter and occupy these caves. But what happens to caves when the water source that has contributed to its formation dries up? Does cave formation stop?
Some caves, such as ones found in Malaysia, no longer have outside sources of water but they still manage to grow larger—much larger—over time. How do they do it? Amazingly, they expand by virtue of chemical processes made possible by their living denizens.
In the case of the caves of Malaysia, and the extensive Gomantong cave in particular, it has been estimated that between 70% to 95% of the total volume of the cave may be attributed to activity of living things in the cave rather than external water sources. Those living things include millions of bats and thousands of birds. These bats and birds do two things continually: they breathe and they defecate. The latter has resulted in 30 foot thick piles of bat and bird guano on the cave floor. Acid in the freshest guano eats away at rock that has fallen from the ceiling. But the most important terraforming ability of these animals comes from their breath.
That’s right. Just by breathing they cause the cave to grow larger. The carbon dioxide from the breath of the bats and birds rises and is absorbed in the humidity of the cave. Water from the air condenses on the cave’s ceiling and walls. There the water and carbon dioxide dissolved in it forms carbonic acid which eats away at the limestone walls and ceiling. The long-term result of this continuous activity is that huge cavities in the walls and ceilings of the cave have been created. At times large portions of the ceiling collapse onto the floor of the cave where they continue to be dissolved by guano.
It might sound strange that living things could have such a dominant influence on the fate of solid rock but lichens, tree roots, and many other plants are known to play an important role in the breakdown of rock on the surface of the Earth. Here we have an animal that can, through its breath and feces, alter the very shape of the cave that it lives in.
How quickly can these cave alterations happen?
I first read about this phenomena in an article by J. Wendel (How bat breath and guano can change the shapes of caves) and was fascinated but what really caught my attention was just how long it took for these biological forces to show their effects. Digging a bit deeper, I found a research article (see reference below) about the same cave that provided more specifics about the rate at which these caves are being altered by their stinky residents.
Researchers estimated that bats and birds in the cave are responsible for up to 1 meter of rock erosion every 300,000 years in the entire cave system and where their densities are very high they could be eroding 1 meter of rock in just 30,000 years. This would be a maximum rate based on the highest amount of carbonic acid that can be retained in water and fill the air of the cave and the fastest rate at which that acid can dissolve the limestone of this cave.
The distinctive form of erosion observed in the cave that results from the breath of bats and birds suggests that tens of meters of rock have eroded since the cave was formed via watery processes long ago. Portions of the cave that are not inhabited by bats nor have been since the origin of the cave have a very different surface morphology providing additional evidence that the presence of bats in the cave have had a profound effect on the morphology of the cave. A paper by Lundberg and McFarlane shows many images of cave ceiling and walls with huge pits carved out by slowing dissolving the limestone. In some places rock has eroded many meters up into the ceiling forming vertical columns up through the ceiling that nearly reach the surface.
The researches have used well-understood observed principles of chemistry to establish the estimated rates of erosion of the cave ceilings and the rocks on the floor of the cave. But to test their estimates, a year ago they placed small pieces of limestone of known size and weight in the cave in various places. They will collect those pieces of limestone in the near future and by examining them the will be able to determine the current rate of erosion in the cave system via these biotic-induced forces.
Over many years, then, bats and other organisms are literally expanding the rooms they live in via their presence in those rooms. Portions of this cave are huge with 100 foot ceilings and up to 30 feet of compacted guano on the cave floor. Carbon 14 radiometric dating of guano in this cave shows that the deepest guano in the piles is more than 40,000 years old. In other caves in the area, guano has been dated to at least 30,000 years old.
You might think that guano would build up rather quickly but millions of insects and trillions of bacteria are hard at work decaying the bat feces as they rain down on the cave floor. As a result, the rate of buildup on the remaining carbon material left after decay is on the order of only millimeters per year at most. Thus the observed rates are consistent with radiocarbon derived dating estimates of tens of thousands of years to produce these enormous piles of decayed guano.
Deep piles of guano, radiocarbon dating that shows increasing ages of guano with depth in the pile, expansion of the cave walls and ceilings without water intrusion from outside, and deeply eroded boulders on the cave floor are compelling evidence of the product of slow processes of cave formation. Cave formation via carbonic acid or even sulfuric acid in limestone rock is a process which can take tens of thousands to hundreds of thousands of years to produce large caverns but that is a “fast process” compared to dissolution resulting from bat breath. Yet again we are faced with another example—among countless similar ones—which testify to the ancient origins of geological features on Earth.
Wendel, J. (2015), How bat breath and guano can change the shapes of caves, Eos, 96, doi:10.1029/2015EO039053. Published on 9 November 2015. https://eos.org/articles/how-bat-breath-and-guano-can-change-the-shapes-of-caves
Lundberg, Joyce, and Donald A. McFarlane. “Microclimate and niche constructionism in tropical bat caves: A case study from Mount Elgon, Kenya.” Geological Society of America Special Papers 516 (2015): SPE516-17. http://specialpapers.gsapubs.org/content/early/2015/06/23/2015.2516_17.abstract
Lundberg, Joyce, and Donald A. McFarlane. “Post-speleogenetic biogenic modification of Gomantong Caves, Sabah, Borneo.” Geomorphology 157 (2012): 153-168. http://www.sciencedirect.com/science/article/pii/S0169555X11003047
Bird, Michael I., Ella M. Boobyer, Charlotte Bryant, Helen A. Lewis, Victor Paz, and W. Edryd Stephens. “A long record of environmental change from bat guano deposits in Makangit Cave, Palawan, Philippines.” Earth and Environmental Science Transactions of the Royal Society of Edinburgh 98, no. 01 (2007): 59-69. http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=1367804&fileId=S1755691007000059
*This is an updated and revised version of a post from 2015.