Thread: Ask a Scientist
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Old 22.03.2021, 19:40
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Re: Ask a Scientist

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Caves frequently form in limestone rock caused by water percolating through the rock and dissolving away minute quantities of the rock. Over millions of years, sufficient material is removed that caves form. Sometimes huge caves. Whole networks of caves.
Sometimes it's faster than millions of years. Sedimentary rock less a million years old can have human size caves See Florida and Yucatan Peninsulas coastal caves.

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Well, stalagtites and stalagmites form inside the cave. These are formed by the same water that has been trickling down through the same layers above and thus that actually formed the caves in the first place. But rather than continue to remove limestone, they actually reverse the process and deposit some and form stalagmites and stalagtites. Every droplet of water adding a minute amount of material.

I guess that if you wait several more millions of years these stalagmites and stalagtites will meet in the middle to form pillars, and the pillars will grow in girth until all available space is filled and effectively the cave is gone and the original condition is restored.

So why does exactly the same process that creates the cave then end up filling it in again? A shift in temperature maybe? Or in the acidity of the water?Does this mean there are certain periods in the earth's history that are favourable to the formation of caves, and certain periods that are favourable to them filling back in?
Now comes the complicate part. The formation of caves need flow of water and some acidity to dissolve limestone.

Let's tackle the water flow first. The source of water can be rainfall and climate changes a lot along time. So, a decrease in rainfall may be the trigger to switch from growth (dissolution) to fill (stalagmites). An increase in rainfall the opposite.

Back to coastal caves, the sea level has increased 110-120m since the last glacial maximum only 22K years ago. The mix of fresh (rainfall) water and seawater creates some reactivity. So, caves at the mixing zone between fresh and saline water are growing, while caves above water level may be filled with mineral deposits, while caves deep in the saline zone are frozen for the moment. Depending on sea level, growth, fill and nothing is happening at the same time in a coastal cave system.

Back to higher terrains, the terrain surface changes a lot. Erosion is powerful over time. Some caves may be created by a river infiltrating its river bed and growing to the sides, then the river erodes the river bed and those caves are now above water level and start to flll. Now think about continents and islands going up and down due to tectonics. Caves underwater are pushed up above water level and now being filled by deposits.

Not a rule of thumb, but generally caves grow while filled or near partially filled with flowing water. And generally filled with mineral deposits when filled with air. So, more than changes in the Earth, is a change in the conditions of the little limestone block where caves develop (relative to Earth's size).

Now the acidity part. Usually the main source of acidity is CO2. So, bacteria in soil and water can generate acidity. If climate changes and a lush jungle is replaced by a desert, the acidity source is gone and a cave may switch from growth to fill up.

Finally rock mechanics. Caves are cavities that may collapse due to the weight of rock and sediments above. Tectonics may sink a limestone block and sediments start to accumulate above. Eventually the pressure is too high and the cavity collapses.

So, a lot of physical processes competing with each other. Pushing in opposite directions the whole time.

All this read like useless science until people realizes oil/gas are found in paleokarst, and we humans produce a lot of harmful trash that should be stored somewhere safely. That somewhere needs to be safe from erosion and cave development over thousands of years

PS. if you want to read more the keyword is speleogenesis.
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