Any hole within the earth that is a void hosted by rocks is a cave. A cave can be filled with air, with noxious gases, with ice, with fresh water, sea water, or acidic waters. They can be formed at the same time as their host rocks such as lava tubes in a volcanic flow or ice caves in glaciers. They can be formed by the erosion of soft rocks within harder rocks, by the action of waves, floods, or wind. We may be able to enter and explore them, or they may be impenetrably sealed away underground.
Not many caves I’ve visited are as spectacular as this panorama within Carlsbad Caverns, but the world’s great caves all have one thing in common: they are caverns within rocks that are dissolved by water flowing through them. In most cases, the rocks are carbonates (limestones, dolomites or marble), and in most cases the waters that dissolve the carbonates are acidic. Groundwater percolating through soils and reacting with limey soils and rocks is enough to make a weak carbonic acid – weak to you and me, but an acid that can slowly and surely eat through carbonate rocks. In waters filtering through rocks with sulfide minerals present, much stronger sulfuric acid can be created (perhaps aided by bacteria that break down sulfide minerals) and this can then carve away the rock along the pathways of underground streams. If waters are warm (hydrothermal), perhaps emanating from a nearby volcano, all the better for cave formation.
Decorations in caves that originate from underground waters are generically referred to as “speleotherms,” the most common of which are stalagmites and stalactites: Okay, the mnemonic definitions, let’s get this over with … Stalactites hang “tight” from the ceiling. Which means that stalagmites would have to hang mighty tight to cling to the ceiling, so they prefer the floor… Anyway, those acidic waters percolating through the earth carry dissolved calcium in the form of calcium bicarbonate – when water carrying this reaches a void space, dripping ensues, and the calcium precipitates out again as a calcium carbonate mineral aragonite and/or its more stable chemical twin, calcite (all aragonite over time converts to calcite). This precipitation slowly forms the “icycle” shaped stalactite hanging from a cave roof; the precipitation from water that drips onto the floor slowly accumulates into a stalagmite. Slow? As much as half a centimeter a year is lightning fast by stalactite/stalagmite standards.
Even though they’re formed in caves, speleotherms have a lot to say about the world outside the cave. Oxygen isotopes retained in the carbonates of the speleotherms are inherited from the climate conditions of their source waters: if the water source was rainwater, analyses can show the prevailing atmospheric temperatures in these rains. Similarly, the carbon isotope ratios within a stalagmite can preserve the signature, first imprinted by bacteria in the soils where the waters passed, of a grassland or forest environment. Since uranium and its daughter product thorium are highly mobile elements that tend to be transported (in exceedingly small amounts) in these waters, and then precipitate out along with the carbonate minerals of speleotherms, radiometric dating can give precise ages of speleotherms. Stalactites from the ices ages are studied in Israel; some cave formations up to half a million years in age have been documented, Thus, analyses of a stalactite can tell us how old it is, how long it took to form, what was the climate of the earth and changes in the climate of the earth above the cave during the time it was forming, and what major plant environments were found there.
Speaking of ecosystems, aside from the stray spelunker, the community of animals that live in caves are also mysterious. Some trogloxenes live in caves but, like bats, must exit the cave for food. Others, the really strange ones, are troglobites that survive only in caves. These are the insects, spiders, crustaceans, salamanders and fish that have adapted to a dark world by losing their sight, their coloration, and either lose appendages or grow elongated limbs. Microbes and bacteria living in caves can aid its formation by dissolving the rocks on which they grow, and some glow in the dark. These are the pale, quiet, spooky creatures that inhabit caves and add to that mysterious chilling feeling.
Where else can we experience the awe of such alien beauty and the mysterious atmosphere that originated thousands of years ago in a single location – here within our earth? Try a cave, if you haven’t already!
Photo courtesy William Haun: http://
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