Limestone is a common chemical as well as organic sedimentary rock with a unique connection and relationship with rain. Sedimentary rocks are formed by small particles, or sediment, that have been compressed together, and organic sedimentary rocks involve carbon compounds. Limestone is created by evaporation and precipitation processes with the involvement of living organisms. There is a tight link between rain and limestone caves.
Ocean creatures like clams, oysters, and coral use calcium carbonate (CaCO3), or calcite, to form their shells and other structures. When they die, these structures are broken down by water and other factors to form organic sediment. Over time the sediment settles and will form limestone beds, thus limestone is often found near ocean environments. Similarly, in the case of evaporation, the sea water containing the sediment turns to vapor and leaves behind the calcite. It again builds up and becomes compacted. Stalactites and stalagmites found in cave systems are also composed of limestone, but through precipitation.
Rain is infamous for eroding and weathering limestone. Such weathering can be seen in Bryce Canyon National Park in Utah. The flow of water can cause basic erosion, but in addition to that process, rainwater is not always pure water. It contains microscopic particles that it carried with it when it turned to vapor or that it collected after it fell and moved through other surfaces. Often it contains a compound called carbonic acid (H2C03), which is a weak acid. This is another organic compound, but it actually has a reaction with the calcium carbonate contained in limestone. For this reason, limestone is damaged very easily by rain. As the rain erodes limestone beds it forms karst scenery.
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Karst scenery is made up of swallow holes, clints, and grykes. Swallow holes form where the water has pooled and essentially eaten its way straight down through the rock. Clints and grykes are formed when limestone is found in conjunction with other types of rock not as easily affected by rainwater. The rainwater will flow over the impermeable rock, but then dissolve the limestone creating cracks and grooves, leaving chunks of the other rock behind. The cracks are called grykes and the rock left behind are clints. Mammoth Cave in Kentucky is another prime example of karst topography.
Stalactites formed on the roofs of caves and stalagmites on the bottom are often the result of the erosion of nearby limestone beds. As the beds are weathered away by rain, the water gradually collects the calcium carbonate, taking it with it. As the water seeps into the ground and into subterranean caverns it will form these recognizable icicle-like formations. In this case, the limestone is a precipitate of the water. In this manner, rain is involved in both the formation and degradation of limestone, creating this unique natural relationship. Rain essentially changes the form of the limestone, rather than completely destroying it.
