Caves
Caves are naturally formed underground voids that are accessible through openings in the surface of the Earth. They can be either above or below the water table. Those below the water table are underwater caves. If a cave grows too big, its roof will collapse, forming a giant sinkhole. This process happened in Orange County, Indiana where Wesley Chapel Gulf represents a collapsed cave. Natural Bridge, Virginia (which George Washington carved his name into -- the vandal!) also represents a former cave that collapsed.
Echo River emerges from the Mammoth Cave system. The river can be seen on some cave tours at the National Park; here water seeps out of a submerged cavern moving in the direction of the arrow. In karst landscapes, most water travels underground, entering through sinkholes and re-emerges as streams at areas of low elevation. Click for a larger image. (Photo: Rosenberg / IUPUI Earth Sciences.)
Formation
Caves form in very special geologic conditions; however, they are mostly associated with karst landscapes. Why? Well, because caves form most famously by karst processes, the same processes that form sinkholes. Except think on much larger scales and that the voids don’t collapse. Famous caves like Mammoth Cave, Kentucky (a part of a National Park) are formed from karst processes.
I want you to watch this NOVA animation on how caves form. Click on “Launch Interactive” to the right of the picture, then click through the animation. You should review how caves form from rainwater and waves as these involve the interaction of water (as a solvent) on limestone (or other rock held together by carbonate minerals). We learned about this in the Soils We covered lava tube formation in the Volcanoes Module and the example of how caves form from bacteria is interesting but not necessary for you to cover.
Caves are not always large. Many caves explored by professionals contain tight passages like this one in Sullivan Cave in Lawrence County, Indiana. The texture on the floor of the cave (solution scallops) was carved by selective dissolution by the water. Other great photos of karst and karst cave ecosystems are available at the Karst Conservancy. (Photo: Indiana Karst Conservancy.) Formations like stalactites and stalagmites are formed from the dissolution and deposition of carbonate minerals. Water carrying dissolved minerals slowly drip from the ceiling. As the water is waiting to drip, some of it evaporates leaving allow some of the dissolve mineral to reform and stay behind on the ceiling. Thousands of years of this slow drip, deposits more and more minerals to form the cones hanging off the ceilings or stalactites. As the water droplet falls through the air, some water from it also evaporates and more mineral is comes out of solution, piling into a cone on the floor of the cave. (No, you’re not having déjà vu; this text is repeated from the Soils module section on dissolution, chemical weathering.)
