How underground limestone caves form?

Answer 1

Most of the world's caves are Karst features, i.e. primarily in Limestone. Karst caves need three materials: a soluble rock like Limestone or Gypsum, water and Carbon-dioxide (CO2). The last two combine to form a weak acid that dissolves the limestone. For a fuller account: The host limestone needs to be of appropriate physical structure and raised into hills, then subjected to reasonably consistent precipitation for many tens or hundreds of thousands of years. Limestone is a sedimentary rock of which the world's greater proportion was laid down in warm, relatively shallow, seas. The rock was laid in horizontal layers - Beds - separated by Bedding-planes which generally reflect geologically-brief changes in the environment. The suite of beds is known as a Formation, generally named after its "type area". Later continental uplift (tectonic processes) raise the formation along with its underlying rocks, usually tilting and folding it to at least some extent in the process. Since most rocks are brittle they cannot take much stress, and limestone beds crack into grids of fine fractures called Joints. The uplift and folding often also causes Faulting - major breaks with the rock mass one side of the Fault Plane being raised, lowered or moved horizontally past that on the opposite side. (Note: Plane - the "Fault Line" sometimes misused as a political metaphor is that of the fault-plane cutting the land surface.) Now we have the hills, next we need rain-water that has absorbed atmospheric CO2 to create Carbonic Acid (weak, natural soda water in fact!). It may be augmented by acids from the soil, too. This solvent permeates through all those joints, bedding-planes and faults; flowing very, very slowly under considerable pressure applied by its depth, from its sinks on the surface to its springs at the base of the formation. In doing so, it dissolves the limestone (chemical weathering), creating meshes of tiny micro-conduits that over many tens of thousands of years coalesce and capture each other to form cave passages. Once this happens, the rate of erosion can increase - though still to perhaps only a few millimetres per thousand years under generally temperate climates. A cave, or a series within a cave system, that still carries its formative stream is called "Active", and is still being developed. Surface changes such as the valley floor being lowered by erosion, or down-cutting within the cave by its stream, changes the water's route and the original, now dried-out, stream-way is called "Fossil" or "Abandoned". Such passages may be filled with silt left by floods as the main flow gradually abandons them; or may become richly decorated with Speleothems - calcite deposits such as stalactites and stalagmites precipitated from ground-water still oozing through the joints in the limestone above the cave. In time such passages may start to break down as there is no stream to dissolve away slabs falling from the roof as permeating ground-water attacks the rock above. In the end, surface lowering of the landscape as a whole, breaches and destroys the cave. Nothing is permanent in Nature! Caves in limestone are also parts of Karst Landscape. i.e. a landscape developed by the dissolution of limestone, giving surface features like Dolines, Limestone Pavement, and in the tropics, distinctive hills such as those represented in Chinese Willow-pattern images. 'Karst' is from the Slavic word 'Kras', the name for its world type-area. @@@@@ The above is purely an introduction to a vastly more complex and subtle series of processes, of course, and you need to refer to appropriate text-books on geology and cave studies to learn them. The scientific study of caves is Speleology - embracing geology, hydrology, Biology, Archaeology and other disciplines. Simply visiting caves to enjoy them for their scenery and the physical and mental challenges they present, is called Caving, though you can't study a cave unless you can negotiate its obstacles.