Mountains and other rock formations are full of cracks and holes, but are these caves?
Generally, cavers define a "cave" as a cavity in rock that is large enough to be entered by a human far enough to be in total darkness. This excludes cracks and holes (too small), shelter caves and frost pockets (not long enough), and cavities formed in soils (not in rock).
Those that do qualify as "caves" can be defined as follows:
Solution Caves are the most common ones, and the ones that cavers like the best - karst caves found in rock that is slightly water-soluble such as limestone, dolomite, marble and gypsum. These caves were formed by water finding its way underground through porous rock, chemically and physically eroding out larger passages. This also occurs, although rarely, in noncarboniferous rocks such as sandstone, quartzite and salt. The two main cave passage shapes are phreatic (tube-like, formed in completely flooded conditions) and vadose (canyon-like, formed by an underground stream in air-filled passage). Passages can also be classified as active (still have water in them, so still forming) or fossil (dry).
Talus Caves, or boulder caves, are spaces in-between large rocks found at the base of a cliff or depression and are usually the result of a rock slide or collapse. Talus caves that are large enough to enter are rare, and they can be quite unsafe.
Crevice Caves, or tectonic caves, are found where bedrock has split and pulled away, such as on the edge of a cliff, creating an enterable crack or crevice.
Sea Caves, or littoral caves, are created by wave action and are found on the margins of modern or ancient seas.
Lava Tubes are the result of molten lava from a volcanic eruption flowing downhill, where the surface cools first while the lava below continues to flow "out". Often this results in a single subway-tunnel cave, but lava can also form complex, multilevel, or deep caves.
Ice Caves are any of the cave types above that contain year-round ice formations. These caves are in bedrock that remains below freezing all year round (permafrost), and generally do not draught during summer. Ice caves can be found throughout the world at high elevations.
Glacier Caves are melted-out cavities within glaciers, and do not really satisfy our definition of 'cave' as they are not in rock and often are not totally dark because of light coming through the ice. However they can be extensive, both long and deep, but tend to change shape quickly as the surface weather changes.
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