|Image: a so called dolly tub, formed by the flowing water, and connected to the next dolly tub by a deep and narrow canal.|
Erosion is generally the process of weathering for rocks. However, this geologic term is used in speleology in a slightly different meaning, which has historic reasons. The first cave explorers saw cave rivers, which were rather similar to subsurface rivers, gorges and erosions as known before. So they interpreted the caves as a result of the erosion of underground streams. Later they learned, that the chemical solution of rock is much more importants for the formation of caves. So they defined a new term: corrosion. It was meant as the opposite of mechanical weathering by flowing water, solution of rock by chemical processes without the energy of flowing water.
After this modern definition, erosion is only the part of rock destroying processes which is based on the mechanical destruction of rocks. As we know, rock is not destroyed in this process, it is just grinded into smaller and smaller pieces, finally sand, which is then transported away by the water.
Some time ago the process of erosion became an old fashined and outdated explanation. It was only used to explain rare secondary processes, after the cave was formed by solution. However, with more discoveries and explorations, sever caves were found which are formed only by erosion. This is rather clear if the surrounding rock is not soluble, and still there is a river cave inside. The only logic explanation is, that is was formed solely by erosion. A rather famous example is a river cave in the lava filed of Mauna Loa on Hawai'i. Lava is not soluble and the cave is not a lava tube.
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