Erosional Caves

Corrasional Caves

A so-called Geologydolly tub, formed by the flowing water.

Erosional caves are caves which form entirely by erosion, typically by flowing water removing rocks and other sediments. As a result the chemical properties of the rock are irrelevant and such caves can form inside any type of rock. Important are the mechanical properties like hardness, cleavability, or fissility. Initially they need some zone of weakness to allow the water to enter the rock, such as a fault or joint.

Geologists use two different terms for the processes connected with the destruction of rocks. The surface of Earth is subject to weathering, the decomposition of rocks by the atmosphere and water, and erosion is the process of relocation of weathered rocks. Once the rock is decomposed to sand or gravel it is moved by flowing water, by floods, or even by the wind. Those geologic terms are used in speleology in a slightly different way, which has historic reasons. Speleologists are interested in the relocation of rock, how rocks are moved to form caves, so they are actually only interested in erosion. In speleology, the main focus is on the formation of caves, so the only thing that is of interest here is erosion, i.e. the removal of rock to form a cave. Here we distinguish between mechanical weathering and transport in solid form (erosion) from chemical weathering and transport in dissolved form (corrosion).

The first cave explorers saw cave rivers, which were rather similar to surface rivers and gorges. So they interpreted the caves as a result of the erosion by underground streams. Later they learned that the chemical solution of the rock is much more important for the formation of caves. So they adopted a new term: corrosion. What was meant was the opposite of mechanical weathering by flowing water, the dissolution of rock by chemical processes (without the energy of flowing water) and transport in dissolved form. In the mid 20th century the process of erosion became an old-fashione and outdated explanation for cave formation. It was only used to explain secondary processes, after the cave was initially formed by solution. In other words: once there was a cave, the water started to flow inside.

However, during further discoveries and explorations, caves were found which are formed only by erosion. This is quite clear if the surrounding rock is not soluble and there is a river cave inside. The only logical explanation is that it was created solely by erosion. These caves are called erosional cave or corrasional cave (which sounds like corrosion, but there is an a instead of an o). Only caves that have been exclusively formed by mechanical weathering are called erosional caves.