Earth

Caves

How caves form

A bit of geology

The fundamental ingredients to produce karst processes are abundant water rich in CO₂ and organic acids, and a favourable type of rock, but in order to have long and deep cave systems, these conditions are not sufficient.

Pores and fissures

Carbonatic rocks, which are most favourable for karst process, are generally very compact rocks. The granules forming them are very densely packed, the mechanisms by which they were formed led to a primary porosity ( i.e. a percentage of voids) and permeability (i.e. a percentage of interconnecting voids that allow the passage of water) of these rocks that are very low. They are practically almost impermeable (the primary porosity of a limestone generally varies from 1 to 20%; only reef limestones have a higher primary porosity). In these conditions, karst processes can act only on the surface of the rocks, creating surface karst forms, such as karren (furrows and small depressions created by the dissolution of the rock): water has no possibility of filtering underground which is an indispensable condition for the formation of caves deep underground.

In order to be affected by karst processes deep underground, a rock must be characterized by discontinuities, through which water can filter and begin to percolate underground. Discontinuities that are very useful for this purpose are bedding planes, which often characterize limestones (dolomites instead are often massive and do not have any stratifications). Bedding planes form during sediment deposition, however most discontinuities in carbonatic rocks are secondary, of a tectonic origin. In fact carbonatic rocks are very fragile and get fractured easily if they are subjected to mechanical stress. Bedding planes are initially horizontal, but subsequent tectonic deformations can incline and fold them in various ways.