CAVE GROWTH IN RELATION TO ROCK FAILURE, NEAR SURFACE STRESSES, AND CRITICAL ZONE PROPAGATION

At the landscape scale bedrock failure creates the pathways for most significant groundwater movement. At short timescales, in low-soluble rocks, this system can be modelled as non-dynamic. At long time scales it is dynamic, and the propagation of failure deeper into the earth is a corrosion function that leads to migration of the base of the critical zone and denudation of the land surface. At intermediate to long time scales, in soluble rock, the system is dynamic and generates a positive linked mechanical-chemical feedback loop for the growth of caves by groundwater circulation. The progressive creation and opening of deeper rock discontinuities, frequently guided by lithological heterogeneities, serves as a network for water circulation, and as the template for cave growth. Of the tens of thousands of explored caves in the world, the vast majority a) follow initial planes of rock failure, b) are less than 250 m in depth from the land surface, and c) are under 10 Ma age. These observed characteristics are a direct result of the material properties of the rocks and the stresses acting upon them. Cave plan and profile forms depend upon 2 factors: initial porosity type and recharge type. Specific shapes, orientation, and positions in the landscape are controlled by the topographic setting. Stress-displacement modelling shows that minor changes in setting can greatly affect optimal zones for cave growth. Many “deep caves” of the world are actually shallow with respect to roof thickness, and appear controlled by depth of significant rock failure.