Glossary of Karst and Cave Terms
The inception of cave development in carbonate rocks begins if water can move through the bedrock and commence dissolution. The earliest water movement may be due to mechanisms (including ground-water pumping and ionic diffusion effects) unrelated to those dominating later development. Similarly, inception may include physical and chemical dissolution (involving removal of carbonates and mineral impurities by water and by strong acids), as well as by the carbonic acid dissolution that dominates later cave growth. Initial water movement can be along primary pores in the rock (in coarse raffle limestones, oolites or chalk), along relatively thin non-carbonate beds within the succession, or along incipient or open fissures (joints, faults and bedding planes). These potential water routes are initially very narrow and water movement is severely restricted and laminar, allowing only very slow dissolutional growth (see gestation), until enlargement beyond the turbulent threshold (breakthrough) permits faster flow and accelerated cave growth. After establishment of turbulent flow conditions the effects of dissolution are augmented by mechanical abrasion and collapse, which expose new rock. During the early development stages a network of narrow openings is formed. Subsequently, geological factors guide the preferential expansion of favorable routes, which capture more of the local flow and enlarge, at the expense of less favorable openings, to form caves. The less favorable fissures are relegated to a subordinate role in transmitting percolation water or, more rarely, in carrying elements of overflow water during floods. Also during the early stages, all voids are water filled but as permeability increases and true hydraulic flow conditions are established, the upper voids drain freely, forming a water table. Almost all caves therefore originate under phreatic conditions but the overall passage morphology is modified during later growth into vadose or phreatic caves, enlarged from the original phreatic imprint, above or below the water table. Ultimately, cave development evolves towards efficient drainage close to the water table. Passage enlargement then becomes regressive as collapse increases. The stage of a cavernous karst collapsing extensively is relatively rarely achieved, being overtaken at high latitudes and high altitudes by surface lowering, but such collapse can contribute to the chaotic land forms of tropical karst .