The long-lasting uncertainty with the central for karstology notion of karst hinders synthesis of knowledge and the development of a theoretical basis of this scientific discipline. This paper analyses the essence of karst, based on generalization of the modern ideas about regularities of the origin and evolution of conduit permeability in soluble rocks, viewed in the light of ideas of synergetics and non-equilibrium thermodynamics of I.P.Prigogine regarding self-organization in open systems and formation of ordered dissipative structures.

The presence of soluble rocks in the sedimentary environment determines a phenomena of self-organization of the flow structure, which brings the water-rock system into a new capacity-state, namely – karstic.  The property of self-organization of this geosystem realizes via specific (speleogenetic) mechanism of permeability development, which action radically changes (organizes in a special manner) the structure and functioning of the flow system.

The mechanism of self-organization of flow and of the formation of the karst geosystem (speleogenesis) includes: 1) early speleogenesis, positive feedback between flow and the rate of enlargement of initial flow paths (revealing of proto-conduits), 2) speleogenetic initiation: a cascade breakthroughs of proto-conduits to the condition of rapid dissolution kinetics, with accelerated growth of initiated conduits, hydrodynamic competition, respective destabilization and reorganization of the flow pattern and change in boundary conditions, and, 3) speleogenetic development: stabilization of the system at dynamic equilibrium at the expense of increased energy exchange with the environment, and further growth of conduits. As a result of this specific evolution the geosystem acquires ànew, «karstic», capacity and more complex of organization, with the establishment of one more level of permeability, the most contrast one.

The notion of karst is derived from the essence of progressive evolution of the geosystem containing permeable soluble rocks, driven by water exchange and speleogenetic mechanism of self-organization of the permeability structure. Regressive evolution of the karst geosystem includes processes of gravitational destruction and various accumulations, which lead to fragmentation and demolition of relict structures of karst permeability. Based on this new approach to definition of the notion of karst, criteria of distinction between proper karst and similar but not identical phenomena (merokarst, pseudokarst) are discussed.

Venezuelan table mountains (tepuis) host the largest arenite caves in the world. The most frequently used explanation of their origin so far was the "arenization" theory, involving dissolution of quartz cement around the sand grains and subsequent removing of the released grains by water. New research in the two largest arenite cave systems - Churi-Tepui System in Chimanta Massif and Ojos de Cristal System in Roraima Tepui showed that quartz dissolution plays only a minor role in their speleogenesis. Arenites forming the tepuis are not only quartzites but they display a wide range of lithification and breakdown, including also loose sands and sandstones. Speleogenetic processes are mostly concentrated on the beds of unlithified sands which escaped from diagenesis by being sealed by the surrounding perfectly lithified quartzites. Only the so-called "finger-flow" pillars testify to confined diagenetic fluids which flowed in narrow channels, leaving the surrounding arenite uncemented. Another factor which influenced the cave-forming processes by about 30% was lateritization. It affects beds formed of arkosic sandstones and greywackes which show strong dissolution of micas, feldspars and clay minerals, turning then to laterite ("Barro Rojo"). The main prerequisite to rank caves among karst phenomena is dissolution. As the dissolution of silicate minerals other than quartz appears to play not only a volumetrically important role but even a trigger role, these arenitic caves may be ranked as karst.

Flank margin caves form in the diffuse flow field of the distal margin of the freshwater lens on carbonate islands and coasts. Dissolution is governed by superposition of mixing zones at the top and bottom of the lens. Flow velocities in the lens margins and organic decay at the lens boundaries enhance dissolution. These caves grow from isolated initiation points in the lens into chambers that amalgamate to form complex, vertically restricted, globular-chamber complexes. The caves and their deposits accurately represent sea-level position and paleoclimate but must be successfully differentiated from sea caves and other pseudokarst features..

Landscapes on highly quartzose bedrock that exhibit almost identical scale and morphology to those on karstified limestones occur under a range of climates and on most continents. These include ruiniform towers, grikes, stone cities, caves, dolines, smaller surface karren, and silica speleothems.

However, these rocks are much less soluble than most carbonates, and the weathering processes are quite different. However, because chemical solution is demonstratively a critical component in the genesis of these landforms, they may be regarded as karst. This chapter summarizes the processes of karstification in quartz sandstones and then reviews the incidence of these landforms around the world.

The practice of referring to certain morphologically karst-like phenomena as ‘pseudokarst’ is problematic, because it ignores basic principles of sound classification, logical naming conventions and accepted geomorphic classifications and terminology. These problems have compounded the difficulty in establishing an accepted classification of ‘pseudokarst’ types. The practice embodies a karst-centric perspective which should be avoided in favour of using conventional geomorphic terminology for non-karstic features. We illustrate this by providing existing conventional terms for many ‘pseudokarst’ types reported in the literature.

In the recent work of Aubrecht et al. (2011) the presence of “unlithified or poorly-lithified beds” of sands in the quartz-sandstone stratigraphic succession is proposed as a key factor for speleogenesis in the Venezuelan tepuis.In this comment we observe that in the cited work the geologic history of the region, in terms of sedimentation environment, diagenesis and low grade burial metamorphism, has not been considered. Furthermore, the peculiar “pillar flow” columns that Aubrecht et al. describe as a proof of the unlithification are lacking in many other different cave systems in the same area.

Four critical points are discussed: the burial metamorphism of the Mataui Formation, the significance of the Schmidt Hammer measurements, the cave morphologies and the role of SiO2 dissolution. Finally we suggest that weathering, in its wider significance, is probably the triggering process in speleogenesis, and there is no need to invoke a differential diagenesis of the sandstone beds. ©

In the recent work of Aubrecht et al. (2011) the presence of “unlithified or poorly-lithified beds” of sands in the   quartz-sandstone stratigraphic succession is proposed as a key factor for speleogenesis in the Venezuelan tepuis.   In this comment we observe that in the cited work the geologic history of the region, in terms of sedimentation   environment, diagenesis and low grade burial metamorphism, has not been considered. Furthermore, the   peculiar “pillar flow” columns that Aubrecht et al. describe as a proof of the unlithification are lacking in   many other different cave systems in the same area.   Four critical points are discussed: the burial metamorphism of the Mataui Formation, the significance of the   Schmidt Hammer measurements, the cave morphologies and the role of SiO2 dissolution. Finally we suggest   that weathering, in its wider significance, is probably the triggering process in speleogenesis, and there is no   need to invoke a differential diagenesis of the sandstone beds

Tjuv-Antes grotta (Tjuv-Ante's Cave) located in northern Sweden is a round-abraded sea cave ('tunnel cave'), about 30 m in length, formed by rock-water abrasion in a dolerite dyke in granite gneiss. Abundant speleothems are restricted to the inner, mafic parts of the cave and absent on granite parts. The speleothems are of two types: cylindrical (coralloid, popcorn-like), and flowstone (thin crusts). Coralloids correspond to terrestrial stromatolite speleothems in which layers of light calcite alternate with dark, silica-rich laminae. The dark laminae are also enriched in carbon and contain incorporated remains of microorganisms. Two types of microbial communities can be distinguished associated with the speleothems: an Actinobacteria-like biofilm and a fungal community. Actinobacteria seem to play an important role in the formation of speleothem while the fungal community acts as both a constructive and a destructive agent. A modern biofilm dominated by Actinobacteria is present in the speleothem-free parts of the dolerite and located in cave ceiling cracks. These biofilms may represent sites of early speleothem formation. Because of its unusual position in between two types of host rock, Tjuv-Ante's Cave represents a unique environment in which to study differences in microbe-rock interactions and speleothem genesis between the granite and dolerite host rock. Our study shows that the mafic rock is superior to the granite in hosting a microbial community and to support formation of speleothems.