First a void and then the rock

Caves formed by biologic processes
Among the primary caves, the most widespread are caves forming when corals and coralline algae grow forming encrusted coral reefs: the growth of these organisms is neither uniform nor homogeneous and voids of various shapes and sizes are created, which are often  large enough for a man to visit. All those who have  done scuba-diving  near a coral reef must have observed these caves. Most  are practically anfractuosities or recesses and, at times, small irregularly shaped channels or galleries are formed, through which it is possible to cross the reef from one side to the other. The  entrances of the galleries may be at different levels (e.g. the Dahab Blue Hole in the Red Sea belongs to this category – as we will see later, the term “blue hole” is used in an improper manner in this case), furthermore these cavities are rarely larger than a few tens of metres. The presence of large size cavities in fossil reefs, in sub-aerial continental environments is very rare. During the transformation of the reef into rocks, in fact, the primary cavities are generally filled with sediments that fossilize them completely.
Volcanic caves
Primary caves can form during the cooling process of a lava flow. Volcanic caves are created when a lava flow, generally basaltic lava that is very fluid, cools on the surface forming a solid “crust”  underneath which liquid lava continues flowing. At the end of the eruption lava stops flowing outwards, the last emissions flow underneath the solidified crust and come out at the  base of the lava flow, leaving channels that are real tunnels with a circular or elliptical shaped cross section.
Lava tubes can reach remarkable lengths, as in the case of  the Kazumura Cave in Hawaii, that is over 60 km long, with depth of over 1100 m. These are prevalently horizontal caves, with a very modest slope, however, when they stretch over long distances, they can reach remarkable depths.
Rocks are generally very dark in colour, with a vitreous appearance due to the rapid cooling, near the walls characteristic steps form due to a supra-excavation  that lava flows carries out on the floor of the caves, and also features similar to stalactites and stalagmites are formed due to the dripping of lava as it cools. In fact, on the ceiling the heat released by the cooling flow causes a new melting process of the rock, which drips downwards solidifying in stalactites-like features (whose origin however is completely different)  while the drops falling on the floor form curious slender and contorted “stalagmites” (similar to the towers built by children on the beach, with droplets of sand and water).
These caves, due to the particular mechanism by which they are formed, are always  very close to the surface: their ceiling often crumbles and fall, and therefore lava tubes are scattered with external pits, in the form of  small shafts, which often have a circular section, known as sky-lights.  This type of caves always forms in continental environments, and therefore in sub-aerial conditions. If a lava flow  comes into contact with sea water, the violent cooling that follows gives rise to explosions that shatter the rock and the formation of lava tubes stops. For different reasons, however, the sea can subsequently flood part of these caves when these form near the coast as,   for example, in the Canaries, on Lanzarote island, in the complex Atlantida system, which has a flooded part that is over 1600 m long. Here, the pressure created by water occupying the galleries helps preserving the tunnels. These are on the contrary very fragile on the surface due to the short thickness of the ceiling and are subjected to rapid degradation due to the caving in of the vault.
Caves formed by cooling
Much smaller size primary caves in volcanic rocks can also form in particular effusive rocks, i.e. basalts, when a rapid cooling creates a “column” like structure, with the formation of large columns several metres high, characterized by a hexagonal cross-section. Collapses along the cooling fissures may give origin to suggestive cavities, specially near the coast where collapses are facilitated by the wave action: a very well-known example is the famous Fingal’s cave on the Island of Staffa in Scotland.

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