school energy&environment

First the rock and then a void

Unlike primary caves formed at the same time as the rock in which they are formed,  the origins of secondary caves  imply different processes acting on rocks that already exist. At times these processes take place many millions or tens of millions of years after the formation of the rock.
Eolian caves
These caves do not have a great lenght, no more than a few metres, and are formed by the abrasive action of the wind and due to particular weathering processes in arid and desert zones, or near the coasts on tender rocks that are particularly flaky, such as poorly cemented sandstone or rocks such as granite, particularly subjected to weathering processes due to hydrolysis (which weathers  feldspathic minerals forming clay and transforming the rock into a  sand made of quartz crystals). Cavities in this category include, for example, the famous “tafoni” in Sardinia. Due to their origin and due to the nature of the rocks containing them, these caves generally have short lives and are not very interesting for both cavers and speleologists.
Tectonic caves
A large number of secondary caves have a tectonic origin, due to collapses  and breaking down along joints, fractures or faults that weaken a rock. Faults and fractures form due to the effect of tectonic deformations affecting rocks deeply buried inside the Earth’s crust: when rocks are uncovered by erosion, the presence of large hollow spaces or high walls determines the opening of breaking surfaces, with the consequent collapse of blocks that can be of a remarkable size. This process can lead to the formation of large caves that are however rarely very deep and long. These caves are characterized by typical geometric squared walls formed along the surface of the fractures or fault that determined the break-down, and large heaps of crumbled material and fallen blocks on the floor. These caves often are large, high and narrow rooms. These caves do not form in underwater environments where the water pressure decreases the possibility of break-down and detachment of blocks, but  these types of cavities are commonly found near large mountain walls, often at the base of cliffs along the coast line, where the action of waves can contribute to break-down processes and water can easily erode and take away the material from the floor. These caves may form in any type of rock and generally they are not of any particular speleological interest.
Sea caves
All sea caves are secondary caves. It must be pointed out that caves of marine origin are relatively few and generally they are not very long or deep, their length being not much more than a few tens of metres, and their depth being quite modest, not much more than a few metres. Caves occupied by sea water, but whose development is more complex and  whose depth is greater, are, as will be seen hereunder, of another origin, even though the sea that invades them can contribute to modify them in various ways. Sea caves, in the strict sense, are created  by the mechanical  action of waves that shatter the rock with  their striking force, by the erosion carried out by the debris they carry, and mainly by the chemical corrosive action that sea water, specially when mixed with meteoric waters, can carry out on rock, together with the biological action of sea organisms. Generally, these cavities are only a few metres or tens of metres in size, even though at times they may have large portals at their opening. They tend to have a sub-horizontal stretching and form a few metres above or below the  water surface.  The origin of notches or  “solchi di battente” is similar to that of sea caves and can often be a continuation of the same. These are a typical coastal formation and are often clearly visible, like a marked indentation at the base of walls and cliffs, at sea level. The presence of sea caves at levels that differ from the present sea level can be a precious instrument to reconstruct the evolution of sea level oscillations. Sea caves of this type can form in different types of rock, however their size grows to an interesting one only in rocks that are particularly sensitive to the corrosive action of sea water, such as carbonatic rocks (which are described in greater detail in the next chapter). Also sea caves can actually be classified in the large category of karst caves.