Secondary caves are caves which were formed later than the surrounding rock. The term Postgenetic Cave is a synonym, the Latin translation of later than the surrounding rock.
Secondary caves are formed after the surrounding rocks, when the forming of those rocks is already completed. The formation of the rock is the primary stage, the rock is formed without caves. The cave is formed in a secondary stage. Typically, there is a long time span between those two processes, for example caves formed in 200 Million years old Jurassic limestone, which were formed during the last 2 Million years.
The distinction between primary and secondary caves dates back to the 19th century, when geologists discovered that some caves were already formed during the formation of the rock, i.e. they are the same age as the rock, and others were only formed much later, i.e. they are much younger. However, this is only a partially helpful distinction, because probably far more than 90 % of all caves are secondary caves, and primary caves can thus almost be called exotic. Nevertheless, the term is still very widespread in the literature today and serves as a basic characterisation of a cave.
The term says nothing about the processes leading to the formation of the cave. The most important process is the removal of part of the rock, either by dissolution in dissolved form, or by weathering and erosion (removal in solid form). Tectonic caves, on the other hand, are formed by the movement of rock packages, and there are even quite exotic caves that are due to removal by animals, keyword animal construction. The number of caves of these different types is quite variegated. Well over 90 % of all secondary caves are formed by solution in combination with erosion.
The exact way, the rock is dissolved depends on the chemistry of the rock and the water, on the temperature and maybe the pressure. Karst caves are formed by surface water with outside temperature using CO2 as agens. For solution processes at depth with little influence by surface water, the term hypogenic cave has recently become established. Here, the chemical and biochemical solution based on sulphur is particularly important, and often the water is also warm or hot, which supports the formation of caves. Cave forming processes supported by high temperature are called hydrothermal processes.