STALACTITES, STALAGMITES AND SPELEOTHEMS IN GENERAL.
The dissolving of the rock by underground waters in the presence of carbon dioxide (CO2) is the process that has led to the formation of the cave (Karstic modelling). This same chemical process is responsible for the cave’s exuberant natural decor (stalagmites, stalactites, etc.) due to the fact that this chemical reaction is reversible, meaning it can go in one direction or the other. So, the same calcium carbonate that has been dissolved by the infiltration of water is deposited inside the cave when the chemical reaction is reversed.
This is how spectacular limestone deposits are formed, such as the well-known stalagmites (that grow on the ground), stalactites (that hang from the ceiling), columns (joining of stalactites and stalagmites), flowstones (layers of limestone that have developed on walls and/or floors), and the hangings or flags (that create twisted shapes that hang from the ceiling). All these precipitated limestone rocks are known by geologists as speleothems. All these speleothems, that we could define as classics, grow following vertical paths imposed by the force of gravity, which controls the dripping and flow of the filtering water.
It is also worth noting a speleothem that is quite rare within the Earth's Karst caves. These are called discs or shields (circular sloping surfaces) and they are plentiful in the Caves of Artá, reaching some truly spectacular sizes. Some examples can be seen in the vicinity of the Queen of Columns, as well as in the Theatre and Hall of Flags.
Equally, the bulb-like crystalline overgrowths are of great interest. They can be seen in some of the halls, such as Purgatory, where you can find the formation known as the Baptistery, or the sector comprising the Theatre and the entrance to the Hall of Flags. These speleothems feature horizontal lines corresponding to episodes of phreatic flooding of the cave, which occurred during phases of high sea levels that took place throughout the Upper Miocene and Pliocene periods, between 3 and 6 million years ago.
Visitors often ask about the growth rate for these speleothems. This is an aspect about which it is hard to generalise although, in general, they usually grow at a rate of less than a few millimetres per millennium. However, no significant formations can be seen in the Artá caves which are still in the growth stage. In contrast, the deposits are quite ancient and inactive. They were formed at least in the Lower Pleistocene period or in the Pliocene period, and in some cases in the Upper Miocene period. This is the case for some examples that can be found coated by the above mentioned phreatic precipitated limestone.
In conclusion, there are many physical and chemical factors that influence the formation of speleothems. The most important factors include pluviometry (abundance or lack of rain), temperature, concentration of CO2 in the cave's atmosphere, etc. These factors control both the scale of the ornamentation of the speleothems and the speed at which they form.