There are mysterious natural phenomena that can be seen all over the world. The Moeraki Spheres are huge boulders found along some coastal areas of New Zealand. These rocks are formed from the sediments of the seabed that have aggregated together due to a process similar to that of the creation of pearls.
The spheres are millions of years old and, given their workmanship, they are simply a wonder of nature. These are concretions, a common geological formation in marine environments, lakes, or where water is present, such as in caves.
In geology concretions are defined as the formations, generally calcareous, that calcium carbonate forms in solution by depositing itself in the crystalline form of calcite. The discipline that provides useful data to scholars on concretions is speleology.
There are different types of these formations that are called with various terms, some of which are best known: stalactites, stalagmites, columns, crystals, tubs, coralloids, pisolites, eccentric. Many sectors of science are used for research in the environmental field and, in the context of caves, the most effective element of is certainly represented by concretions: they represent the richest and most reliable archive of information on the Quaternary.
From a chemical point of view, their formation always follows the same process: the water circulating in the microcracks and porosities of the rock contains numerous dissolved salts, mainly calcium bicarbonate. This is a relatively very slow circulation under conditions of pressure and absence of air.
When the water comes into contact with the voids of the cavity and with the air, it finds itself in physical conditions of lower pressure and the calcium bicarbonate is transformed into carbonate. Pure calcium carbonate is white, and in fact there are very white concretions.
Often, however, the concretions contain other salts that give characteristic colors. The presence of mineral impurities, although the main cause of the coloring of concretions, is not the only factor: other causes may be the structure of the crystal lattice or the presence of organic materials and / or contaminants.
The concretions have a very slow growth. Their evolution is highly variable and depends on many factors: quantity of water, degree of saturation, genetic mechanism, climate and morphology of the karst system and much more.
Growth typically ranges from a few microns / year to fractions of a millimeter under normal conditions. In environments such as thermal caves, faster growths are recorded, up to 100 mm / year. The concretions of sulphates and chlorides grow faster and at a variable rate.
The growth of ice concretions depends on the speed of water dripping and the air temperature. The concretions are formed both in a vadose and in a flooded environment. The growth of various types of concretion depends on the conditions of formation, and on the deposition mechanism.