Imagine that the level of the oceans is much higher than it is today and that much of today’s Europe is underwater. Extending along the present-day arc of the Carpathian Mountains from Silesia in Poland to the Iron Gate in Romania is the Miocene Sea. As a result of the cooling of the climate, the level of oceans decreases by about 60 meters and in the Carpathian Foredeep, between the Carpathian Mountains and the uplands of central Poland, a closed reservoir with very high salinity is created. The watercourse cuts deep into the land, forming narrow bays and lagoons, and rock mass movements and subsidence cause the seabed to drop steadily.
Under these conditions, about 13.6 million years ago, salt deposits were formed. Slow precipitation of salt deposits takes place – evaporation accompanied by intensive evaporation of water caused by dry climate. The progressing evaporation together with the subsidence of the seabed and the erosion of the coastal parts of the basin meant that more terrigenous material was supplied to the basin along with the sea waters, as a result of which excesses of gangue was deposited within the salt beds.
Saline sedimentation in the Carpathian Foredeep probably lasted at least 200,000 years. Deposits that were formed in the Carpathian Foredeep extend over a length of 300 km and width of 100 km. These also include the Wieliczka deposits, which stretch for 10 km in length, with a width from several hundred metres to 1.5 km.
The picturesque underground landscapes of the Wieliczka mine owe their beauty to nature, which sculpted the rocks in unusual, varied and complicated forms. It is worth knowing that the Wieliczka Salt Mine has a unique geological structure, not found anywhere else in the world. What caused this?
After the sedimentation of rock salts, there were strong rock mass movements associated with the uplifting of the Carpathians. The tectonic activity of the Carpathian Mountains led to the folding of salt layers, separating them from the ground and shifting them at least a dozen kilometres to the north. As a result, the salts were lifted up to the surface, which enabled their early discovery and exploitation.
The Carpathians, acting as a large bulldozer moving rock masses, formed the deposit into a very unusual structure. The basic feature of the Wieliczka deposit is its two-fold construction. It consists of two parts: the upper lump deposit and the lower bedded deposit.
The lump deposit is unique in the world. As the name suggests, it is made up of lumps of rock salt, which are characterised by a variety of sizes, from the size of a basketball, to huge blocks with a volume of up to 100,000 m3. After reaching the underground, the miners dug corridors in which they searched for lumps of salt, and when they found them, they extracted the salt, creating underground chambers. The most beautiful chambers in the mine are those exploited in lumps of salt, one of which is St Kinga’s Chapel which tourists visit on the Tourist Route.
The bedded deposit that makes up the lower part of the deposit was folded and formed in the form of tectonic scales collapsing to the south. The salt layers that make up this part of the deposit are different from each other. The most valuable among them is the so-called fore-shaft salt deposit, exploited since the first decade of the 16th century. One of the most beautiful chambers that belongs to this deposit is Pieskowa Skała – tourists visit it while exploring the Tourist Route.
An interesting experience, which we recommend to everyone who visits the “Wieliczka” Mine, is the comparison of the workings in the lump and the bedded deposits. A keen-eyed tourist, with a little help from a guide, will certainly notice the differences between them and will be able to recognise which chamber was carved in the lump deposit and which one in the bedded.
Types of salt
Rock salt is a common name for halite, a mineral with the chemical formula NaCl (sodium chloride). It is a rock which is characterised by an excellent cleavage, good heat conductivity and easy solubility in water. The purest types of rock salt are transparent or almost transparent. However, the rocks with the admixture of claystones take on a green colour – and such salt is the most common in the “Wieliczka” Salt Mine.
Experienced miners and guides can identify different types of salt at a glance, but this can be a difficult task for tourists. Therefore, we will present below the most interesting and most important types of Wieliczka salt:
Typical green salt – the most common type of salt in a lump deposit. The characteristic green colour is due to the admixture of claystones. The content of sodium chloride in it ranges from 93–98%.
Stained-glass green salt – belongs to the lump deposit, but is not as widespread as typical green salt. Sodium chloride content is about 30–80%, usually about 50%. In the old days, salt was not suitable for food and was not exploited due to the amount of admixtures. The name of this rock is taken from the similarity to a stained glass window, in which the “glass” is salt dissolved due to moisture in the air, and the “frames” are formed from claystones and anhydrites.
Fore-shaft salt – discovered in the 16th century, this is the most valuable rock salt from the bedded deposit. Its sodium chloride reaches 99%. It is honey-coloured. When broken with a hammer, it gives off a characteristic bituminous odour.
Spiza salt – occurs in the bedded deposit in many forms, including coarse grained, banded, laminated and grey. Their common feature is the admixture of sand, silt and claystones. The exception is the “eagle” spiza salt of unusual purity.
“Eagle” spiza salt – with an exceptionally small admixture of other minerals. In the olden days it was intended for the royal table. Its name is derived from the emblem of the Kingdom of Poland. The eagle’s mark was applied to barrels before they were delivered to the royal court.
“Cracking” salt – a very rare type of spiza salt. Natural gas is trapped between its crystals. When struck by a hammer, it breaks into small pieces and is accompanied by a silent crack. A similar sound can be obtained by dissolving it in water.
Secondary crystallisation salts
Secondary crystallisation salts are a very picturesque element of the underground landscape. Tourists can see various forms of this type of salt on the sightseeing route. It is worth knowing that they are younger than the rock salt in the Wieliczka deposit, some of which were formed in the geological times, millions or hundreds of thousands of years ago, and others are created in modern times.
Secondary crystallisation of salt is always associated with brine leaks. Depending on the prevailing conditions and the location of the leak, it can take a variety of forms.
The most common are “salt cauliflowers” – spongy growths of salt, which usually occur on the ceiling or on the wall of a gallery or chamber. Another form are the salt stalactites, which can be seen on the ceiling of the workings. Slightly less common are stalagmites and “salt stains”, which are formed only in places of more intense brine condensation. When salt takes the form of narrow dripstone tubes, it is referred to as “salt pasta” or the silvery “St Kinga’s hair”. Where drops of salt water fall down and form the so-called brine mist, “Christmas trees” are made of halite crystals that are not fully formed. “Salt growths” are formed in underground lakes and salt water reservoirs.