Calcareous tuff

Toba columns at Lake Mono, California.

The calcareous tuff, travertine, or rough stone, the latter term in the field of industrial rocks, is a limestone rock very porous, a variety of silt, formed by the precipitation of carbonates from bodies of fresh water at room temperature, on numerous occasions on living plants.

Tuff formation cycle

Process of calcification of the moss generating toba.

Tobaceous building active, growing. Rock formation "Christmas Tree" (Cañón del Sumidero, Chiapas, Mexico).

Rainwater is poorly mineralized and contains a very low amount of carbon dioxide (CO₂). As these waters cross the ground, they are charged with CO₂ produced by the biological activity of plants and bacteria, and can dissolve calcareous rocks. Thus, highly charged CO₂ water can dissolve the calcareous rocks of the aquifer during its underground journey. It then takes with it dissolved calcium (Ca²⁺) and carbonate (HCO₃^-) ions.

When it comes out of the subsoil in springs, waterfalls or rivers, it is when the water degasses (loses CO2), due to turbulence or due to the biological action of plants when using CO₂ in photosynthesis, and precipitates calcium carbonate (CaCO₃) in the form of calcite.

2HCO3− − +Ca2+▪ ▪ CaCO3+CO2+H2O{displaystyle 2HCO_{3}^{-}+Ca^{2+}rightleftarrows CaCO_{3}+CO_{2}+H_{2}O}

The small crystals are deposited in the form of a calcareous crust on the plants present in the source, spring or river. They are mainly the mosses, stems or any other vegetable that serve as support. The superposition of these successive layers forms the rock called tuff.

When the vegetal support dies and disappears, it leaves the place it occupied before empty and the negative of that vegetal remains on the rock, which is responsible for the cavernous-looking porosity of the tuff.

The calcareous tuff is brown (in the Levantine area of Spain) or cream (in the Balearic Islands). In construction it is used as an ornamental rock. It is generally found in coastal areas of the Valencian Community (Costa Blanca), Murcia, the Balearic Islands... and more specifically forming "beaches" next to the same sea. Its way of extraction is similar to that of marble, carving and forming large blocks where they are later prepared according to commercial needs.

It is also widely distributed throughout the peninsula associated with river and lake systems. It abounds especially in the Alto Tajo, in the area of Puente San Pedro, in the Guadiana Alto, in the area of the Lagunas de Ruidera and in the Alto Ebro, where tuff is the etymum of many place names and some localities: Valle de Tobalina, Tubilla del Agua, Toba de Valdivielso, Tobera, Tubilleja, etc.

It should not be confused with pumice or volcanic or rough tuff, common in the Canary Islands, made up of volcanic materials that when emitted hot, weld together and form a porous stone of medium consistency and color from red to yellowish white.

Uses

Side facade of the Church-Fortaleza of Sant Bertomeu in Javea (Alicante), built the entire building in Tusca stone.

This rock is used as a construction material, in the form of ashlars, reaching sizes of 40 × 20 × 50 cm, and a weight of 30 to 40 kg, but due to its porosity, it filters a lot of moisture, causing times the appearance of mold on the walls (a stone of this size will double its weight if water leaks). Due to its easy handling and to the fact that it has a mineral formation that breaks up when struck, it is used as ornamentation such as an arch in a portal or arches in an open area similar to a porch or naya, pillars, chimneys (arch of the main opening), etc. There is the trade of "tosquero": stonemasons, almost sculptors, who with the appropriate tools prepare the pieces of tosca that can form an arc, giving them the necessary curvature and adding, according to customer requirements, volutes, raised edges to form the full arc. An arch (or formwork) is formed by equal even pieces (one piece for each side of the arch of equal size and turn) closed by a single piece called a "clave", which is the only one that the mason who places it can be cut for final fit.

For better protection and to prevent moisture from seeping in, a certain type of varnish can be applied, such as that used for marine vessels, to fill in the pores and cover the stone with a light plastic layer that does not make it lose its showiness.

On the Costa Blanca, its extraction without authorization is currently prohibited because it is located in areas of public domain (Coast Law), importing that from the Balearic Islands, which is lighter (cream or pale brown) and more fragile than the Valencian