Brazilian semi-ornamental rock basalt

Basalt is the commercial name of a family of peculiar volcanic rocks, which takes place specifically in southern region of Brazil, in the States of Rio Grande do Sul and Santa Catarina. They are applied widely to civil construction in special respects of ornamental use, often for floor, wall, and pavement.

In Brazil, the rocks for construction use are divided roughly into two categories: so-called "ornamental rocks", which are applied with surface polish, as granite and marble; so-called "semi-ornamental rocks", which are used without polish, as slate. In this sense, basalt is classified as a special group of semi-ornamental rocks.

Brazilian federal government and several state governments have recognised ornamental rocks as a part of natural subterranean resources, so they have published colour-paged catalogues of ornamental rocks in order to stimulate their production. In comparison with the ornamental rocks, the semi-ornamental rocks were not so attractive and were less known. Among the semi-ornamental rocks, slate is traditionally used in Brazilian construction industry. Recently, special types of gneiss are growing up in their production and consumption. In spite of the wide application to construction and great economic importance, the semi-ornamental rocks are still less evaluated. Especially, basalt is regional and specific rock product, so little is known even within Brazilian territory. The present article introduces the geologic setting, mining method, general use, ornamental application, and artistic use of the semi-ornamental rock basalt.

In the State of Rio Grande do Sul, south Brazil, there are early Cretaceous rhyolitic pyroclastic flow deposits characterised by ultra-high grade welding and extremely developed secondary flow. They are underlain generally by early cretaceous Parana Flood Basalt (about 125 Ma), but sometimes intercalate basaltic lava flows between their cooling units. The total thickness of the pyroclastic flow deposits is about 350 m at Atlantic coast region, but it becomes thinner to the west, and finally they disappear in an interior region of South American Continent. The eruption centre is supposed to be present in Namibia, West Africa. At least five cooling units of the pyroclastic flows have been recognised and each one is little more than 50 m thick.

The lithological aspects and construction material utilities of this welded tuff is highly variable within one cooling unit according to the level (Fig. 1). The basal part of a cooling unit is composed of glassy obsidian. In this part, the rocks are physically brittle, and vertical fractures are well developed. Therefore, the rocks are not suitable for noble construction material or road gravel. However, because of its deeply black colour (Brightness=18, after quantitative rock colour evaluation of Motoki), they are applied to the construction of multi-colour wall and ornamental pavement (Fig. 2). The middle-upper part of a cooling unit is of dark grey (Brightness = 45) and mechanically hard and solid. In some few localities, there are highly developed sub-horizontal parallel fractures originated probably from secondary flowage of the welded tuff. This part is useful for exploitation of rocks as construction material, and many quarries are in operation. The materials called commercially as basalt in the markets correspond to the rocks of this level. Because of weathering and consequent iron mineral decomposition, a part of the rock is coloured in light yellow to dark red. They are applied to semi-ornamental use (Fig. 2). However, in many places the above-mentioned sub-horizontal fractures are less developed, being useless for semi-ornamental rock exploitation. Therefore, the rocks are crushed to make high-quality gravel. At the top level of a cooling unit, the rocks became darker in colour and mechanically less resistant. They are extracted as low-quality gravel.

Before 20th century, basalt was extracted and used only for bulk block use, as territory division walls. Entering into the 20th century, commercial mining and civil engineering applications started, e.g. urban street pavement and wall construction. Nova Prata city, State of Rio Grande do Sul, is the centre of basalt production. At present, Nova Prata and the neighbouring 16 municipalities have more than 400 basalt quarries in operation. Most of them are of small family business scale, but a dozen of them are of large enterprise scale.