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Basalt is the viable name of a family of unusual volcanic rocks, which takes place distinctively 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 approximately into two categories:

  • "Decorative rocks", which are applied with surface polish, as granite and marble; so-called.
  • "Semi- decorative rocks", which are used without polish, as slate. In this sense, basalt is classified as a special group of Semi- decorative rocks.

Brazilian federal government and several state governments have recognized decorative rocks as a part of natural bottomless resources, so they have published color-paged catalogues of decorative rocks in order to motivate their production. In comparison with the decorative rocks, the semi-ornamental rocks were not so attractive and were less known. Among the semi-ornamental rocks, slate is conventionally used in Brazilian construction industry. Recently, special types of gneiss are rising up in their production and consumption. In spite of the broad application to construction and great economic significance, the semi-ornamental rocks are still less evaluated. Particularly, basalt is regional and specific rock product, so little is known even within Brazilian region.

The present article introduces the geologic setting, mining method, general use, decorative application, and artistic use of the semi- decorative rock basalt.

In the State of Rio Grande do Sul, south Brazil, there are early Cretaceous rhyolitic pyroclastic flows deposits characterized by ultra-high grade welding and tremendously developed secondary flow.

They are underlain generally by near the beginning 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 at last they vanish 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 recognized and each one is little more than 50 m thick.

The litho logical aspects and building material utilities:

  • The basal component of a cooling unit
    The litho logical aspects and building material utilities of this welded tuff are highly variable within one cooling unit according to the level. The basal element of a cooling unit is composed of glassy obsidian. In this part, the rocks are physically fragile, and vertical fractures are well developed. Therefore, the rocks are not appropriate for fine construction material or road gravel.

    On the other hand, because of its totally black color (Brightness=18, after quantitative rock color evaluation of Motoki), they are applied to the construction of multi-color wall and decorative pavement.

  • The middle-upper component of a cooling unit
    The middle-upper element of a cooling unit is of dark grey (Brightness = 45) and mechanically hard and solid. In some few localities, there are extremely developed sub-horizontal parallel fractures originated most likely from secondary flowage of the welded tuff. This component 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 resulting iron mineral decomposition, a part of the rock is colored in light yellow to dark red. They are applied to semi-ornamental use.

    On the other hand, in many places the sub-horizontal fractures are less developed, being useless for semi-ornamental rock exploitation. For that reason, the rocks are crushed to make high-quality gravel.

  • The top level of a cooling unit
    At the top level of a cooling unit, the rocks became darker in color and mechanically less resistant. They are extracted as low-quality gravel.

    Prior to 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.

Basalt production centre
Nova Prata city, State of Rio Grande do Sul, is the centre of basalt production. At present, Nova Prata and the neighboring 16 municipalities have more than 400 basalt quarries in process. Most of them are of small family business scale, but a dozen of them are of huge enterprise scale.

In case of decorative rocks, e.g. granite and marble, they are extracted from the outcrops of the quarries in blocks using diamond wire cutter, later they are sliced in 2 cm thick to make polished rock plates in large factories.

On the other hand, in case of basalt, the quarry mining and industrial reforming processes are widely different. At the level in which the horizontal parallel fractures are well developed, the rocks are mechanically firm, but not as strong as granite. Therefore, they are easily "cut" by manual works using a chisel and a hammer. Drilling machines and diamond cutter are unnecessary.

The parallel sub-horizontal fractures
According to the interval of the parallel sub-horizontal fractures, three different types of commercial products are shipped from the quarries. Where the parallel fracture interval is close, from 5 to 10cm, the rocks are cut to make square flooring stones (40 x 40 x 6cm), called locally "laje". The flooring stones are the most important products of basalt and applied mainly to building floor and urban street floor pavement. The pavement made up of basalt flooring stones with decorative pattern sometimes takes place in the cities of this region.

  • Parallelepipedo: At the stage in which the fracture interval is wider, from 10 to 15 cm, parallelepiped pavestones (10 x 10 x 20 cm), called in the vicinity "parallelepipedo ", and are mainly produced. They are, in fact, considered to be less fine products than the parquet (flooring) stones, and used for non-ornamental pavement of roads.
  • Alicerce: At the stage where the fracture interval is very wide, more than 15 cm, fundamental stones (18 x 25 x 50 cm), called in the vicinity "alicerce", are mainly produced. Moreover, the sub-horizontal parallel fractures are a fundamental factor for quarry installation and procedure.

During afore mentioned procedure, a large amount of rock morsels of rough shape are dejected. Some of these rock fragments are large enough; in this case, they are useful to special type of street floor pavement. The small fragments are crushed and shipped as gravel for common construction.

Some of the rocks extracted from the quarries are submitted to superior manual cutting in small manufactories to make high quality and just-sized flooring stones and wall stones. Few factories cut and slice them, using machines in order to make plates of 2.5 cm thick. The products are often used for floor and step with resin surface protection, called popularly varnish painting.

Slight-color rocks
General color of basalt is dark grey, but in rare cases, they are slightly colored in yellow, pink, green, blue, purple, etc. The color grade of these rocks is very low (40< Saturation <60), therefore they are classified to be "slight-color rocks".

Some civil constructors of Nova Prata apply them to make peculiar and beautiful color-mosaic walls. In a unique case, basalt is used for construction of wine ripening deposit, so-called "cantina" house. This rock fort maintains constant low temperature and high humidity environment of underground wine deposit during every seasons of the year.

Decorative objects
In addition to construction material use, basalt is useful to artistic ones. At Nova Prata, many city monuments are made up of basalt. Decorative objects, as table and chair, and small goods, as penholder and ashtray, are typical materials of the region. One of the extraordinary peculiar goods is rock "picture" of pseudo fossil.

Underground water penetrates along initial fractures of basalt and disseminates iron and manganese oxide and hydroxide in the fractures. When the rock-fracture is opened by a hammer strike, a picture-like natural art image appears.