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Basalt, a peculiar family of Brazilan porphyry

Motoki, A.*, Vargas, T. *, and Zucco, L.L.** 2004.

* Departamento de Mineralogia e Petrologia Ignea, Universidade do Estado do Rio de Janeiro (DMPI/FGEL/CTC/UERJ)
** Colégio Téecnico da Universidade Federal Rural do Rio de Janeiro (Ctur/Ufrrj)

English version text with the fieldwork data updated on July 30, 2004, based on bilingual article in English and Spanish: Motoki, A.; Vargas, T.; Zucco, L.L. (2003). El basalto, piedra semi-ornamental brasileña. Litos, 66, 52-63. May. 2003, exposed at

Basalt is the commercial name indicating a peculiar variation of porphyry volcanic rock, which is extracted as construction material from south most of Brazil, States of Rio Grande do Sul (Fig. 1). They are applied widely to civil construction in special respects of ornamental use, often for floor, wall, and sidewalk pavement. In spite of the commercial name, this rock is not mafic and basic basalt, but felsic and acidic rhyolite or dacite. To distinguish from scientific name, the commercial name “Basalt” is written in capitalised Italic.

Domestic mining of the Basalt started at the beginning of the 20th century according to the arrival of Italian immigrants to the region of northwest area of the state of Rio Grande do Sul. In decade of 1950, the mining grew to commercial scale in order to provide construction material for urban street pavement and wall construction. After 1980, the Basalt mining and related rock-processing industries increased notably. At present, Nova Prata and the neighbouring 16 municipalities have more than 300 Basalt quarries in operation. Most of them are of small family business scale, but a dozen of them are of large enterprise scale. According to the information of the Syndicate of Quarry Extraction and Industry of Nova Prata and Neighbours, the annual production of 2002 was 15500 m2 in square tile flagstone, the typical product of the Basalt.

Brazilian federal government and several state governments have recognised natural stones, regionally called “ornamental rocks”, as a part of natural subterranean resources, publishing colour-paged catalogues of rocks in order to stimulate their production. In addition of granites and marbles, quartzites, slates, mylonite, and Basalts are less know, but they have notable economic contribution, especially in exportation to Europe.

In this region, there is felsic volcanic formation underlain by early Cretaceous Paraná Flood Basalt (about 135 Ma), namely Plamas Unit. This unit is made up of pyroclastic flow deposits characterised by ultra-high grade welding and extremely developed secondary flow. The horizontal extension of this felsic volcanic rock layers is about 150,000 square km and the maximum flow length is about 500 km, being a largest pyroclastic flow formation of the world. The total thickness is about 400 m at Atlantic coast, but it becomes thinner to the west and finally disappears in interior region of the continent. The eruption centre is supposed to be present in Namibia, West Africa. During eruptions of the early Cretaceous, the Atlantic Ocean was not present yet, and South America was in direct contact with Africa. About eight cooling units of the pyroclastic flows have been recognised and each one is little more than 50 m thick, which corresponds to one large volcanic eruption. The Basalt ore takes a part of this welded tuff unit.

The lithological aspects and construction material utilities of this welded tuff are similar to the other variations of porphyry, being highly variable within one cooling unit according to vertical level (Fig. 2). The basal part of a cooling unit is composed of glassy rock. Because of relatively brittle behaviour, this rock is mechanically not suitable for construction material. However, its deep black and dark red colours make them possible the application to multi-colour wall. The black rock shows the lowest brightness, B=18 according to colour specification by computer, being of the darkest Brazilian ornamental rocks.

The middle-upper part is highly devitrified, being composed of fine-grained rock, therefore mechanically hard and firm, with excellent characteristics in mono-axial compression (325 MPa), bending (40MPa), water absorption (1.37%), and surface hardness (7 in Mohs). General colour of this rock is dark grey, B=45. In some localities, such as Nova Prata, Paraí, Casca and Vila Flores, 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 by handwork, using chisel, hammer, and crowbar without heavy machinery (Fig. 3), therefore many quarries are in operation. Such a geologic condition is limited within uppermost one or two cooling units of the pyroclastic flow deposits.

According to the interval of the parallel fractures, 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 tile flagstones (30x30cm, 35x35cm, 40x40cm, and 50x50cm; Fig. 4), applied mainly to building floor and urban sidewalk ornamental pavement (Fig. 5). The surface of the flagstones sometimes show dendritic pseudofossil texture. The irregular shaped tabular rock fragments are cut to make small square tabular stones for garden pavement, so-called cube (Fig. 6). Because of weathering and heterogeneous devitrification, same rocks are slightly coloured in yellow, green, blue, purple, and pink (Fig. 7). Because of low colour saturation, S=5, they are classified to be slight-colour rocks.

At the level in which the fracture interval is wider, from 10 to 15 cm, parallelepiped pavestones (10 x 10 cm, 10 to 15 cm thick) are mainly produced, and used for street general pavement. Where the fracture interval is very wide, fundamental stones (25 x 50 cm, 10 a 20 cm thick), called locally "alicerce", are produced. In any case, the sub-horizontal parallel fractures are a fundamental factor for traditional quarry installation and operation.

In massive outcrops, such as of Garibaldi, Bento Gonçalves, Farroupilha, and São Marcos, large blocks are extracted and they are sliced and polished to make ornamental rock slabs. Their deep green or deep violet in colour and flower-like spherulite texture is highly characteristic. Such type of quarries occur stratigraphically in one or two cooling units of lower level.

In a special case, the Basalt is used for construction of wine ripening deposit, so-called "cantina" house (Fig. 8). This rock castle maintains constantly of low temperature, high humidity, and dark environment in underground wine deposit during every seasons of the year.

In addition to construction material use, Basalt is applied to artistic ones. At Nova Prata, many city monuments are made up of Basalt. Ornamental objects, as table and chair (Fig. 9), and small goods, as penholder and ashtray, are characteristic materials of the region. One of the special peculiar goods is rock "picture" of pseudofossil (Fig. 10).

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