Basalts
Original title: Basaltos
Describes basalt, the dark, hard volcanic rock common in the lower parts of Madeira, its three local varieties and their use in building. It also notes basalt columns and dykes visible on Madeira, Porto Santo and the Desertas.
Dark, compact and hard volcanic rocks, formed of labradorite feldspar and augite. They are relatively poor in silica but contain more lime and magnesia than trachyte, which is rich in silica.
According to the geologist James Smith, of Jordan Hill, three kinds are known in Madeira: compact basalt (pedra viva (‘live stone’: compact basalt used for building) or alvenaria (local name for compact basalt used as building stone)), vesicular basalt (cantaria rija (hard dressed stone of vesicular basalt)) and scoriaceous basalt. Tuffs often alternate with the basalts, and compact basalts are frequently seen resting on layers of scoriaceous basalt. When it occurs in thick layers, compact basalt often splits vertically into columns of polygonal section, as is often seen in quarries and can also be observed near Gorgulho (‘weevil’), at Tem-te não Caias (Porto da Cruz) and at Faial.
Compact basalt is used in every kind of building, although the hardness of its grain means it cannot be dressed.
Vesicular basalt is found mainly in the neighbourhood of Câmara de Lobos (‘seals’ den’), and is used throughout the island for door jambs, cornerstones, steps, etc.
Scoriaceous basalt occurs everywhere, often interspersed with compact basalt, but it is not used in building.
The basalts, very common in the lower zone (coastal belt) of Madeira and also found on Porto Santo (‘holy harbour’) and the Desertas (‘deserted islands’), generally contain crystals of augite and olivine, the former black, the latter greenish. They often occur as dykes, vertical or oblique, cutting through layers of conglomerate or of basaltic rocks that have split.
In this article
Place
Places mentioned · 07
- Câmara de Lobos
Main area where vesicular basalt used for door frames and steps is found.
Parish west of Funchal. The Elucidário traces its name to Zarco's discovery voyage and describes its churches, chapels and collegiate chapter, early settlers, fishing community and renowned vineyards. It also records the parish's localities, boundaries, levadas and population.
- Desertas
Santa Cruz. Islands where basalts also occur.
An island group near Madeira that was already separated from it at the last eruptions and shares endemic species with it. It was the main orchil-harvesting area, and Wollaston found 89 beetle species there. The islands were investigated by scientific expeditions, including the Endurance scientists in 1914.
- Faial
Santana. Parish where columnar basalt can be seen.
Faial is a north-coast parish of Santana municipality created in 1550, whose mountain springs feed the Levada da Serra. It contains Chícharos and the Diferença chapel, owns the eastern part of Pico Ruivo, and is noted for cheese-making and columnar basalt.
- Gorgulho
Funchal. Locality near which columnar compact basalt can be observed.
Coastal locality and islet west of Funchal in São Martinho, with a ruined fort. It has lava flows containing rock pools, columnar basalt nearby and a municipal seawater pool built in 1932. It was the planned site of a glass and tile works.
- Porto da Cruz
Machico. Parish containing the Tem-te Não Caias basalt columns.
North-coast parish created in 1577 and noted for good-quality wines. It was transferred from Santana to Machico municipality in 1852 and was the seat of a capitania-mor. It was also a fishing port where some sperm whales were landed.
- Porto Santo
Island where basalts also occur.
An island of the archipelago, settled alongside Madeira and forming one of the three captaincies. It has always constituted a single parish, dedicated to Nossa Senhora da Piedade, whose church was repeatedly sacked. The Elucidário records the Moorish pirate raid of 1617, its own Misericórdia and a rainwater channel built on this drought-prone island.
- Tem-te-não-caias
Machico. Site in Porto da Cruz where columnar basalt is visible.
Site in Porto da Cruz noted for the most typical columnar basalt wall on the island.
People
People mentioned · 01-
Scottish geologist of Jordan Hill whose classification of Madeiran basalts into compact, vesicular and scoriaceous types is followed in this entry.