Dykes
Original title: Diques
Explains volcanic dykes, walls of rock formed when molten material filled earthquake fissures, and lists notable examples across Madeira, such as those at Curral das Freiras, Cabo Girão, Garajau and Penha de Águia.
Dykes and their occurrence in Madeira
Dykes (diques) are walls of volcanic rock formed when molten material penetrated fissures opened by earthquakes in pre-existing ground. Where dykes are abundant in a locality, this shows that volcanic activity was felt there with great intensity.
In Curral das Freiras (‘nuns’ fold’) and other valleys of the interior of Madeira there are a great number of dykes, and at Cabo Girão there are both vertical and horizontal dykes.
Dykes sometimes taper to a point at the top. There are also some, such as the one at the hillock of Piedade (‘pity’) on the Ponta de São Lourenço, that are cut off or intercepted at the base by horizontal strata.
The basalt dyke that forms a knoll (cabeço) or nipple at the top of Garajau (‘tern’) can be made out on either side of this cape, and is double on its eastern side. There are also double dykes at Ribeiro Frio (‘cold brook’), on the seaward side of Penha de Águia (‘eagle crag’) and elsewhere, which probably result from a fresh penetration of lava into the same fissures.
Some dykes stand isolated because the surrounding ground has been destroyed by denudation. The peaks of Empinos, near Curral, are formed of isolated dykes, and at Ribeiro do Juncal (‘rush-bed brook’), at the head of the Ribeira da Metade (‘halfway stream’), there is a dyke in the same condition, about 45 metres high and 18 wide.
It is supposed that in most of Madeira’s dykes the molten material was forced upwards from below. In the Piedade dyke already mentioned, however, and in another at Ribeiro dos Piornais (‘broom-thicket brook’), between Caniço (‘reed’) and Funchal, the opposite seems to have occurred, since both are interrupted or cut off at the base.
In this article
Place
Places mentioned · 13
- Cabo Girão
Câmara de Lobos. Sea cliff showing both vertical and horizontal dykes.
Sheer sea cliff about 600 m high (listed at 589 m) and a recommended viewpoint. Its cliffs contain dykes, and it is one of only two places where C. dissectum grows. A rockfall from it killed Henrique Alemão in his boat.
- Caniço
Santa Cruz. Reference point for locating the Piornais dyke.
Caniço is a parish in the municipality of Santa Cruz, lying on the boundary between the captaincies of Funchal and Machico. Its early settlers included Gaspar Rego, Vasco Fernandes Rego and Álvaro de Ornelas, whose family founded morgados there; its alcaide and judge were appointed by the Funchal senate. The parish includes the Garajau statue, the Jesuits' Socorro residence and the Quinta do Barão da Conceição.
- Curral das Freiras
Câmara de Lobos. Interior valley with a great number of dykes.
Small inland parish set in a deep valley believed to be an extinct volcanic crater and Madeira's main eruptive focus. The estate was bought for the nuns of Santa Clara, who fled there in 1566 and gave it their name, and it was created a parish in 1790. It was earlier a pasture and a refuge for runaway slaves and criminals, and it became a julgado of the Funchal comarca in 1927.
- Empinos
Peaks near the Curral formed by isolated dykes.
Peaks near the Curral formed by isolated dykes.
- Funchal
Reference point for locating the Piornais dyke.
The capital of the archipelago and seat of the principal captaincy, where settlement began; it was made a vila c.1451–52. Its bay, the widest in the archipelago, lies between Ponta do Garajau and Ponta da Cruz and offers sheltered anchorage, though its harbour had never been studied oceanographically. The Elucidário discusses it as a health resort with climate data and records the January 1906 disturbances, the Danish prince's visit in May 1846 and recent volcanic cones in the area.
- Garajau
Santa Cruz. Cape topped by a basalt dyke forming a knoll, double on its eastern side.
Garajau is a headland closing Funchal bay to the east, 2.9 miles from Funchal, named after the terns found there by the discoverers. It is topped by a basalt dyke, bears a chapel of Nossa Senhora das Neves and the 1927 Sacred Heart statue, and had a telegraph from 1839 to 1847. It is noted for volcanic bombs, a surviving dragon tree, the only wild norça, Senecio incrassatus and deep-water shells, and is recommended as a viewpoint.
- Penha de Águia
Santana. Seaward face shows double dykes.
Truncated-pyramid sea cliff nearly 600 m high on Madeira's north coast, praised for giving its name to a valley of unrivalled beauty. Stuebel considered it the remnant of a destroyed secondary crater, and its seaward face shows double dykes. It is a breeding site of the boieiro and gave its name to an entail.
- Piedade
Calheta. Hillock on Ponta de São Lourenço whose dyke is cut at the base by horizontal strata.
Piedade is an inhabited locality in the parish of Jardim do Mar, Madeira. The Elucidário records there the chapel of Nossa Senhora da Piedade and the manor house of the Couto Cardoso entail, founded in 1730 and rebuilt in 1825.
- Ponta de São Lourenço
Machico. Headland where the Piedade dyke lies.
The Ponta de São Lourenço is the long, narrow eastern headland of Madeira in the parish of Caniçal, said to be named after the caravel São Lourenço, with a lighthouse on its islet. The Elucidário describes its geology, seasonal vegetation, islets, fossil calcareous bodies at Piedade, land snails and use as pasture. The steamer Forerunner sank there in 1884, and a submarine bank lies between it and the Desertas.
- Ribeira da Metade
Santana. The Ribeiro do Juncal lies in its upper reaches.
The Ribeira da Metade is a stream on Madeira and a tributary of the Ribeira do Faial; its valley is overlooked by the Ribeiro Frio viewpoints, and the Ribeiro do Juncal lies in its upper reaches. It marked one limit of Lançarote Teixeira's land grant. The rare grasses Festuca albida and Anthyllis lemanniana grow in its ravine, and it was suggested as a site for growing cinchona.
- Ribeiro do Juncal
Machico. Has an isolated dyke about 45 m high and 18 m wide.
Has an isolated dyke about 45 m high and 18 m wide.
- Ribeiro dos Piornais
Funchal. Stream between Caniço and Funchal with a dyke cut at its base.
Stream between Caniço and Funchal with a dyke cut at its base.