Lava tubes 1Formation of lava tubes
Lava tubes
Original title: Canais de Lava
Describes lava tubes (lava channels) in Madeira's recent basalt flows, explains how they formed, and lists examples such as the cave near the mouth of the Ribeira de São Vicente and the collapsed channel of the Fojo near Funchal.
1Formation of lava tubes
In various parts of Madeira there are caves hollowed out of basaltic lava, generally of the most recent date. They formed by a mechanism identical to the one that gave rise to similar formations in certain volcanic regions of the globe, such as the Azores and the Sandwich Islands.
A narrow stream of still-fluid lava, hemmed in by mud or scoria, slid down a slope or a ravine. Meanwhile its exterior, losing much of its heat by radiation, solidified, leaving the core at a temperature still high enough for its fluidity to let it keep moving.
The result was a channel in which container and contents were of the same nature, with the one essential difference of their unequal temperature. Once the pasty core had drained away, there remained this kind of volcanic conduit. The most perfect example in Madeira is the cave near the mouth of the Ribeira de São Vicente, which may measure some 80 metres in length and some 4 to 5 in width.
2Other examples and features
At Montado dos Pessegueiros (‘peach-tree pasture’), a locality in the same parish (freguesia) of São Vicente, and at Porto Moniz there are also channels of this kind, in flows of recent lava. On the south coast, both to the east and to the west of Funchal, traces of tunnels of this nature can be seen, most of them at sea level.
The Fojo, a deep chasm hollowed out of friable tuff behind the coastline and a little to the east of Ponta da Cruz (‘point of the cross’), appears to be the result of the erosion and collapse of a volcanic channel that ran there in remote ages and opened into the sea. Its ruins, now piled up in confusion as blocks of blackened lava, show smoothed surfaces with the long, parallel striations that characterise the walls of lava tunnels. The sea water, having passed through a nearby cave formed by marine erosion and having eaten away the rocks beneath the channel, is thought to have caused this collapse, helped by the rainwater that flows into the pit, wearing away the tuff and widening its extent.
Between the Fojo and Ponta Gorda (‘broad point’), not far from the Muralha dos Franceses (‘Frenchmen’s wall’), similar traces of other collapsed channels can be seen, surrounded, like that of the Fojo, by yellow tuff of volcanic mud.
Hanging from the roofs of some of these tunnels, like stalactites or resembling Prince Rupert’s drops, are curious excrescences of vitrified lava, testifying to the fluidity of the rock under the action of a very high temperature. Specimens of these excrescences, remarkable for their size and bizarre shape, were found in a cave at Ribeira da Janela (‘window stream’).
In this article
Place
Places mentioned · 11
Abroad
- Azores
Portugal. Cited as a volcanic region with similar lava tubes.
The Azores, an Atlantic archipelago that shares plant species with Madeira. The Elucidário records settlement there by Madeiran families such as the Bettencourts, the 1801 ban on Azorean wines in Madeira, and imports of Azorean tobacco. It also notes that Azorean farmers were brought to Madeira to teach maize-growing methods.
- Fojo
Funchal. Deep pit in tuff thought to be a collapsed lava tube.
The Fojo is a natural excavation west of Ponta da Cruz in the municipality of Funchal. It is a deep pit in tuff, linked to the sea by an underground channel, and is thought to be a collapsed lava tube. It is recommended as a coastal scenic spot.
- Funchal
Traces of lava tubes occur along the coast east and west of the city.
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.
- Montado dos Pessegueiros
Porto Moniz. Locality with lava tubes in modern lava flows.
Montado dos Pessegueiros is a locality in the parish and municipality of São Vicente, Madeira, with lava tubes in modern lava flows. It is one of the two remaining sites of the fern Dicksonia culcita (feto abrum) and the only one where it is frequent. It was suggested as a site, in the Ribeira do Inferno, for growing Cinchona succirubra.
- Muralha dos Franceses
Santana. Collapsed lava channels are seen nearby.
Collapsed lava channels are seen nearby.
- Ponta da Cruz
Funchal. The Fojo lies a little east of this point.
This Ponta da Cruz is a headland in São Martinho regarded as the western limit of Funchal bay. It was once the site of a small redoubt and lies near a fish-canning factory.
- Ponta Gorda
Santana. Remains of collapsed tubes lie between it and the Fojo.
Ponta Gorda is a point in São Jorge parish where a quay was built between 1904 and 1910. Remains of collapsed lava tubes lie between it and the Fojo.
- Porto Moniz
Location of lava tubes in modern lava flows.
North-western parish and municipality with a quay and a tolerable port that serves as a good landing when southerly winds close the south coast. It has one of the oldest Magdalene chapels in northern Madeira, whose feast falls on 22 July. It was the birthplace of Dr Manuel Gomes Jardim.
- Ribeira da Janela
Porto Moniz. A cave here yielded remarkable glassy lava drips.
Ribeira da Janela is a parish on Madeira's north-west coast in the municipality of Porto Moniz. It is named after the island's longest and most abundant stream, which rises on the Paul da Serra and feeds the Rabaçal levadas, and after a window-like islet near its mouth. Its uplands include Fanal, and its forests were damaged by fire in 1919.
- Ribeira de São Vicente
São Vicente. Near its mouth lies the most perfect lava tube in Madeira, about 80 m long.
Ribeira de São Vicente is a major stream rising on the plateau and crossing most of São Vicente parish to the sea. The São Vicente chapel stands on a basalt rock at its mouth, and near the mouth lies the most perfect lava tube in Madeira, about 80 m long.
- Sandwich Islands (Hawaii)
United States. Cited as a volcanic region with similar lava tubes.
Cited as a volcanic region with similar lava tubes.