Geology

The formation of the islands that make up the Madeira archipelago goes back to the Tertiary era, in the Miocene. Submarine uplifts formed columns of emergence, in which volcanic activity continued into the earliest Quaternary. In successive periods of upheaval these altered the islands’ outline, Madeira and Porto Santo (‘holy harbour’) branching apart at a depth of 2,000 m and surfacing in the Atlantic 21 miles apart.

The absence of well-defined craters, long since collapsed or reshaped by natural agents, is a characteristic proof of their long period of formation.

On Madeira the main eruptive focus lay in the central part of the island, at Curral das Freiras (‘nuns’ fold’). It was immense and violent, producing partial reliefs in stages that rose from 300 m to 600 m and from there to 1,200 m in a great deluge of lava. This marked and defined the backbone of the central chain, piling up scoria engulfed in fresh torrents and rising progressively to altitudes greater than those the island shows today.

Adventitious and secondary vents opened, some partly within the central massif, others breaking out at points far distant from it. The outer slope of the cones is at times very steep, so that the lavas do not always solidify in regular layers, and instead scatter as blocks and fragments.

Long periods of quiescence were followed by greater paroxysms. During one of the spells of calm, soils formed through the natural breakdown of the rocks, and some plants were thus able to grow: this early flora was discovered in the lignite of São Jorge, buried by later volcanic emissions. In one of the periods of upheaval came the uplift marked by the marine fossils of São Vicente, lodged at the head of an erosion valley at an altitude of 360 m, where they form an enclave.

The first subaerial formations appear in the Pliocene, with a great outpouring of basaltic lavas radiating from the original focus. Sometimes denser, sometimes more fluid, they solidified more quickly at the surface and were covered with scoria in a flexible crust that cracked and bristled. They spread further to the north and were more broken up to the south, running and flowing down different gradients that tend towards the horizontal near the sea.

A new basaltic period followed, with a great projection of scoria and the formation of conglomerates, which blocked and diverted the lava flows discharged irregularly in every direction.

The trachytic lavas followed the basaltic ones after Madeira had reached a moderate relief. Fissures and vents opened in them, determining the mountain chain along the island’s spine. North of the Curral these lavas rose with a high proportion of alumina, covering depressions and accumulating between obstacles.

Trachydolerites alternate with trachytes and extend chiefly to the west and north. It is worth noting that lavas of this kind in the Boaventura (‘good fortune’) valley are younger than those of the upper formation of the Curral.

The essexites, which welled up from adventitious fissures and were forced out by pressure from great depths, look like ancient rocks but are recent in formation. They are found embedded at Porto da Cruz (‘harbour of the cross’), the Curral and Ponta do Sol.

Many peaks in the interior of the island are embedded in lavas younger than those that formed them, which run along their feet and slopes. Others were drowned in lavas and in blocks fallen from the nearest calderas, as can be seen in the sections cut by the Socorridos and Boaventura streams.

The main crater of formation, which at its widest measures 2,500 metres in diameter and is broken in its collapsed southern segment, arched from west to east through the peaks Grande, Empinos, Arranhamento, Jorge, Casado, Torrinhas, Lage, Eirinhas, Coelho, Ruivo, Torres and Cidrãos Cedro. The adventitious vents within the same crater gave rise to the peaks of Roda, Meio and Cerro, and to the upland commons (montados) of Cidrão and Paredão.

The Paul da Serra (‘upland marsh’), considered until recently to be a crater choked by internal collapse (the opinion of Stuebel), is seen in the most recent studies as a lava bank, with tuffs and scoria identical to those of the Lombo Gordo (‘broad ridge’) formation in the Curral (the opinion of Gagel).

Indeed, a line of heights with small depressions carved by erosion links these two points by way of Bica da Cana, Pináculo, Monte Medonho, Rocha Negra and Pico do Ferreiro.

Had all the volcanic activity been concentrated in the central caldera, Madeira’s highest point would rise, at Pico Ruivo (‘russet peak’) or another peak, to an altitude of 2,500 metres.

The last volcanic surge appears outside the central zone: at the ends of the island (Porto Moniz and Caniçal), after the Desertas (‘deserted islands’) had already separated, and in the southern region, at Câmara de Lobos (‘seals’ den’), Funchal and Caniço, where the most recent cones are found.

Basalts

Basalt is a rock of uniform appearance, blackish, heavy and hard.

Its variations in colour and hardness depend on its accessory elements and on its former state of fluidity. It occurs in large beds, ranging from thin layers to enormous rock masses, lavas from different periods of formation, and it predominates in the lower region.

Basalts show contraction cracks caused by cooling, and they break into large prismatic blocks of polygonal section, mostly irregular hexagons, as can be seen in quarry workings. These masses, stacked in long prisms, form handsome walls of columnar basalt, which are common on the banks of deep streams; the most typical face is that of Tem-te-não-caias (‘hold on, don’t fall’), in the valley of Porto da Cruz.

Where the flow is thinner and gently sloping, lamellar basalt is found, contracted into parallel faces that come apart under the violent blow of the pick. Columnar basalt is called pedra viva (‘live stone’: compact basalt used for building). Its density varies, so it needs different charges of explosive to be split into blocks, which are then partly dressed for use as building material. Lamellar basalt yields the slabs used for kerbs, levadas (irrigation channels), washing places, paving, etc. The flagstones of the churchyard at Monte were taken from a quarry, now worked out, which, from the nature of its rock, gave its name to the locality (sítio) of Lajinhas (‘little flagstones’). These basalts cannot be carved.

When basalt is fractured, it is sometimes found to be covered by a coating or thin greyish layer, caused by the alteration of iron salts. This creates a line of least resistance, the casco (‘shell’), which masons strip off, and they reject such basalt for more important work, such as tanks, etc.

Basalts contain small bubbles formed by gases, whose arrangement and shape indicate the direction and speed of the flow. In larger, egg-shaped bubbles, geodes sometimes form, in which aragonite, exuded from the basaltic mass as it cooled, crystallises. It is known as madre de pedra (‘mother of stone’) because it is found inside the rock.

When it is full of a great number of vesicles, basalt is known as pedra favada and pedra de porco (‘pig stone’: rough vesicular basalt used to scrape pig hides), from the use made of it at pig-killing time to scrub the animal’s hide after singeing and strip it of its bristles.

Fine-grained, somewhat vesicular basalt forms the hard dressed stone, which can be worked well and is widely used in buildings, coats of arms, bridge voussoirs, millstones, hand mills, large troughs, etc. Fine carved work can be seen on the façade of the chapel of São Luís (St Louis), in the old Rua do Bispo, and on the portico of the Angústias (‘sorrows’) cemetery.

Trachytes

These are rough rocks of microlitic texture and a certain fluidity. They sometimes look like spheroids decomposed in concentric layers, like a tunicate bulb, and are dirty brown or greyish in colour. They form an amorphous matter of feldspathic elements and contain, visible to the eye, small scattered crystals of sanidine and magnetite.

Trachytes have been confused with porphyry and, when truncated, with volcanic bombs. In Madeira, trachytes are found in the zone above 300 m in altitude, where they decompose, as can be seen in road cuttings, showing a darker nodule.

Trachydolerites

They show a very characteristic appearance of former fluidity, as can be observed in a lava flow in the valley of Boaventura. They are richer in alkalis than trachytes and sometimes contain granules of peridot.

Their colour varies from light to dark grey, passing through yellowish tones, speckled with lighter dots. Trachydolerites alternate with trachytes at some points in the high region and on the slopes of Pico do Serrado, Rabaçal and Porto da Cruz.

Essexites

These are plutonic rocks, composed mainly of plagioclase, olivine and magnetite. Two kinds are distinguished: porphyritic essexite, dark grey to brownish, dotted with magnetite and diopside of greasy vitreous lustre, as at Penha de Águia (‘eagle crag’), the Massapez brook and Ponta do Sol; and melaphyric essexite, grey, porous and fine-grained, with relatively large crystals of augite and microliths of olivine, as at Ponta Delgada, Ribeira Brava and the stream of São Vicente.

In the Soca brook, at Porto da Cruz, the first bed of essexite looks different from other rocks of the same nature, not only those found in Madeira but all others known. For this reason Dr Gagel, in view of the quantitative and qualitative analysis of this rock, and having consulted the greatest authorities on the subject, gave it the name madeirite, and included a fine colour microphotograph of it in his valuable work Studien über den Aufbau und die Gesteine Madeiras.

Dykes

During the period of upheaval the rocks cracked, and fresh lavas and scoria filled the open fissures, forming veins between the walls of the fracture. Where these walls were less resistant than the intruded mass, they broke down under the action of the weather, leaving the vein unsupported and exposed.

Exposed dykes predominate in the central region. The jagged outline of the Torres (‘towers’) and Torrinhas (‘little towers’) resembles the ruins of fantastic castles, an immense curtain wall (cortina, the defensive wall of the city) toppled by the forces of nature. Like a lone sentinel, on the way up to Pico Ruivo, one comes upon the picturesque dyke known as the Homem em Pé on the Achada do Ferreiro (‘the blacksmith’s plateau’), at an altitude of 1522 m; it seems to grow taller as the ground at its base erodes.

Among the many dykes that cut through the coastal rocks, those set into the sheer walls of Cabo Girão deserve notice, among them the Poço do Eiró, which is perfectly vertical.

The arrangement of the dykes and their transverse, convergent and even intersecting directions reflect the effect of different centres of disturbance at widely separated times, which often had their foci at points now submerged beneath the sea.

Lava channels

The lavas cooled more quickly at the surface, and where they touched the walls and bed over which they flowed, than in the interior, which therefore stayed fluid for longer. As a result they became hollow when the flow ended, forming underground channels. The most characteristic lava channels are the Fojo (a collapsed channel), near Ponta da Cruz (‘point of the cross’); those of Cavalum, on the Ribeira de Machico (‘Machico stream’); and that of Cardal, beside the bridge of the town (vila) of São Vicente.

Volcanic bombs

These are contorted pieces of lava, either hurled out by the calderas, in which case they are pear-shaped or lenticular, or forced through fissures, in which case they look like dough pressed through a strainer. They occur among conglomerates and, singly, near the most recently formed cones. Olivine bombs are fragments of a peridotitic rock lying at depth and torn loose under pressure in volcanic explosions; they are bound up with basalts or form conglomerates, as is most often found at Porto Moniz.

Tuffs

These are consolidated volcanic muds, highly variable according to the nature of the materials that formed them, and generally scoriaceous.

The commonest tuffs yield pedra mole ('soft stone': soft yellowish tuff and its soil), yellowish in colour; lime predominates in the paler kinds, while the darker ones contain oxidised iron salts. It is used only for rough infill and retaining walls.

Soft cut stone (cantaria mole) is coarse-grained, heterogeneous, and greyish-brown or dark magenta. Filter stone (cantaria de filtro) is dark grey and used for filtering water in wells and basins. Oven stone (cantaria de forno), reddish-brown, is used to make braziers and the lining of ovens. These soft stones are also used as building material, although they are eroded by time and the action of natural agents.

Scoria

Scoria derives from volcanic froth that solidified rough, rugged and vesicular. It sometimes settled in layers, or built up mounds, cones and humps on the mountain spurs; in that case it consists of masses thrown out of the caldera in an explosive surge.

Its colours vary widely, blackish tones predominating. It is used as an outer facing for garden tanks and cascades and, in recent times, inside the partition walls of houses, in place of wood shavings.

Volcanic ash

Volcanic ash consists of lava very finely divided by pulverisation during explosions. It forms microlithic aggregates of amorphous appearance, which nevertheless contain countless small dark crystals rich in augite and iron oxides, and, in the more recent ashes, concretionary aragonite – Ribeiro Seco, Gorgulho, etc.

Sometimes carried by water down gentle slopes, the ashes lie in black layers between the tuffs and are deposited in cavities, forming cave sand (areia de furna), which is used in the mortar for the interior rendering of some dwelling houses. Ashes also occur in the matrix of the tuffs as an integral part of the soft stones. The so-called Monte sand (areia do Monte) is a purplish aggregate of ash which, once mechanically ground to particles, is used in the same way as cave sand.

Pumice

Pumice is a spongy obsidian lying over the volcanic muds and forming aggregates with them where they flowed viscously – Santa Luzia, Pilar, etc. – or bedded in layers parallel to those of the tuffs, as can be seen in orderly sequence on the coast at Penha de França, at the mouth of the Ribeiro Seco.

Greenish pumice of recent appearance is found at Porto dos Frades, on Porto Santo, and seems to have been carried there by ocean currents.

Clays

Clays form a mass made up of a great variety of earthy products derived from the decomposition of the aluminium silicates in the rocks, especially the trachytes. Old, hardened clays containing ferric hydroxides and carbonates lie interbedded and baked between layers of volcanic products; these are called laterites.

Clays containing sand form the barros (loams), a type of ground that gives its name to several localities called Barreiros. Some clays are coated with a layer of iron hydroxides that looks like scrap iron – São Jorge, Santana, etc. – and others with carbonates and sulphates – Porto Santo.

Massapez (heavy ferruginous clay soil) is an earth containing a large quantity of brownish ferruginous clay, which retains the water it absorbs for a long time, while salão (reddish clay soil of volcanic origin) is reddish and less dense. These terms likewise give their names to several localities where the two earths are found. The purplish earths derive from the decomposition of trachytoids at high altitudes, and the yellowish earths from the disintegration of the tuffs called pedra mole (‘soft stone’: soft yellowish tuff and its soil).

On the colouring mineral earths and clays there are two curious reports from the time of the captains-general Dom Diogo Coutinho and Dom José da Câmara, concerning the use of red ochre (almagre), ochres and pozzolana, and proposing their application and industrial development.

Alagoas

This name is given to several localities where the ground is marshy and forms ponds (alagoa, seasonally flooded hollow) in the rainy season. They arise either from depressions caused by erosion in low-lying places or from the remains of old secondary eruptive centres found at high altitudes. To this latter cause belong the ponds of Porto Moniz, Fanal de Cima and Fanal de Baixo, and that of Santo da Serra. On the Pico da Cancela, at Caniçal, there is one in the shape of a horseshoe, and therefore an old breached pond.

Limestones

They derive from the remains of submarine life, the shells and carapaces of small animals, mechanically and chemically transformed into rocks that surfaced or were torn loose by volcanic action.

Calcareous products ejected by the craters in the form of muds are often found – Ponta da Cruz, Queimadas de Machico, etc.

The limestones at present worked for the kilns are found on the Ilhéu da Cal (Lime Islet), on the island of Porto Santo, on Selvagem Grande and, in the north of Madeira, at São Vicente. From some unworked deposits stone could be obtained for other industrial purposes.

Calcium carbonates in concretionary forms and stalactites line the walls of some caves, especially on the Ilhéu da Cal.

Calcite in the form of rhombohedra, pure white, or in yellowish granular varieties, is found at São Vicente; aragonite, of the orthorhombic system, in needles and diverging sheaves and in beautiful columnar and acicular forms, occurs at Madalena do Mar and Boaventura.

The ancient action of waters charged with dissolved sulphates on the limestones produced gypsum, interbedded in layers of sediment or in concretions: compact in some places, coarsely crystalline in others. In its pure form it constitutes the selenite found on Selvagem Grande.

Concretions

Rainwater, charged with carbonic anhydride and dissolving the calcium carbonates, penetrates the tuffs and loose sands, forming little channels through shrinkage, which branch according to the direction of infiltration.

This is how the curious calcareous-sandy arborescent formations of Piedade, at Caniçal, and of Mornos, on Porto Santo, were formed, as well as those in some tuffs at Covões, in Caniço, and among tuffs and ashes at Fojo and Praia Formosa.

The fanciful shapes into which the calcareous sands have sometimes cemented, especially at Piedade, earned them the popular name of bonecos (dolls).

Other views hold that the Piedade and Mornos deposits are of plant origin, classing them as fossils. (See Calcareous bodies.)

Erosion

Over time the relief of the Madeira group has been considerably altered. The chief agent has been the action of rainwater and the sea, whose sometimes prodigious work wears away the different materials, mechanically and chemically, and carries them off to the sea.

The rocks gradually decompose, lose their balance, lean and collapse, turning into successive debris, transitory deposits governed by the weight and speed of the currents. The torrential rains that accompanied the various periods of volcanic formation caused progressive erosion and carved out the valleys with varying degrees of energy, according to the nature of the bed. Meanwhile the currents – now squeezed between the cliffs, now dropping from one level to another in falls that scoured deep rock pools at their base, now overflowing and then spreading out to form the flats and the meadowlands – went on depositing and removing their load according to the contributions of their confluences, their flow and the obstacles they met.

The varying severity of the winters, which sometimes causes floods, twists and diverts the course of the waters, as shown by old beds and traced on the high banks of the cliffs by the remains of transported material.

Our valleys are magnificent sections that show the order in which the layers forming them were laid down, the irregularities of their arrangement and the influence of the causes of unconformity that produced their present shape.

The action of the sea has greatly reduced the outline of the shore and shifted the coastline, which in some places has been cut almost sheer. It has opened small bays, inlets and coves, enlarged and diminished the beaches, and opened up rocks and cut them off as islets. Even in historical times the north coast has been visibly eaten away, while the south coast has grown appreciably.

The trachytic rocks of the Ilhéu da Fonte da Areia, on Porto Santo, are hollowed out into bowl shapes, in which sea salt and sands of various debris are sometimes deposited.

In the 16th century the springs of João Diniz gushed out onto the pebble beach (calhau). The Bânger pillar was built on a shoal, and in the 18th century people crossed by canoe between it and the city’s defensive wall. The chapel of the Misericórdia of Machico stood at the mouth of the stream and is now about 200 metres from the sea. The Ribeira de São João is gradually silting up the cove of Pontinha. One need only see how the streams become choked in the winter floods, and look at the muddy fringe of the sea, to understand readily the work done by the waters.

Fajãs

When rock faces collapse through denudation, they form heaps, terraces or aprons (pataréus, poios or sapatas) at the base of the fall. When these are large, they make up land with a natural slope, inland or on the coast: Fajã Escura, Fajã das Galinhas, Fajã dos Padres, Fajã do Mar, etc.

Lugar de Baixo, in Ponta do Sol, is a growing fajã (strip of flat land at the foot of a cliff), much enlarged by the flash flood (aluvião) of 1803. When rock faces fall into the streams, they force them to twist and divert their beds.

In the 17th century, water seeping between gently sloping sedimentary layers of different kinds caused a slow landslide, or avalanche, of ground that pushed out into the sea in the parish (freguesia) of São Jorge.

Sands

Sands come from the crushing and rolling of different materials. Their colour and nature reveal their origin: black sand comes from basalt; greyish-brown sand from trachytes; dark yellow sand from tuffs; and pale yellow sand from shelly limestones. The sands contain microlithic crystals from the decomposed rocks, and olivine crystals are abundant during the great high tides at Praia Formosa.

Calcareous sands sometimes form a sandstone, a hardened aggregate, or are cemented together with particles of different materials, and are then called ariusco (cemented calcareous sandstone of Porto Santo) – Porto Santo. S.

The publications of the Mineralogical and Geological Museum of the University of Coimbra, Memórias e Noticias no. II, 1940, include a detailed study of the geology of the Savage Islands (Selvagens) entitled Uma Missão Geológica a bordo do navio Hidrográfico Carvalho de Araújo, from the pen of Dr J. C. de Morais, director of that museum.