OVI-WAC GEOLOGY

A guest post for Deskarati by Alan Mason –

OFFSHORE VOLCANIC ISLANDS OF THE WEST AFRICAN COAST

Introduction

Two of the most popular European tourist destinations are the archipelagos of Madeira and the Canaries. To a lesser extent, the more southerly archipelago of the Cape Verde Islands is also visited by the more discerning tourist. All three island groups have three things in common.

They all lie in the eastern Atlantic Ocean, (Figure 1) close to the mainland of Africa, but separated from it by distances of nearly a hundred miles or more. Their climate is of the oceanic type which ameliorates the hot, dry conditions of the neighbouring African countries lying to the east along the same latitudes. Thirdly, each archipelago has been formed almost entirely by geologically recent volcanic action, though this activity has now largely ceased.

Figure 1

For the tourist who takes a lively interest in their surroundings it is a pity that there is so little available in the way of explanation of the geological origins of these archipelagos. Neither are there descriptions of geological structures readily visible to the interested observer. It may well be that such information is available within the university libraries but these are largely inaccessible to the general reader.

There is a need for an account for the general reader written by a competent geologist, whether amateur or professional. Nowadays, wikipedia can supply some of this lack, but even here the subject is tackled by three separates articles on the individual archipelagos.

This short article is presented as a stop-gap to fill the perceived need. The author has no qualifications in geology and has only visited the main island of Madeira. The various sources are acknowledged in footnotes.

1. Continental Drift

The Atlantic Ocean is slowly widening by a few centimetres per year so that the continents of North and South America are gradually moving away from the continents of Eurasia and Africa. The driving force for this process is the upwelling of magma from the Mid-Atlantic Ridge, creating new crust deep in the ocean depths, and to a lesser extent, new land above sea level in places like Iceland.

It is an interesting historical fact that the geomagnetic research on the mid-Atlantic submarine ridges in the early 1960s was the catalyst which finally drove the world of academic geology to accept the Theory of Continental Drift. This idea had been put forward sixty years previously by Alfred Wegener. His evidence was based on geography, geology and palaeontology.

As the idea of moving continents was too bold for most geologists, they discounted Wegener’s evidence without attempting to explain it in any other convincing way. The theory was accepted by many geographers and biologists, but rejected by geologists, because they could not see a geological mechanism by which continental drift might occur.

Figure 2. Alternately magnetised polarities are indicated by blue or red stripes.

2. Geomagnetism of the Ocean Floor

The pattern of deep ocean mid-Atlantic ridges showed geomagnetic properties from the iron-containing minerals present in the lava. It was found that the magnetic polarity of the ridges continually alternated. (Figure 2) This was due to the known fact, from other sources, that the earth’s geomagnetic field reverses polarity at regular intervals, approximately every one million years.

It was also clear that the magnetic pattern of ridges on one side of the mid-Atlantic line was a mirror image of the ridges on the other side. The evidence was ineluctable. The Atlantic Ocean was spreading at a clearly measurable rate. A back-calculation would enable us to work out when the process started.

Once the Theory of Continental Drift had become generally accepted it was found to explain a whole lot of other puzzling geological phenomena, like the existence of deep oceanic trenches close to the edges of continents, and the “rings of fire”, of extreme vulcanicity, at the continental edges. It became a powerful tool in explaining processes like orogeny (mountain building), and the formation of oceanic crust by seafloor spreading.

3. The Eastern Atlantic Ocean Floor

Since the sixties much research has been done on the ocean floor and the map of the findings shows an incredible degree of complexity in the pattern of ridges and faults. (Figure 3) The deep trench within the Mid-Atlantic Ridge shows clearly, as well as a large number of transform faults at right angles to the main trend.

The archipelagos of Madeira, the Canaries, and Cape Verde are picked out with red outlines and are seen to be seamounts, standing on vast bathymetric swells. The seamounts are undersea mountain peaks rising from the ocean floor as a result of volcanic activity, and the accumulation of solidified lava flows. Where several peaks break the surface of the waters they produce archipelagos of small islands. The bathymetric swells are enormous banks of fluid magma acting as reservoirs for the volcanoes.

These bathymetric structures do not seem to be directly part of the general process of seafloor spreading from the Mid-Atlantic Ridge. It is more likely that they have been produced by a local buckling of the seafloor crust allowing magma to rise from below the lithosphere to create enormous bathymetric swells, and zones of seamounts close to Africa. The general process is illustrated in Figure 4.

For example, Madeira lies on the Tore – Madeira Ridge, “a bathymetric structure of great dimensions oriented along a NNE to SSW axis that extends for 1000 kilometres…The origins of the (structure) are not clearly established , but may have resulted from a morphological buckling of the lithosphere.” (From wikipedia on “Madeira”; the location of “Tore” is something of a puzzle as it does not appear on quite large atlases, and it is important to understanding the size and location of Madeira’s bathymetric base.)

Figure 3

Figure 4. The Origin of Offshore Volcanic Islands of the West African Coast

4. The Time Scale (Figure 5)

The offshore volcanic islands are all relatively new, and seem to have arisen at much the same time, geologically speaking. None of them are much older than about 20 million years. The Madeiran vulcanicity began about 5 million years ago in the Miocene period, continuing into the Pleistocene until about 700, 000 years ago. The Canaries Archipelago was also formed in the Miocene between 14 and 9 million years ago. The last eruptions were believed to have occurred 3500 years ago.

The vulcanicity which created the Cape Verde Archipelago began in the early Miocene about 20 million years ago, and reached a peak at the end of the period, 7 million years ago when the islands reached their maximum size. Today, most of the islands show no vulcanicity. An active volcano on one of the islands of the Archipelago, (Fogo) erupted as recently as 1995.

Figure 5

As may be expected, the individual histories of volcanic activity on the three archipelagoes are periodic, erratic and localised, so that it is better to consider them separately from now on.

5. The Madeiran Archipelago

The archipelago lies 323 miles (520 Km) off the coast of Africa. It consists of three main groups of islands, Madeira itself, the Desertas Islands and Porto Santo. (Figure 6A) As explained earlier, these islands are seamounts rising from the abyssal plains of the eastern Atlantic seafloor, some 23, 000 feet. This is from 17, 000 feet to sea level and another 6, 000 feet to the tops of the mountain peaks of Madeira. This represents a colossal outpouring of material from below the oceanic crust.

The ages of the volcanic rocks of the archipelago have been determined and it is suggested that they have been formed by the oceanic crust moving across a “hotspot”. This is a stable rising plume of hot magma which periodically bursts through the crust to produce formations of basalt or other materials as it is cooled by seawater. The oldest rocks were the first part of the crust to pass over the hotspot. Figure 6B indicates the possible route of crustal movement over the hotspot in the archipelago.

Figure 6A                                          Figure 6B

6. A Shield Volcano

Madeira is an example of a “shield volcano”, that is a type of volcano built almost entirely of fluid lava flows. In consequence it has a low profile and rounded shape like the shield of an ancient warrior. In this way it lacks the typical steep conical shape of the more typical cinder volcano, created by ash, and debris from the interior. (Figure 9)

My own observations in the mountains of central Madeira, is that the periods of basalt flow must have been interrupted by periods of thick ash deposits, presumably from more typical volcanoes. The perimeter of the island is very heavily eroded and with steep, deep valleys and this would only be possible if it was mostly composed of weakly consolidated tuffs (compacted volcanic ash). Had the deposits been mainly basalt as they are in the interior they would be highly resistant to erosion by rainfall and runoff.

Figure 7                                             Figure 8

Figure 7 shows layers of basalt about 4-6 feet thick (1-2 metres) separated by strongly banded layers of tuff. A dyke is a deposit of lava which has broken through, and intruded itself into a fault or split in earlier deposits, frequently at a right angle to them. The basalt and tuff layers in Figure 7 are almost horizontal because they are only a few million years old and there has been no opportunity for major geological upheavals. The dyke, by contrast, is an almost vertical crack through the pre-existing horizontal layers. The sharp contrast between the blue-grey basalt and the underlying red tuff is seen in Figure 8. The colour is probably due to high levels of iron oxides in the ferric state, Fe2O3.

Figure 9

7. Five Stages in the Development of Madeira (from wikipedia article) see Figure 10

1. Base Formation

This was the major activity creating the main shield volcano, with large eruptions and huge sheets of basalt. It ended three million years ago.

2. Peripheral Formation

There was a reduction in the amount of material being ejected, and the development of platforms and dykes at the margins. This ended 740, 000 years ago.

3. High Altitude Formation

Volcanic activity continued with a rain of ash, bombs and fragments from the interior of the volcanoes.

4. Paul de Serra (translates from Portuguese as “The Marsh in the Mountains”)

This is a region of high undulating plains, (4, 000 feet) above the deeply eroded peripheral valleys. It is composed of basalt sheets, and ended about 550, 000 years ago.

5. Recent eruptions

This occurred on minor peripheral islands and ended 6, 500 years ago.

These five stages are illustrated diagrammatically overleaf in Figure 10

Figure 10

8. Geological Features of Madeira

Valley Erosion

Many of the more obvious geological features of Madeira rarely appear in any of the lavishly illustrated colour guide books of the island. . However, even the landscape studies can be useful to the student of geology. The view below of the valley of the Ribuera de Januela shows the deeply dissected landscape typical of most of the river valleys of the periphery of the island. It also shows a thickly wooded region, once common to the whole of Madeira, so that its Portuguese name means, “the woods”.

Figure 11 Ribuera de Januela

Amphitheatre Erosion

One of the geological accounts mentions two large amphitheatres carved out by the forces of erosion, but without actually naming them. Both are on the south side of the island. One is the great hollow in which the capital, Funchal is situated. The old town is in the centre, low down, close to the shore. All the newer hotels catering for the tourist trade are high up on the rim of the great hollow, with spectacular views of the city below and the Bay of Funchal.

The other amphitheatre is Curral das Freiras, (Nuns’ Valley), a popular excursion for tourist coaches, partly because the drive there is very demanding with narrow roads and hair-raising precipices. (Figure 12)

Figure 12 Curral das Freiras, (Nuns’ Valley),

Sao Lourenco

Sao Lourenco is a small town on the north-west coast and it shows many signs of the previous vulcanicity. Volcanic bombs and pyroclasts can be seen embedded in the soft cliffs of tuff and they are being gradually exposed by weathering. Along the shore are many pillow lavas. These are softly rounded shapes caused when lavas are extruded underwater and rapidly cooled by the sea. (Figure 13) On weathering they show a characteristic concentric pattern of cracks. (Figure 14)

Paul de Serra

My personal nickname for this region is “The High Plains” because in the height of summer they are blessed refuge from the heat. They are at an altitude of 4, 000 feet and appreciably cool. They are not really plains, but constitute a gently rolling country in marked contrast to the rest of the island.

(Figure 13)                                          (Figure 14)

Everywhere else on Madeira there are steep gradients. At the edges of the Paul de Serra the ground falls away steeply into the deeply eroded river valleys. The Portuguese name means, “The marsh in the mountains”. Much of the region is covered in heather moorland and scrub, but I do not recall having seen any marshes.

The rainfall in this region is higher than the rest of the island because of the altitude. It is not uncommon to be up on the Paul de Serra and discover that one is above the cloud base, and all the coastal regions are hidden from view. In winter it can be quite cold and wet, like an unlucky Scottish summer holiday. One of the joys of the region is that tourists are rarely seen there, perhaps because “there is nothing to see”.

(Figure 15)                                                        (Figure 16)

Figure 17 gives a good idea of the level nature of the Paul de Serra and its light cover of heather and scrub. The Ribeira Grande (Great River) is in its upper courses here and is cutting through the hard basalt to a pebbly layer below. In the distance the river is beginning to cut itself a steep-sided small valley.

The gas cavities in the basalt have a characteristic shape, from which the technical term “amygdaloidal” almond–shaped is derived.

The High Peaks

Again this is my own name for the region. At the eastern end of the long plateau which includes the Paul de Serra, are a cluster of peaks whose heights are all around 6, 000 feet. They include the highest, the Pico Ruivo (6, 107 feet – 1862 m), Pico das Torres, Pico do Areeiro, Pico Cidrao, Pico Cedro, Pico Casado, Pico Grande and Pico Ferreiro (5, 190 feet – 1582 m).

The local authorities have constructed a wide and well designed path which enables tourists to gain access to some of these peaks in comparative safety. Figure 17 features the Pico das Torres on the walk from the Pico do Areeiro. The tourist path is shown clearly at the Rocha Negra (Black Rock) in Figure 18.

(Figure 17)                                                 (Figure 18)

Figure 19

9. The Canaries Archipelago

There are seven main islands in this group. (Figure 20) As in the Madeiran Archipelago, the oldest islands lie to the east and the youngest lie to the west. This indicates a crustal plate moving from west to east over a stationary hotspot, which has been named, “The Canarian” or “The Canaries Hotspot”.

Figure 20 (The Canaries Archipelago)

Twenty million years ago the crust lay further to the west. As the crust passed across the hotspot, an upwelling of magma created Fuerteventura. (Figure 21) Over succeeding millions of years the hardened rocks of Fuerteventura moved further east, and the island became volcanically quiet while the hotspot now lay beneath a more westerly part of the crust.

Volcanic activity as recent as 1.2 million years ago created the island of El Hierro, the youngest member of the island group. All this fits in with an eastward-moving plate created by activity at the Mid-Atlantic Ridge.

(Figure 21) The Canaries Hotspot

Lanzarote and Fuerteventura

The Canaries Hotspot is believed by geologists to have arisen some 60 million years ago building up undersea deposits but only produced islands above the ocean surface in the Miocene about twenty million years ago. (See Figure 5). The oldest of the islands, and the most low-lying of the group, are Lanzarote (670 metres or 2, 198 feet) and Fuerteventura (812 metres or 2, 664 feet) as they have been subject to 20 million years of erosion in comparison with their neighbours.

Gran Canaria

Gran Canaria is an almost circular island, typical of its volcanic origins. Although now deeply eroded, the central peak, Pico de las Nieves, (Peak of the Snows) still rises to a height of 1949 metres, or 6, 394 feet. About 80% of the island’s bulk was formed during the Miocene period (14 – 9 million years ago) over about 200, 000 years of eruptions. Some thousand cubic metres of basalt were emitted and then were subject to erosion for the next 4 million years. This is known by geologists as “The Old Cycle”.

A second series of eruptions, (“The Roque Nublo Cycle”) occurred between 4.5 and 3.4 million years ago. This shorter cycle is also estimated to have also produced about a thousand cubic metres of basalt, and most of the existing peaks today are a result of the erosion of this material. This second period of slow basalt emissions ended dramatically with violent eruptions of pyroclastic flows. These were fiery hot gases, lava foams, and debris travelling at several hundred miles an hour, from the vents, and engulfing everything in their path.

Tenerife

The largest island in the group is Tenerife and being only a few million years old it reveals some remarkable statistics. Its topmost peak, Mount Teide, is the highest mountain in Spain, (3, 718 metres, or 12, 198 feet). Now, this is largely a political statement because the Canaries just happen to be a part of the Spanish kingdom.

However, it is remarkable that a volcanic peak on a small and relatively insignificant offshore island overtops all the peaks of the Spanish Sierras, including those of the Pyrenees whose highest top is the Pico d’ Aneto at 3, 404 metres or 11, 168 feet. It is either a measure of the remarkable mountain-building properties of active volcanoes, or of the erosive power of water in continental highlands, depending on one’s point of view.

The Teide volcano on Tenerife is the world’s third largest volcano, situated on a volcanic oceanic island. Like its neighbours on Lanzarote, La Palma, and El Hierro it has shown activity within the period of historical records, that is the last 500 years.

Its present quiescent state is attested by the many tourists to the island who take a motor coach close to the mountain top and complete the journey to the summit by a funicular railway. Before this, an ascent to the summit required many hours on mule back, with water and provisions, up the track through the black sands and boulder-strewn cinder fields.

There is a large circular rim and an internal hollow or caldera at the summit, which, though largely inactive, remains warm and capable of releasing steam and sulphurous vapours, often condensing as elemental sulphur.

10. The Cape Verde Archipelago

The name Cape Verde is an anglicisation of the Portuguese name “Cabo Verde” derived from the nearby Cap Vert (Green Cape) on the Senegal coast of Africa, and is not a reference to the colour of the islands, which are actually dry and yellow-brown because of the infrequent rainfall.

Figure 22 The Cape Verde Archipelago

 The archipelago is a horseshoe-shaped cluster of ten main islands (Figure 22) some 570 Km (350 miles) off the coast of West Africa. Three are flat, sandy and dry, while the rest are rockier, higher and covered in vegetation. The highest peak in the archipelago is the Pico do Fogo, (2, 829 metres or 9, 281 feet) on the island of Fogo.

Figure 23 The Cape Verde Hotspot

The island group is entirely formed as a result of volcanic activity. The process began in the early Miocene about 25 million years ago and reached its peak at the end of the period, about 8 million years ago when the islands reached their maximum sizes. (See Figure 5) It has been attributed to a hotspot associated with a bathymetric swell that formed the Cape Verde Rise. This is one of the world’s largest undersea protuberances, rising some 2.2 Km above the abyssal plain in a semi-circular region of 1200 square Km. (See Figure 3)

The oldest islands lie to the east and are about 20 million years old, and the youngest, in the west, are about 8 million years old. (Figure 23) However, there are pillow lavas exposed on Santiago that are 125-150 million years old, but these were probably formed in the deep ocean long before the beginning of eruptions which broke the surface of the waters to produce dry land.

Today, active vulcanism is restricted to the island of Fogo, (Fire Island), which last erupted in 1995. The volcano, Pico do Fogo, (Figure 24) has a caldera or bowl shaped summit which is 8 Km (5 miles) in diameter. The rim of the caldera lies at height of 1, 600 metres (5, 249 feet), and there is a more recent cone within in it which rises to 2, 829 metres (9,281 feet).

The caldera or bowl was created by subsidence once the original magma chamber had been emptied by eruption. A deeper magma chamber lies 8 Km (5 miles) below the volcano and this provided the material which created the newer and higher cinder cone within the older caldera. (See Figure 23)

Figure 24 Scale Cross Section of Volcano Pico do Fogo

11. Pyroclastic Deposits

All the islands of the archipelagoes described here are composed of the eroded remains of old volcanic cones and the “pyroclastic debris” associated with eruptions. Figure 25 gives a simple schematic representation of the formation of pyroclastic materials. They consist of ash or tuffs as an enveloping matrix, and within the ash are characteristic fragments.

Pumice is a light rock filled with gas cavities – a kind of “foam lava”. There are sharp-edged, angular fragments from the volcanic vent and from the shattering of hardened lavas. Scoriae are light, gas-filled rocks which were soft enough when hitting the ground to flatten into pancake shapes. Finally there are characteristically rounded, tapering volcanic bombs of lava close to solidifying temperatures when they penetrated the ash layers. These deposits are readily identified by a geologist or well-informed amateur observer, but Figure 26 gives some idea of the difficulty of identifying pyroclastic materials under field conditions.

Figure 25 Formation of Pyroclastic Materials

Figure 26 Pyroclastic deposits at Sao Vicente, Madeira

12. In Conclusion

The Galapagos Islands of the eastern Pacific Ocean share much in common with the OVIWAC archipelagoes under discussion here. They too have been formed by volcanic action in relatively recent geological times. Darwin had visited them in the early 1830s, while on the “Beagle” voyage, collecting specimens and formulating his ideas on evolution. Some ten years later the American writer Herman Melville, author of “Moby Dick” went to these remote islands in the early 1840s and wrote of his experiences in essays published in 1853, just four years before Darwin published his joint scientific paper with Alfred Russell Wallace.

Melville knew the islands as, “Los Encantadas” (the enchanted isles) instead of the more common name “Los Galapagos” (the tortoise islands), but it is clear that he was far from enchanted by them. A few extracts are presented here as a good illustration of the appearance of a volcanic archipelago. The “Aracama” he alludes to is possibly the bleak Atacama Desert in northern Chile, noted for its salt deposits.

“Take five and twenty heaps of cinders dumped here and there in an outside city lot: imagine some of them magnified into mountains, and the vacant lot the sea; and you will have a fit idea of the general aspect of the Encantadas, or Enchanted Isles. A group rather of extinct volcanoes than of isles; looking much as the world at large might, after a penal conflagration.”

“On most of the isles where vegetation is found at all, it is more ungrateful than the blankness of Aracama. Tangled thickets of wiry bushes, without a fruit and without a name, springing up among deep fissures of calcined rock, and treacherously masking them; or a parched growth of distorted cactus trees.

In many places the coast is rock-bound, or more properly clinker-bound; tumbled masses of blackish or greenish stuff like the dross of an iron-furnace, forming dark clefts and caves here and there, into which a ceaseless sea pours a fury of foam; overhanging them with a swirl of gray, haggard mist, amidst which sail screaming flights of unearthly birds heightening the dismal din.”

SOURCES

Figure 1 (Map of Africa) Reproduced from “The Faber Atlas” D. Sinclair & L. Dudley Stamp 1961

Figure 2 (Mid Atlantic Ridge) Modified and coloured from “Physical Geology” J.R.L. Allen 1975

Figure 3 Reproduced from “The Atlantic Ocean Floor” National Geographic Magazine.

Figure 4 (Origin of Offshore Volcanic Islands of West African Coast) Author

Figure 5 (The Time Scale of Island Formation) Author

Figure 6 (Movement Over the Madeiran Hotspot) Author, redrawn from “The Faber Atlas” Sinclair & Stamp 1961

Figure 7 (Basalts & Tuffs) Author Field photographs of local geology

Figure 8 (Red Tuff at Contact) Author ditto

Figure 9 (Formation of a Shield Volcano) Author

Figure 10 (Geological History of Madeira) Author

Figure 11 (Ribuera de Januela Valley) Reproduced from “Madeira – Dream Island” Francisco Ribeiro 1993

Figure 12 (Curral das Freiras – Nuns’ Valley) Reproduced from “Madeira – Dream Island” op. cit.

Figure 13 (Pillow Lavas 1) Author Field photographs of local geology

Figure 14 (Pillow Lavas 2) Author ditto

Figure 15 (Paul de Serra) Author ditto

Figure 16 (Amygdaloidal Gas Cavities) Author ditto

Figure 17 (Pico das Torres) Author ditto

Figure 18 (Rocha Negra) Author ditto

Figure 19 (Map of Sites) Modified from “Madeira the Complete Guide” John and Susan Farrow (Hale 1994)

Figure 20 (The Canaries Archipelago) Author, radically redrawn from “The Family World Atlas” Verlag, 2008. (Data from Dr. Ken Bristow in a letter published in the “Daily Mail” 10 Jan 2011 and from wikipedia article on Gran Canaria.)

Figure 21 (The Canaries Hotspot) Author

Figure 22 (The Cape Verde Archipelago) Author, radically redrawn from “The Family World Atlas” Verlag

Figure 23 (The Cape Verde Hotspot) Author, redrawn from “The Family World Atlas” Verlag

Figure 24 (Scale Cross Section of Volcano Pico do Fogo) Author,

Figure 25 (Formation of Pyroclastic Materials) Author

Figure 26 (Pyroclastic deposits at Sao Vicente, Madeira) Author Field photographs of local geology

“In Conclusion” Quotations from “Billy Budd, Sailor, and Other Stories” Herman Melville, Penguin, 1967

Figure 27 (Hawaiian Hotspot Today) Daily Mail 10 January 2011 – ALAMY.

Figure 27

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