The Edinburgh Geologist
by Michael F. Billett & Alistair Cowden
The volcanic island of Madeira situated 600 km west of the Moroccan coast (Figure 1), is not only interesting from a geological point of view: its inherent natural beauty and equable climate made it a very pleasant place to spend three weeks geological mapping last summer. The Madeiran archipeligo comprises the main island and its satellites Porto Santo and the Desertas Islands. They lie between the Azores to the north-west and the Canary Islands to the south, approximately 1500 km from the Mid-Atlantic Ridge (Figure 1). The volcanic nature of the island produces dramatic scenery with steep gorges radiating from the rugged central mountains, the highest peak being Pico Ruivo at 1861 m. The island also possesses some of the most spectacular coastal scenery in the world, with seacliffs reaching up to 600 m. The vegetation on the island is varied with vineyards and banana plantations in the coastal regions, lush deciduous vegetation on the mountain slopes, and eucalyptus and pine woods in the cooler mountain regions. In striking contrast to the rest of the island the eastern peninsula of Sao Lourenco is a flat semi-desert due to the influence of the Sahara.
The geological history of Madeira dates back to the Miocene when the main island-building stage took place. Intermittent and more localised volcanism probably continued up into pre-historic times. Since its discovery in 1419 by the Portugese mariner Joao Zarco the island has been volcanically quiescent, unlike its neighbours, the Azores and the Canaries. The volcanic succession has been divided into four main complexes by Mitchell-Thome, 1976 (Figure 2). These are:
4: Recent lavas
The spatial distribution of the volcanic complexes is somewhat tenuous, due to the uncertain nature of the criteria which define the complexes. However Complex 3 overlaps Complex 2 and lies unconformably upon the basement complex within the area studied. The occurrence of interbedded marine sediments within the volcanic pile provide a useful but incomplete dating tool. The youngest sediments are Quaternary reef deposits on the Sao Lourento peninsula, which testify to continual uplift during the evolution of the island.
The work that we carried out last summer (1979), was mainly concerned with the central mountain region (Figure 2), in which a local volcanic stratigraphy was compiled. Our main objective was to recognise the basement complex (Complex 1 of Figure 2) and consequently unravel the early volcanic history of the island.
The main results of our fieldwork are summarised in the stratigraphical sections in Figure 3, which attempt to correlate our observations from a number of localities. This led to the recognition of three distinct volcanic units belonging to the basement complex (1) uncomformably overlain by a younger group of volcanics of Complex 3. The lowermost unit (A) is a thick sequence of altered waterlain tuffs which is overlain by a massive pile of pyroclastics and thin basaltic flows. The pyroclastics from Unit B like some of the lava flows, possess red weathered surfaces indicating sub-aerial exposure and emergence of the island above sea-level. The third unit (C) consists of huge thicknesses of vent agglomerates, tuffs, and repeated basalt flows. The presence of a diachronous volcanic boulder bed containing 1 m blocks of basalt, provides evidence for extensive: erosion at this time. Unit C also contains an interesting suite of gabbroic nodules in some of the pyroclastic beds. Complex 3 is separated from Unit C by an irregular unconformity, and consists of a thick pile of ankaramitic trap basalts and interbedded air-fall tuffs, rather similar to the Tertiary trap basalts of Skye and Mull. The whole volcanic pile, excluding the trap basalts, is extensively cross-cut by a network of predominantly east-west trending basic dykes.
This systematic study of the volcanic succession has shown that the 'basement complex' cannot simply be regarded as a uniform pyroclastic pile, but as a laterally heterogeneous series of pyroclastics related to a complex volcanic cycle. The first stages in this cycle were the eruption of submarine tuffs, followed by the construction of a classic strato volcano complete with soils, lava flows, agglomerates, and volcanic bombs (Figure 4). Unit C represents a more explosive phase of volcanism before a period of volcanic quiescence, uplift, and tilting which led to the erosion of the older volcanics. The last major stage in the volcanicity, and volumetrically the most important, was the eruption of the trap basalts and associated tuffs onto the pre-existing landscape throughout the island. A number of cinder cones and thin lava flows scattered throughout the island are the most recent evidence of volcanic activity.
We would like to thank the Clough Fund of the Edinburgh Geological Society and the Weir Fund of the University of Edinburgh for financing our fieldwork in Madeira, and expanding our geological horizons, besides improving our sun tans. As a footnote it should be said that the South Orkney Islands (November 1979 edition Edinburgh Geologist) is not the only place to have mythical graptolites. During our stay in the mountains we were shown by a group of excited locals a fine specimen of 'Dendroidea'. If this were true it would have grave consequences for current sea floor spreading theories of the Atlantic. However after intense geological examination it became clear that it was nothing more than a dendritic iron stained surface much to the consternation of the locals.
The only introductory article upon the geology of Madeira of use to readers who wish further background is by Mitchell-Thome and consists of a chapter in his book The Geology of the Middle Atlantic Islands, published by Gebruder Borntraeger of Berlin, 1976, and is available in Edinburgh University Library.
Michael Billett and Alistair Cowden were formerly undergraduates of the University of Edinburgh. At the time of publication, they were at the Department of Geology, University College, London and Department of Geology, Chelsea College, London
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