On occurrence of vitrophyres in Macaronesia

International Symposium on the Activity of Oceanic Volcanoes. Ponta Delgada, 4-9 August 1980.Vitrophyres, defined as rocks in which more than 50% of the groundmass is glass, irrespective of composition and whether porphyritic or not, occur in Macaronesia. In the literature, vitreous rocks are ofter referred to, but where chemical analyses are lacking and only inadequate petrographic descriptions are given, it is not always clear what proportion of such may actually be vitrophyres. On the other hand, references are given to vitrophyres in the region. Vitrophyres certainly are present in the Azores, Canaries and Cape Verde archipelagoes, but are more doubtful in Madeira and the Selvagens. In Tenerife, these rocks are most voluminous though more widespread in Gran Canaria. In the former, the Teide-Viejo volcano complex comprises essentially obsidian-type rocks. It appears that the Macaronesian vitrophyres are of Quaternary/Sub-Recent age, though those of Gran Canaria and La Palma belong to Middle-Lower Miocene. As not all islands have been given the same petrological attention, it is possible that further investigations would show vitrophyres to be more abundant than at present known.info:eu-repo/semantics/publishedVersio

Structural features of the sedimentary rocks of Macaronesia

The sediments within the five Maraconesian archipelagos of the Azores, Madeira, Selvagens, Canaries and Cape Verde are referred to. Lithology, structural features, outcrop areas, thicknesses, maximum dips, inland extents, highest elevations and stratigraphic ages are remarked upon. The very uneven distribution of sedimentaries within the various archipelagos is noted, and a possible cause for the great thickness and older age of such in the Canaries and Cape Verde, as also their scarcity in the Azores, is given.info:eu-repo/semantics/publishedVersio

On some unusual valleys in Macaronesia

A valley can be defined as: «Any hollow or low-lying land bounded by hill or mountation ranges, and usually traversed by a stream or river which receives the drainage of the surrounding heights». It is commonly a linear depression, the result usually of fluvial erosion. […]. ln all such features, valleys of volcanic islands are akin to those in headwaters areas of continental valleys, and in general it may be said that such island valleys are more 'simple', more 'straightforward' in characteristics and patterns. Thus when we find valleys in these islands which depart from the usual traits, this must arouse our interest, for certain factors have interfered with the more 'normal' evolution. Never can one ignore the climatic regime pertaining to a region. Macaronesia extends from latitudes 40°-15° N, from longitudes 31°-13° W. Our definition states that «usually» a stream traverses a valley, but this does not hold for most of these Atlantic islands, yet in dry and wet seasons, the valley is always there. Average -annual rainfalls vary from 950 mm .in the Azores to 260 mm in Cape Verde; average annual temperatures .from 17.7 °C in the Azores to 24.5 °C in Cape Verde. […].info:eu-repo/semantics/publishedVersio