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Photo ID: 987
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Photo Title: Los Azulejos, Gran Canaria, Canary Islands.
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Keywords:
los azulejos, mogan, gran canaria, atlantic islands, oceanic islands, volcanic islands, macaronesia, rocks, rock formations, miocene, miocene rocks, caldera de tejeda, caldera, volcanic, volcano, shield volcano, stratovolcano, ignimbrite, pumice, lava, phonolite, trachy-phonolite, margins, intra-caldera, fractures, fissures, magma, magma chamber, hydrothermal, alteration, gases, emission points, steam fumaroles, hyrothermal vents,
Description:
The most dramatic event in the volcanic history of Gran Canaria took place 14.1 million years ago, when a cataclysmic volcanic explosion sent some 80 cubic kilometres of ash, rock fragments and burning clouds of pulverised magma into the air, covering 400 Km2 of the island in a composite rhyolitic – trachytic – basaltic ignimbrite. Within hours the shield volcano which had formed the basis of Gran Canaria had been destroyed and in its place, an elliptical caldera estimated to have been 18 x 28 kilometres in size and 1000 metres deep had formed; the Caldera de Tejeda. Over the next million years, up until some 13 million years ago, explosive eruptions continued, largely filling in the caldera and covering much of the south, south west and west of the island with up to 20 layers of highly differentiated felsic trachytic and rhyolitic ignimbrites and lava flows. The extent of the area covered over the rest of the island is largely unknown since this remains buried under more recent material. These are known as the Mogan formations. The last of these eruptions included in the Mogan Formation are known as the Montaña Horno formation, which are the rocks which form the Azulejos (The turquoise rocks in this image) and part of the rocks above these, which were emplaced between 13.3 and 13 million years ago. Then at around 13 million years ago, there was a resurgence of the island’s volcanism comprising of magmas of a chemically different composition, these being known as the Fataga formation, and which instead of the eruptions being from ring fractures inside the caldera’s periphery, the focal point of activity moved to the centre of the caldera, with the formation of a high stratovolcano. The ignimbrites and lava flows comprised of silica undersaturated nepheline trachytes and phonolites, emitted from this volcano between 12.5 and 9 million years, were to completely bury most of the remains of the caldera and overflow its flanks, building huge accumulations of lava flows which reached the coastline to the north and south of the island. These flows also comprise the upper layers above the Azulejos, which culminate in the 1412m high Montaña del Horno. This stage was accompanied and followed by the intrusions of syenites into the caldera basement and a large cone sheet swarm within the central caldera area, lasting up until 7.3 million years, this again greatly transforming the caldera area by uplifting much of its central core area. Following this, there has been little or no volcanic activity at all in the south western area of the island and the erosion that has now lasted over 8 million years has once again revealed part of the ancient caldera wall, notably from Berrezales near Agaete, to Mogan, but also within the Barranco’s of Arguineguin, Fataga and Tirajana and in the area of Temisas, where more recent activity did occur. The most prominent outcrop of this caldera wall is exposed here, at Los Azulejos, on the road from Mogan to San Nicolas de Tolentino. The vividly coloured yellow, turquoise and pink rock formations, which form the Azulejos, together with some of the overlying red ignimbrites, were located on the inside of the outer margins of the caldera, close to the top of the caldera wall, this being very evident in the Azulejos formation just to the west of the small cafeteria on the road side. Here the various coloured layers can be seen to gradually be less inclined down into the caldera towards the top, showing how the caldera was gradually filled in, with the heavy pyroclastic deposits pinching out against the caldera wall. The coloured layers are composed of hydrothermally altered pumice deposits interbedded with thin layers of ignimbrites dated as 13.29 million years old and overlain by thicker layers of red hydrothermally altered rhyolitic ignimbrites dated as 13.16 my The final top layers composed of green-grey phonolitic ignimbrites and lavas emplaced between 12.4 and 12 million years, are however unaltered, this then suggesting that the hydrothermal alteration of these lower rocks took place between the end of the Mogan formation and the beginning of the Fataga formation between 13 and 12.5 million years ago. This hydrothermal alteration was caused by the heat escaping from the newly emplaced, shallow Fataga formation magma chamber, through fissures within the very porous, water saturated pyroclastic and epiclastic deposits that had filled in the caldera to the brim, resulting in widespread steam fumaroles and hyrothermal vents. As the resultant chemical and moisture rich, 200-250 degree centigrade gas came into contact with the pumice and ignimbrite deposits close to the outer margins of the caldera, it altered the composition of the minerals contained within these rocks, changing their colour and composition forever, creating one of Gran Canaria’s greatest natural wonders. This hydrothermal alteration may have occurred very widely across the caldera, however since it was mostly buried by the later Fataga lavas and ignimbrites, intruded into and uplifted and then severely eroded, then in part covered by the later Roque Nublo volcano’s materials, then again severely eroded to such a depth that it has exposed the plutonic syenites in the central section, any other hydrothermal materials are no longer visible.
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