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Photo ID: 480
|Photo Title: Cone sheet swarm, Caldera de Tejeda, Gran Canaria.|
cone sheet, dikes, cone sheet dikes, trachyte rock, trachyte dike, presa del parralillo, gran canaria, canary islands, canyon lands, eurasian buzzard, buteo buteo insularum, artenara, mesa de acusa, barranco de la aldea, san nicolas de tolentino, tejeda formation, miocene, caldera, caldera de tejeda, volcano, volcanic, stratovolcano, ignimbrites, lava flows, igneous intrusion, intrusive complex, silica undersaturated, trachytes, phonolites, nepheline phonolites, fataga formation, syenite stocks, trachyphonolitic, radial dikes,
The canyon lands of the Caldera de Tejeda, situated in the centre-west of Gran Canaria form some of the most spectacular landscapes of all the Canary Islands, an unspoilt wilderness of exceptional beauty. Although some areas are covered in pine forest, this area is mostly of a hot and arid, very rugged terrain of deep canyons and high ridges, where the rocks are shades of whites, yellows and reds and where the only sounds are of the winds blowing through the dry grasses and the “piuuuuh” of the Eurasian Buzzard (Buteo buteo insularum) which are relatively common here, often seen drifting high above the rocky ridges.
These canyon lands form part of the most dramatic geological history of the island of Gran Canaria. It began some 14.1 million years ago, when a cataclysmic volcanic explosion created an elliptical caldera estimated to have been 18 x 28 kilometres in size and up to 1000 metres deep, a caldera which was only to be refilled again over the next million years, as explosive eruptions continued emitting layers of ignimbrites and lava flows.
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 trachytes and nepheline phonolites, emitted from this volcano at intervals of around 50,000 years, between 12.5 and 9 million years, were to completely bury 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.
It is during this period when the early paleo-canyons began to take their shape, owing to the increased erosion caused by the new high central volcano and the long time periods between eruptions.
The important fact is that this stage was accompanied and followed by the large scale intrusion of some 500 trachytic cone sheet dikes, hypabyssal syenite stocks and trachyphonolitic subordinate radial dikes that formed a 12 km diameter intrusive complex within the Miocene Caldera de Tejeda between 12.3 and 7.3 million years ago, intruding into the lavas and ignimbrites that had previously filled in this caldera.
These dikes intruded concentrically around a central axis of radial symmetry and dip between 30º and 50º towards the centre. Ages of dikes ranging from 11.7 to 7.3 million years suggest more or less continuous intrusive activity during both volcanically active and inactive phases. Intrusions were emplaced at average intervals of 5-10,000 years, whilst explosive eruptions took place at 50,000 year intervals. Formation of the Tejeda cone sheets most likely resulted from deformation processes due to resurgent doming, initiated by the recurrent replenishment of a flat, laccolith-like shallow magma chamber. Magma supply exceeding the volume that could be compensated for by up-doming of the overlying caldera fill resulted in the formation of cone shaped fractures. As the fractures open they are immediately filled with high pressure magma from the top of the chamber, creating the conical sheet form that gives them their name.
The emplacement of this cone sheet is believed to have caused a structural uplift of the core zone by as much as 1,400 metres, this core zone being just west of where Roque Bentaiga stands today and where syenites have been located at 1200 metres above sea level.
Following this there was a long period with little or no volcanic activity, when erosion really began to carve into the rock formations.
Then between 4.6 and 3 million years ago there was yet another new period of very large scale volcanic activity, the Roque Nublo Cycle which was again to build a stratovolcano in the centre of the island. The lava flows and breccia sheets emitted during this time were to fill in the previously carved out canyons, forming new ridges of volcanic materials. Finally after all these events, there was to be no more volcanic activity in the south-western area of the island and for over 3 million years now erosion has been the only factor involved, shaping this entire area into that of a very beautiful, rugged and mostly inaccessible landscape and revealing the full extent of the hundreds of cone sheet dikes. This image shows clearly the cone sheet dikes dipping some 45º down to the left side of the image. At the top right of image the horizontal rock formations are composed of Roque Nublo lavas, which remain un-eroded.