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Photo ID: 793
|Photo Title: Dike complex below Roque Betaiga, Gran Canaria, Canary Islands.|
roque bentaiga, gran canaria, canary islands, macaronesia, atlantic islands, oceanic islands, volcanic islands, volcanic monolith, caldera de tejeda, shield volcano, stratovolcano, roque nublo volcano, roque nublo breccia, lava flow, pyroclastic breccia, ignimbrite, strombolian cone, fissure vent, strombolian cone, extensive erosion, volcanic cycle, erosive cycle, cone sheet, syenite, ignimbrites,
This image is of the rock formations just below the Roque Bentaiga, taken from the village of El Espinillo. These formations, which form the base of this monolith, are geologically very complex since they are the result of many volcanic and erosive cycles dating back over 12 million years, this area having repeatedly been the focal point of much of Gran Canaria’s most dramatic volcanic events. From the very beginning, some 14.5 million years ago, the central emission point from which the original shield volcano that constitutes Gran Canaria was formed, is believed to have been in the area of the Mesa del Junquillo, just 5 km to the west.
Then, at 14.1 million years ago, a cataclysmic volcanic explosion largely destroyed the island, creating an elliptical caldera estimated to have been 18 x 28 kilometres in size and up to 1000 metres deep, the Caldera de Tejeda. The centre point of this caldera, where the initial collapse of the “cauldron block” into the empty magma chamber 4 –5 km below was located again in this area, probably between the Roque Bentaiga and the Mesa del Junquillo.
Following this, over another million years or so, explosive eruptions continued, emitting layers upon layers of ignimbrites and lava flows, which were to eventually refill the caldera. Then at around 13 million years ago, there was a resurgence of the island’s volcanism comprising of magmas of a chemically different composition 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, which it is believed to have been slightly further to the south-east, around Cruz Grande, this period known as the Fataga Formation lasting from 12.5 to 9 million years ago. The Lava flows and ignimbrites of this period were to cover a very large part of the island, with the possible exception of this area, since there are no remains of either at this point.
This stage was however accompanied and followed by firstly the intrusion of plutonic syenite stocks in this area below Roque Bentaiga and followed by a large cone sheet swarm, within the central caldera area, lasting from 12.3 to 7.3 million years ago, this again greatly transforming the caldera area by causing a structural uplift of the core zone by as much as 1,400 metres, this core zone being here, where the Roque Bentaiga stands today. Interestingly again there is no evidence that these cone sheet dikes acted as emission points and only failed to reach the surface.
Following this there was a long period with no volcanic activity, and then, between 5.3 and 2.87 million years ago there was 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 pyroclastic breccia sheets emitted during this time were to largely fill in the previously carved out canyons, forming new ridges of volcanic materials.
Incredibly, in addition to this, during the Roque Nublo period, a series of strombolian volcanic cones had formed on the flanks of the main volcano, one of these, the most important one, thought to have been over 1 km in diameter, was located here, just below where the Roque Bentaiga stands today and exactly on top of the focal point of the previously emplaced plutonic and cone sheet intrusions.
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.
The formations in this image (below Roque bentaiga) are comprised of a complex mixture of layers of fallout pumice, epiclastic materials and the Caldera de Tejeda intra caldera ignimbrites, which were intruded into by syenite stocks and then structurally uplifted by as much as 1400 metres by the cone sheet swarm, prior to the formation of the Roque Bentaiga. This area is also criss-crossed by numerous dikes of younger Roque Nublo age.
Interestingly there are also breccias that seem to correspond to an older eruptive centre, although no evidence of this centre has ever been found as mentioned above. If it were to have existed, then all the materials emitted have been completely eroded away. Some of the outcrops of solid rock are plutonic syenites.