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Caldera de Tejeda fault, Barranco de Tirajana, Gran Canaria.

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Photo ID: 622
Gallery ID: 62
Photo Title: Caldera de Tejeda fault, Barranco de Tirajana, Gran Canaria.
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Keywords:
caldera de tejeda fault, caldera de tejeda wall, caldera de tejeda, caldera, miocene caldera, barranco de tirajana, gran canaria, atlantic islands, volcanic islands, atlantic ocean, oceanic islands, canyon, barranco, gorge, erosion, trachy-rhyolitic ignimbrites, mogan formation, ring fractures, fataga formation, phonolite, lava flows, pyroclastic flows,
Description:
This part of the Barranco (canyon) de Tirajana is specially interesting in that the erosion has revealed part of the most dramatic geological history of the island of Gran Canaria. Some 14.1 million years ago, there was a catastrophic volcanic explosion, which created an elliptical caldera, 18 x 28 kilometres in size and up to 1000 metres deep, the Caldera de Tejeda. Over the following million years eruptions continued at intervals of 30 – 50,000 years from ring fractures around the caldera’s periphery, largely refilling this caldera with trachy-rhyolitic ignimbrites and rare lava flows, which is known as the Mogan formation. 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. The ignimbrites and lava flows emitted from this volcano between 13 and 9 million years ago, at intervals of around 50,000 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. This image is of the south western wall of the Barranco de Tirajana, just south of the reservoir, where the Caldera de Tejeda wall has become visible as a result of extensive erosion, this being in the centre of the image, where the palm trees run down inside a gully. To the left is the outside of the caldera, with Miocene pre caldera, shield volcano basaltic rocks forming the lower layers. These are overlain by 13.97 million year old Rhyolite lava flow ‘VL’, which is the steeply inclined layer to the left of the orange rocks and below the track (thin grey line)) To the right of the gulley is the interior of the caldera, which has been down-faulted by at least 50 metres. The lower basaltic rocks are not visible, but the continuation of the Rhyolite Lava flow ‘VL’ from the left, here forms the lower layers, which are then in turn overlain by the orange coloured layers of rhyolitic ignimbrites, which pinch out at the caldera wall. The rocks above the orange ignimbrites on the right and all the rocks above the track are younger Fataga Formation Trachy-phonolitic ignimbrites and lava flows. The bottom of the canyon is at 300 metres asl, the track at 500 masl and the mountain top at 1100 masl. The lavas and ignimbrites on the right, the intra caldera rocks, are more vividly coloured owing to hydrothermal oxidisation resulting from the hot, water saturated, mineral rich gases that had been escaping from fissures within the caldera. The unaltered rocks of the rhyolite lava flow on the outside of the caldera are grey colour, whilst the oxidised intra caldera rhyolites are a dark pink colour.
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