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Photo ID: 983
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Photo Title: La Calderilla, Gran Canaria, Canary Islands.
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Keywords:
la calderilla, gran canaria, canaria islands, atlantic islands, oceanic islands, macaronesia, phreatomagmatic maar, pyroclastic spatter cone, barranco de guayadeque, la caldera de los marteles, quaternary eruptive centre, fracture zone, echelon fissure, la calderilla formation, la calderilla eruptive centre, strombolian eruption, pyroclastic cone, ground water, explosive eruptions, lapilli, scoria, bombs, olivine-basanite, roque nublo hauyne phonolite lavas,
Description:
Located at the extreme south-eastern tip of the island’s highlands and at the head of the Barranco de Guayadeque, are a series of 7 late Quaternary eruptive centres formed along two fracture zones or echelon fissure. (Geological Field Guide Gran Canaria, H.U. Schmincke - 1990) Five of these are pyroclastic spatter cones with associated small lava flows, which emerged on high ground, whilst the two others that opened up within valleys, within less than 3 km of each other are phreatomagmatic maars; La Caldera de los Marteles and La Calderilla. These eruptive centres are believed to have formed more or less synchronously around 85,000 – 100,000 years ago and form part of the La Calderilla formation. The La Calderilla eruptive centre is composed of two of the seven eruptive centres, which are believed to be on a separate fissure to the 5 eruptive centres associated with the Caldera de los Marteles. La Calderilla emerged from the newly formed upper source of the Barranco de Guayadeque, following the closure of the previous valley by the earlier emplacement of the Caldera de los Marteles. La Calderilla eruptive centre consists of a 200 metre diameter Maar of 60 metres in depth and a pyroclastic cone to the south-east which is at 1840 metres above sea level both of these having formed simultaneously, only that one of the two ascending columns of magma was to interact with ground water setting off a series of explosive eruptions and the other not. The maar results from a series of both phreatomagmatic and strombolian eruptions. The deposits surrounding the crater show alternating layers of pale grey-brown welded tuff of phreatomagmatic origin and layers of fine grained black, grey and reddish lapilli, courser grained black lapilli, scoria and bombs up to 1 metre in size of strombolian origin. The phreatomagmatic tuff contains lithics from the shattered base rock structures, predominantly composed of Roque Nublo hauyne phonolite lavas with large feldspar crystals of up to 3cm, but also of basalt, basanite and trachyte. There are also vitrious fragments and individual olivine and pyroxene crystals. The pyroclastic cone located on the south east rim of the caldera was formed by purely strombolian eruptions and has a small crater open to the south east, from which a lava flow was emitted, which flowed for some 3 Km down into the Barranco de Guayadeque. The lava is composed of an olivine-basanite which has been dated as 85,000 years old. This image is of the La Calderilla caldera or maar. The light coloured rock formations to the top left are welded tuff of phreatomagmatic origin, whilst the dark coloured rocks at the back (western wall) of the caldera are a Roque Nublo hauyne phonolite lava flow, which was broken through by the explosive eruptions.
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