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Roque Nublo breccia, Gran Canaria, Canary Islands.

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Photo ID: 537
Gallery ID: 59
Photo Title: Roque Nublo breccia, Gran Canaria, Canary Islands.
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Keywords:
roque nublo, gran canaria, canary islands, ayacata, roque nublo stratovolcano, volcano, volcanic cone, asymmetric cone, gravitational collapse, collapse, catastrophic, edifice failure, volcanic, volcanic edifice, volcanic eruption, explosive eruptions, vulcanian, phreatomagmatic, low temperature, unusual, matrix, trachytic, phonolite, altered, zeolites, smectites, alteration, hot, vitric, condensed, water vapour, diagenetically, cemented, magma, water, interaction, eruption columns, tephra fountains, high-density, heterogeneous, ignimbrites, crystals, vesiculated, pumice, ash, lithics, lithic rich, layered sheets, pyroclastic, pyroclastic flows, ash flow, roque nublo breccia, breccia, non-welded ignimbrite, prehistoric, canyons, lahars, lava flows, tephrites, phonotephrites, tephri-phonolites, phonolites,
Description:
These rock formations were deposited at some time between 3.9 and 3.0 million years ago, during a period when explosive vulcanian-phreatomagmatic eruptions dominated the Roque Nublo volcano’s activity. During this period the highly explosive eruptions emitted a series of dense, relatively low temperature, water vapor and lithic rich, pyroclastic flows, resulting in a rather unusual type of non-welded ignimbrite, (also termed block and ash flow) in which the juvenile components (matrix composed of trachytic to phonolitic pumice fragments and ash) were largely altered into zeolites and subordinate smectites. The alteration took place in situ and immediately after the deposition of the deposits when they were still hot, by the reaction between the vitric juvenile components and the condensed water vapor, which diagenetically cemented the pumice rich block and ash flow. Some of the Roque Nublo breccias are composed of lahars and others are of pure phonolitic pumice, indicating the possibility of plinian type eruption columns. The combined effect of the magma-water interaction and the high content of lithic fragments is sufficient to explain the characteristic low emplacement temperature of the Roque Nublo ignimbrites (< 500ºC) and the fact that the eruption columns rapidly became too dense to be sustained as vertical eruption columns and were transformed into tephra fountains which fed high-density pyroclastic flows. These heterogeneous ignimbrites containing varying proportions of crystals, vesiculated pumice, ash and up to 55% Lithics (rock fragments) were to form massive layered sheets of breccia-type pyroclastic deposits known as the Roque Nublo breccia, of up to 60 metres thick in places and extending up to 25 km in length as they were channeled down a number of prehistoric canyons, towards the coastline. Many of the breccia sheets are interlayered with lava flows composed of tephrites, phonotephrites, tephri-phonolites and phonolites. This image is of a close up view of Roque Nublo breccia located at Ayacata, on the road side to Presa de las Niñas. This rock possibly forms part of the Ayacata Formation, which is comprised of broken parts of the breccia sheets which became dislodged during the final catastrophic collapse of the southern flank of the stratovolcano.
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