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Rhyolitic ignimbrite P1, Barranco de Tirajana, Gran Canaria.

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Photo ID: 461
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Photo Title: Rhyolitic ignimbrite P1, Barranco de Tirajana, Gran Canaria.
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Keywords:
rhyolitic ignimbrite p1, barranco de tirajana, gran canaria, canary islands, silicic ignimbrites, explosive eruption, caldera, miocene, caldera de tejeda, collapse caldera, lava, magma, differentiate, crystalisation, magma chamber, reservoir, rhyolite, ignimbrite, rhyolitic ignimbrite, salami, anorthoclase, broken, crystals,
Description:
This part of the Barranco (canyon) de Tirajana is especially interesting in that the erosion has revealed part of the most dramatic geological history of the island of Gran Canaria, the first deposits of silicic ignimbrites created by a catastrophic explosive eruption 14.1 million years ago. The initial formation of the island of Gran Canaria took place between 14.5 and 14.1 million years ago, during which time successive accumulations of very voluminous, fluid basaltic lavas, almost continuously extruded from several eruptive centres arising from fissures radiating out from a central vent located in the centre-west of the island, rapidly constructing an immense shield volcano reaching possibly 2000 metres altitude and which was somewhat similar in size to today’s island, this being known as the Tejeda volcano. These magmas originated from a reservoir situated below the oceanic crust at some 14 km depth. Towards the end of this period, a new magma chamber was to form at a shallower depth, situated within the shield volcano itself at some 4 – 5 km depth, whilst at the same time new eruptive vents were to open, fed directly from the sub crustal reservoir. As a result the magmas contained in the new shallower magma chamber, were only to be replenished periodically by small quantities of basalt, and now almost isolated these magmas began to differentiate. Over some tens of thousands of years this basaltic magma chamber was to become a multi zoned chamber with a cool and highly viscous crystal-magma mush comprised of rhyolite in its top part overlying layers of trachytes, trachyandesites and trachybasalt. At some time around 14.1 million years, a new conduit connects the sub crustal magma reservoir with this shallow chamber, suddenly injecting large amounts of fresh basalt into this one, mixing with the differentiated magmas, tremendously increasing the pressure and temperature within the chamber and releasing the gases that were previously in equilibrium within the residual mixture. As a result, the overlying rocks begin to dome upwards and outwards, creating a series of ring fractures, from which a froth of pulverized rhyolitic magma and gas begin to erupt. Within minutes a cataclysmic explosive volcanic eruption is under way. Within hours, as the highly explosive eruptions continue and the magma chamber is almost emptied, synchronously, the summits of the island now fractured and unsupported begin to collapse into this chamber. The entire center of the shield volcano falls piecemeal into the magma chamber as a huge cauldron block and acting as a piston suddenly increases the pressure on the remaining magmas and the recently injected basalt, which provoked the eruption. This basaltic magma now also begins to erupt, the pressure so high that the magma is pulverized to such an extremity that the result is a very strange and rarely seen basaltic ignimbrite containing rhyolitic components as well as a great quantity of lithic fragments. Within hours the most dramatic event in the volcanic history of Gran Canaria is over, the volcanic explosion having emitted some 80 cubic kilometres of ash, rock fragments and burning clouds of pulverised magma into the air, covering 400 Km2 of the island in a composite rhyolitic – trachytic – basaltic ignimbrite. The shield volcano has been destroyed and in its place is an elliptical caldera estimated to have been 18 x 28 kilometres in size and 1000 metres deep; the Caldera de Tejeda. The Ignimbrite known as P1 or composite flow P1 is a mixture of three major component magmas and forms a triple layer of ignimbrites representing the contents of the magma chamber in reverse. The lowermost deposits or first erupted are composed of rhyolites, followed by deposits of mixed rocks and overlain by an upper layer or last erupted unit of basaltic composition. These are further subdivided into four rhyolitic, two mixed rock and three basaltic units of different composition and structure. This image shows part of the first erupted component of this composite flow in the foreground, the rhyolitic ignimbrite, which is to be found in small outcrops along the base of the south-western wall of the Barranco de Tirajana. This unit is known as the ‘salami’ since when broken open the rock is of a pink colour and largely composed of broken white anorthoclase crystals.
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