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Photo ID: 942
|Photo Title: Caldera de Los Marteles|
caldera de los marteles, gran canaria, canaria islands, atlantic islands, oceanic islands, macaronesia, phreatomagmatic maar, barranco de guayadeque, quaternary eruptive centre, fracture zone, echelon fissure, basanite, strombolian eruption, kugel-lapilli, la calderilla, barranco de la madre del agua
Located at the extreme south-eastern tip of the island’s highest elevations 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 Caldera de los Marteles (this image) is believed to have formed prior to La Calderilla 3 km away. This circular crater of 550m in diameter and of 80 m in depth is located at 1459m above sea level. It emerged from within the previous upper course of the Barranco de Guayadeque, closing it off from its upper source close to the Pico de Las Nieves, which today forms the Barranco de la Madre del Agua, which can be seen from the roadside above the caldera where there is a dam, this known as the Presa de Cuevas Blancas. (which never has any water in it)
Following the closure of the valley erosion formed a new canyon to the south-west, whose source was now in the area of La Calderilla, which is today’s Barranco de Guayadeque.
The Caldera de los Marteles is a Maar and was formed by a series of violent phreatomagmatic explosions, when an ascending column of magma came into contact with ground water, the resulting extreme increase in pressure shattering the rock strata and opening a large circular depression from which both phreatomagmatic surges and pyroclastic fountains would be erupted. Following the initial phases of eruption, the crater area may have been significantly deeper, within which there may have been a pooling of magma in the form of a semi open magma chamber, into which the waters from the valley would have flowed into, resulting in violent surges of explosivity. The initial phreatomagmatic explosions produced layers of grey lithic rich ash, overlain by finer grained layers. The interesting point about this eruption is that these initial layers are then overlain by a dense layer of large (up to 15 cm diameter) composite lapilli containing lithics, (kugel-lapilli) formed during the transition between purely phreatomagmatic eruptions and later strombolian eruptions. This may have been a consequence of the diminished supply of water as the canyon became closed by the eruptive deposits, the explosivity of the freatomagmatic surges being much less powerful and being more a combination of phreatomagmatic surge together with strombolian fountaining, where the quantity of vesiculated magma was now much greater than the lithic fraction. (Fragments of previously formed rock)
These layers are best observed along the eastern rim of the maar, left of picture, not seen here) where they overlie a previously emplaced Pliocene/early Quaternary nephelinitic tephra cone. Finally with the water supply from the canyon cut off by the new volcanic deposits the final eruptions were purely strombolian, these producing layers of finer lapilli. The narrow southern rim of the maar, (left of picture) above the Barranco de Guayadeque is composed mostly of finer lapilli from these later strombolian eruptions.
The steep rocky escarpment to the west side of the crater (over the almond tree) is composed of post Roque Nublo olivine nephelinites (Llanos de la Pez formation). There is also a small outcrop of Roque Nublo hauyne phonolite lava, located on the northern rim of the Caldera, below the car park area and a small nepheline-basanite dike, which cuts through the post Roque Nublo lavas and the phreatomagmatic deposits of the caldera. The nepheline-basanite with abundant olivine phenocrysts has not been dated, although it is thought to be of very recent age, 10,000 years or less. Note the ditch on the far side of the crater floor, this is ongoing compaction of the unconsolidated crater fill. On the north-western rim of the caldera is one of the 4 other eruptive centres associated with the Marteles eruption, believed to be along the same fissure, this is the pyroclastic spatter cone known as Morro de la Caldera. The Caldera de Los Marteles emitted lavas of basanitic composition with groundmass plagioclase.
SiO2 – 43.20%, Al2O3 – 11.90%, Fe2O3 – 3.96%, FeO – 8.52%, MgO – 11.20%, CaO – 11.40%, Na2O – 3.13%, K2O – 1.50%, TiO2 – 3.80%, P2O5 – 0.80%, H2O – 0.20%.