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Photo ID: 596
|Photo Title: Caldera de Bandama, Gran Canaria, Canary Islands.|
caldera de bandama, pico de bandama, gran canaria, canary islands, oceanic islands, volcanic islands, caldera, maar, holocene, magma, water, phreatomagmatic eruption, strombolian eruption, explosive eruption, basanite, basanite lava, lava, cinder cone, fissure, fissure vent, pyroclastic deposits, pyroclastic, pyroclastic fall, porphyritic, phenocrysts, olivine,
The Caldera de Bandama has been dated as 1970 years old (Radiocarbon dated +/- 70 years) and is one of a large number of volcanoes of Holocene age, which are the latest eruptions associated to the rejuvenation stage of Gran Canaria.
Although referred to as a caldera, the enormous depression measuring almost 1 Km in diameter and 200 metres depth, is a ‘Maar’, a maar being defined as a broad low rimmed crater formed during powerful explosive eruptions involving the interaction of magma and water.
The Bandama volcanic edifice is actually made up of two parts; the Caldera de Bandama and a cinder cone, the 575 m high Pico de Bandama, which originally formed as two cinder cones standing together prior to the formation of the caldera. The eruptive sequence began as a line of several small strombolian eruptions, emitted along a single fissure, which after some time were to form into two distinct eruptive vents, each building a separate cinder cone with associated lava flows, firstly in the area where the caldera is today, followed later by the second cone to the north, this where the Pico de Bandama stands today. Whilst the eruptions from the northern vent were purely of a strombolian nature, the ascending magma below the southern vent had began to come into contact with groundwater, setting of a series of explosive phreatomagmatic surges, together with the ongoing strombolian activity.
Although eruptions continued in the area of the caldera, most of the activity became temporarily focused to the north of the fissure, producing the large pyroclastic cinder cone which still stands today, together with large areas of pyroclastic fall deposits and a lava flow which flowed out from the base of the volcano, down into the Barranco de los Hoyos to the north east.
The formation of the caldera itself came towards the end of the eruptive sequence of the Bandama edifice, when a new source of ascending magma within the fissure below the caldera area, encountered another and perhaps larger aquifer, setting off a tremendous phreatomagmatic explosion, completely destroying the remains of the cinder cone, and sending a pyroclastic flow over 10 Km to the south. During this time, the rock strata below the volcano, shattered by the extreme pressures, collapses in on itself, forming the caldera.
Finally, strombolian eruptions were to continue for a short time, the evidence for this being the thin layer of lapilli which is to be found at the top of the Maar ring above the phreatomagmatic deposits.
The silica undersaturated lavas of the Bandama volcano are of a basanite composition with a silica content of 41.8% and a sum of alkalis of 3.8%. The texture is porphyritic with abundant phenocrysts of olivine. In addition, the lava flows contain some ultramafic and mafic xenoliths and cumulates.
In this image the lower rock formations at the back of the caldera are of 9.7 million year old phonolite lavas originating from the Miocene Caldera de Tejeda. The upper rock formations, above the thin line of vegetation and below the maar ring are 3.0 –3.9 million year old Roque Nublo volcano trachy-phonolitic pyroclastic breccia. The top maar ring is composed of ash, rock fragments and blocks from the phreatomagmatic explosions, interbedded with pyroclastic fall deposits of lapilli.
To the left of image below the rock wall and above the erosive slope is a thin white layer, this is the Las Palmas Detritic Formation, a layer a conglomerates which most probably was the aquifer which caused the explosions.