Francisco-Jose Perez-Torrado

Cenozoic volcanism II: the Canary Islands

The Canarian archipelago comprises seven main volcanic islands and several islets that form a chain extending for c. 500 km across the eastern Atlantic, with its eastern edge only 100 km from the NW African coast. The islands have had a very long volcanic history, with formations over 20 million years old cropping out in the eastern Canaries. Thus all stages of the volcanic evolution of oceanic islands, including the submarine stage as well as the deep structure of the volcanoes, can be readily observed. Rainfall and vegetation cover are relatively low, with the exception of the island of La Palma, favouring both geological observation and rock preservation. Furthermore, the absence of surface water has promoted groundwater mining by means of up to 3000 km of subhorizontal tunnels (locally known as ‘galerias’). These galerias are especially numerous in Tenerife, La Palma and El Hierro, and allow the direct observation and sampling of the deep structure of the island volcanoes without requiring expensive and indirect geophysical methods.

Geochronology and stratigraphy of the Roque Nublo Cycle, Gran Canaria, Canary Islands

The Roque Nublo Cycle, of Pliocene age, is the second main cycle of subaerial volcanic activity in Gran Canaria. During this cycle a stratovolcano at least 2500 m high developed in the central part of the island; its volcanic products covered an area of about 540 km2. The detailed stratigraphy of the Roque Nublo Cycle, together with a magnetostratigraphic study and six new K–Ar dates, which complement 22 previously published ones, have led to the reconstruction of the volcanic history of this cycle in which six main stages have been distinguished. The revised chronology of the Roque Nublo Cycle implies an overlap in age with the earliest rocks of the third cycle of subaerial activity in Gran Canaria. No volcanic hiatus occurred between the second and third cycles, as was previously thought to be the case.

Evolution of ocean-island rifts: The northeast rift zone of Tenerife, Canary Islands

The northeast rift zone of Tenerife presents a superb opportunity to study the entire cycle of activity of an oceanic rift zone. Field geology, isotopic dating, and magnetic stratigraphy provide a reliable temporal and spatial framework for the evolution of the NE rift zone, which includes a period of very fast growth toward instability (between ca. 1.1 and 0.83 Ma) followed by three successive large landslides: the Micheque and Guimar collapses, which occurred approximately contemporaneously at ca. 830 ka and on either side of the rift, and the La Orotava landslide (between 690 +/- 10 and 566 +/- 13 ka). Our observations suggest that Canarian rift zones show similar patterns of development, which often includes overgrowth, instability, and lateral collapses. Collapses of the rift flanks disrupt established fissural feeding systems, favoring magma ascent and shallow emplacement, which in turn leads to magma differentiation and intermediate to felsic nested eruptions. Rifts and their collapses may therefore act as an important factor in providing architectural and petrological variability to oceanic volcanoes. Conversely, the presence of substantial felsic volcanism in rift settings may indicate the presence of earlier landslide scars, even if concealed by postcollapse volcanism. Comparative analysis of the main rifts in the Canary Islands outlines this general evolutionary pattern: (1) growth of an increasingly high and steep ridge by concentrated basaltic fissure eruptions; (2) flank collapse and catastrophic disruption of the established feeder system of the rift; (3) postcollapse centralized nested volcanism, commonly evolving from initially ultramafic-mafic to terminal felsic compositions (trachytes, phonolites); and (4) progressive decline of nested eruptive activity.

Beachrocks from the island of La Palma (Canary Islands, Spain)

Beachrocks on La Palma Island developed on platform-forming lavas of the Cumbre Vieja volcano. Some of these lavas are related to the 1585 (Puerto Naos), 1677 and 1971 (Echentive) eruptions. Radiocarbon dating of the Charco Verde beachrock gives a conventional age of 33 330 8 490 BP, while that at Playa Chica beach gives a calibrated age of 14 940 8 525 BP. The beachrocks, up to 1.5 m thick and some tens of metres wide, consist of several decimetre-thick horizons dipping 2^15‡ seaward. Petrographically, they can be classified as rudstones and arenites, with volcanic clasts as their main component. The original porosity of the beachrocks was intergranular (and occasionally intragranular) and was partially occluded by cementation and locally by internal sediments. The main cements are fibrous aragonite and micrite high-magnesium calcite (HMC). Spar aragonite, peloidal HMC and microbotryoidal HMC are scarce. The elemental geochemistry of these cements is consistent with a marine origin whereas the isotopic geochemistry indicates precipitation from marine waters slightly modified by meteoric waters. The evolution of beach deposits, and especially the beachrocks in La Palma island, follows three stages: (1) beach deposition, (2) beachrock formation, and (3) beach retrogradation and/or erosion. The studied beachrocks prompt us to make some important considerations. (1) The mean tidal range in the Canary Islands has not varied over the last thousand years. (2) The position of the beachrocks at the present-day sea level would require a combination of eustatic and isostatic movements to keep the sea level stable at the present level over the last thousand years. (3) Volcanic activity supplies the sediment that forms the beaches. (4) A dry warm climate with a very low rainfall (below 250 mm/year) and a high insolation rate (6^11 h/day) favours and favoured cement precipitation and beachrock formation by increasing the water temperature in the intertidal zone and in the inner part of the beaches. (5) The presence of beachrocks in the La Palma beaches prevents the total disappearance of the beaches. @ 2003 Elsevier Science B.V. All rights reserved.

The ongoing volcanic eruption of El Hierro, Canary Islands

El Hierro, the youngest of the Canary Islands (Spain), is no stranger to hazards associated with volcanic activity or to efforts to minimize the effects of these hazards on local communities. As early as 1793, administrative records of El Hierro indicate that a swarm of earthquakes was felt by locals; fearing a greater volcanic catastrophe, the first evacuation plan of an entire island in the history of the Canaries was prepared. The 1793 eruption was probably submarine with no appreciable consequences other than that the earthquakes were felt [Carracedo, 2008]; over the next roughly 215 years the island was seismically quiet. Yet seismic and volcanic activity are expected on this youngest Canary Island due to its being directly above the presumed location of the Canary Island hot spot, a mantle plume that feeds upwelling magma just under the surface, similar to the Hawaiian Islands. Because of this known geologic activity, the Spanish Instituto Geografco Nacional (IGN) has managed geophysical monitoring of the island since the beginning of the 1990s.

Alteration processes of the Roque Nublo ignimbrites (Gran Canaria, Canary Islands)

Abstract The non-welded, lithic-rich ignimbrites of Roque Nublo are the most characteristic deposits of the second magmatic cycle (Pliocene in age) on Gran Canaria. They are very heterogeneous, with 35–55% by volume of lithic clasts, 15–30% of moderately vesiculated pumice, 5–7% of crystals and 20–30% of ash matrix. The juvenile components (pumice fragments and ash matrix) are largely altered. The mineral assemblage that originated by the alteration of vitric components consists mainly of zeolites (chabazite, phillipsite and analcime) and subordinate smectites. The alteration does not show any lateral or vertical gradation in the deposits and does not change in intensity with respect to the distance from the emission center. These alteration processes affect only the Roque Nublo ignimbrites; their associated fall and surge deposits remain unaltered. A geo-autoclave mechanism is proposed to explain the alteration processes, which devitrified the juvenile components of the Roque Nublo ignimbrites and gave the rocks their characteristic strong induration. This implies that the eruptions which led to the formation of the Roque Nublo ignimbrites had an important phreatomagmatic component, and that the resulting pyroclastic flows were too dense and too poorly expanded to permit the water vapour to be separated from the vitric fragments during transport. 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 components and the condensed vapour, and cannot be attributed to late diagenesis or weathering.

The Plio-Quaternary magmatic feeding system beneath Gran Canaria Canary Islands, Spain; constraints from thermobarometric studies

Abstract: A complete framework of chronostratigraphic, petrographic, geochemical and thermobarometric data allows a reinterpretation of the evolution of the Plio-Quaternary volcanism in Gran Canaria. Bulk-rock and mineral chemistry has been used to define P–T crystallization paths, based on clinopyroxene–melt thermobarometry. Mafic magmas mainly crystallized at mantle depths (17–22 km) with secondary fractionation at crustal levels (1.5–8.0 km), suggesting multistage magma ascent. Crystallization of evolved magmas took place at crustal depths (3–15 km). The complexity of the magmatic plumbing system increased in the last 1 Ma, as shown by the presence of reverse zoned clinopyroxenes in the lavas, with green diopside–hedenbergite cores surrounded by brown diopsidic rims. Petrographic and chemical features of such clinopyroxenes support a xenocrystic origin of the green cores, related to magma mixing or mingling processes between pre-existing colder evolved magmas and new batches of hot basic magmas. The evolution of the magmatic feeding system beneath Gran Canaria was affected by the long-term uplift of the island as a result of lithospheric flexure caused by loading of the neighbouring island of Tenerife. This uplift favoured the generation of giant landslides and nesting of the recent volcanic activity (the last 1 Ma) within landslide basins. The association of rifts, giant landslides and nested volcanism with a higher degree of magmatic differentiation is proposed to constrain the plumbing system model.

The Holocene volcanism of Gran Canaria (Canary Islands, Spain)

ABSTRACT This work presents the first detailed map of the Holocene eruptions of Gran Canaria (Canary Islands, Spain). It provides complete and detailed information for all 24 Holocene eruptions of Gran Canaria, improving the knowledge of this recent volcanism and the assessment of volcanic hazards on the island. This map is a synthesis of collated and interpreted field data and topographic maps. We have integrated information obtained from: (1) detailed geological field surveys, (2) morphometric analysis of eruptive deposits, (3) high-resolution digital elevation models, and (4) aerial photographs.

Polygonal feeder tubes filled with hydroclasts: a new volcanic lithofacies marking shoreline subaerial–submarine transition

This paper describes for the first time a new lithofacies that formed in the passage zone during the construction of pahoehoe lava-fed deltas in the north and NE coastal areas of Gran Canaria (Canary Islands, Atlantic Ocean). These lava-fed deltas, which are Pliocene in age, present similar lithofacies associations to those previously described in the literature, except for two distinct features: low abundance of hyaloclastites and a new lithofacies in the passage zone. This new lithofacies, which we propose to name ‘polygonal feeder tubes filled with hydroclasts’, comprises metre-scale lava feeder tubes with decimetre-scale coherent poorly vesicular and aphanitic rinds of polygonal morphology, filled with closely packed hydroclasts generated in situ. It represents the first stage in the interaction between seawater and thick lava flows entering the sea under the influence of a tidal regime, thus marking sea level. This lithofacies should be easily recognizable in ancient successions, and thus represents a useful marker of palaeoshorelines. Supplementary material: Sample preparation and sample geochemistry are available at http://www.geolsoc.org.uk/SUP18785.