Introduction

Volcanism in the Canary Islands

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The volcanic history of the Canary Islands is thought to date back 30Ma. All but two of the islands (Fuerteventura and La Gomera) have experienced active volcanism in historic or prehistoric times, with the last major eruptions occurring in Tenerife in 1909 and La Palma in 1949 and 1971. BBC News reported a surtseyan-type eruption off the southern-most island of the Canaries, El Hierro, on 7th November 2011. That submarine volcano is still active today.

Most of Tenerife is built upon a basement of submarine extrusive rocks. The oldest rocks which outcrop on Tenerife have been dated at around 8.5 Ma. Ancochea et al., (1990) proposed a model for the evolution of the island in stages, each characterized by different forms of volcanism and stages of magmatic differentiation.

1. Shield Building Phase (11 – 4.2 Ma) followed by an erosional unconformity.

2. Las Cañadas Stratovolcano (3.3 – 2.0 Ma and 1.5 – 0.168 Ma)

3. Cordillera Dorsal volcanism (0.9 – 0.43 Ma).

4. Formation of Las Cañadas caldera, the Orotavo and Guimar valleys (1.5 – 0.168 Ma).

5. Formation of Teide and Pico Viejo (168 ka – Present).

Old Basalt Series (8.5 – 3.3 Ma)

The oldest visible geological unit on the island is the Old Basalt Series which comprises predominantly basaltic lavas and pyroclastics, resting on the submarine basement founding the island. These rocks are present in three isolated, deeply eroded, massifs at the corners of the island, the Teno Massif in the north west, the Anaga Massif in the north east and the Roque del Conde Massif in the south. The preserved thickness of basalt in these massifs exceeds 1 km, but the rocks exposed above sea level are just a cap on a huge volcanic edifice which rises from the sea floor. The subaerial basalt in the three massifs may have a volume of 1000 to 2000 km³, whilst the submarine basement may have a volume between 15,000 and 20,000 km³ (Gill and Thirlwall, 2003). The three massifs are likely to have been formed as independent islands between 8.5 and 3.3 Ma.

We have studied the rocks of the Teno Massif at Los Gigantes, Santiago Del Teide and Masca.

The Cañadas Volcano and the Dorsal Ridge Basalts (1.9 – 0.13 Ma)

No surface rocks on the island have been dated in the range from 3.3 to 1.9 Ma, and this period most likely represents an interval of volcanic quiescence and erosion. This interlude ended with the development of a large composite volcano, called Cañadas I, in what is now the centre of the island, at the junction of the three basaltic massifs. The volcano may have been as much as 40 km in diameter rising up to 2500 m above the basaltic basement rocks (Gill and Thirlwall, 2003). Early eruptions (c. 1.9-1.8 Ma) forming the lower stratigraphy of Las Cañadas are mainly basaltic lavas, but are overlain by a complex succession of more highly evolved volcanic rocks including trachytes and phonolites which were erupted between 1.5-0.17 Ma (Marti et al., 1994).

The phonolitic rocks of the Barranco de la Orchilla, a deep gorge near San Miguel on the south of the island, are more evolved rocks, forming during this later stage in the formation of Cañadas I.

The upper part of the Cañadas volcano was partially destroyed by subsidence to form the Cañadas Caldera.

Coincident with the later stages of development of the Cañadas volcano, extensive basaltic emissions (Old Basaltic Series II) along the axis between Las Cañadas and the older Anaga massif created the volcanic spine known as the Dorsal ridge or Cordillera Dorsal. On each side of this ridge, two large depressions occur, the Guimar Valley to the south and the Orotava Valley to the north.

Watts and Mason (1995) present results of marine surveys in the coastal waters of Tenerife which indicate the presence of an extensive bathymetric high off-shore from Orotava. They interpret this as a significant landslide deposit, appearing to confirm the hypothesis of large-scale mass movement for the formation of the Orotava valley. K-Ar dating of the back-wall of the Orotava valley suggests that this landslide occurred between 0.78-0.27 Ma.

We examine the rocks from the plumbing system inside Las Cañadas when we visit Los Roques de Garcia, where it is also possible to view the caldera wall and floor.

The pyroclastic rocks at Chimiche Quarry, La Cisnera, Barranco del Azucar and Tajao Quarry are the products of the Plinian eruption of Las Cañadas volcano. There is a good view of the Caldera wall and floor from the walk near Vilaflor and from the top of Teide. There is a good view of the Orotava Valley from a viewpoint to the east of Teide and as we descend from the Dorsal Ridge at Montaña Negra, before driving down the Guimar Valley.

Modern Basalt Series

Widespread basaltic volcanism followed the collapse of Las Cañadas volcano covering large areas of the lower flanks of Las Cañadas and in particular on the Dorsal ridge. Most of these basalts were erupted from scoria cones which remain well preserved on the Dorsal ridge. The scoria cones often exhibit strongly linear alignment suggesting fissure-fed eruption.

We study a scoria cone near San Miguel de Abona.

Teide Complex

Following the collapse of Las Cañadas 0.17 Ma, recent volcanism built Mount Teide and its lower neighbour, Pico Viejo, which rise up to 1500m above the caldera floor. These cones are built of successive layers of lavas interbedded with layers of ash and pyroclastic rocks produced by explosive Strombolian and Plinian eruptions. Early eruptions from the Teide-Pico Viejo complex consist predominantly of felsic trachyte and phonolite, with later, historic activity reverting to basaltic eruptions.

Pico de Teide and other vents particularly on the Dorsal ridge have erupted several times since the island was settled in 1402. The most recent activity has been confined to the north-western quadrant of the island, and the last eruption occurred in 1909.

We examine the output of Teide and Montana Negro at Icod, Garachico, Roques de Garcia and the trip up Teide.