La Gomera: A short geological guide

Ken Madrell (UK)

The island of La Gomera has an area of 370km², it is 25km in diameter, has a maximum altitude of 1,487m (Alto Garajonay) and is situated approximately 40km west of Tenerife. Unlike the other Canary Islands, La Gomera has experienced a long and continuing eruptive break and is in a ‘postshield erosional stage’. Carracedo and Troll (2016) describe this as the stage when active volcanism has ceased, and erosive and denudational landforms are predominant (p. 39).

The submarine base of the island shows that it rests on a shallower ocean bed than the surrounding islands. The emerged land mass is semi-circular in shape, with a radial drainage pattern from its centre near Alto de Garajonay.

The dating of the island has proved problematic, as some of the earlier measurements placing its age between 15 Ma and 19 Ma have since proved to be inaccurate. More reliable estimates now put its age at between 10 and 11 Ma.

Fig. 1. Roque Argando viewed from Lomo de la Mulata.

La Gomera’s general stratigraphy comprises of three main rock sequences:

A Miocene basaltic shield, including a basal plutonic complex (that is igneous rock formed by solidification at considerable depth beneath the earth’s surface).
A nested felsic (that is, igneous rocks that are relatively rich in elements that form feldspar and quartz) stratovolcano (which is built up of alternating layers of lava and ash).
The youngest Pliocene volcanism.

Fig. 2. Sketch map of La Gomera, showing the main towns and geology of the island.

Key:

Rock sequences	Colour	Geological period	Age in millions of years (Ma)
Young Edifice	Green	Pliocene	2.0 Ma 5.7 Ma
Old Edifice	Upper – White —————- Lower – Blue	Miocene	5.7 Ma 8.6 Ma ————————– 8.6 Ma 10 Ma
Submarine Edifice	Red	Miocene	More than 20 Ma

Fig. 2 shows a sketch map of the general geology of the island. The rocks shaded in red are the Submarine Edifice of the Miocene basaltic shield; those shaded in white and blue are the Upper and Lower Old Edifice and equate with the felsic stratovolcano; and the rocks shaded in green are the Young Edifice relating to the youngest Pliocene volcanism.

The present landscape is a product of its volcanic history and the more recent processes of erosion and denudation. The deep and steep-sided eroded valleys are known as the barrancos (Fig. 3) and are typical of the island’s landscape.

The geological history

The northern sector of the island is the Miocene basaltic shield, underlain by basal mafic (that is, rocks that are relatively richer in magnesium and iron than felsic rocks) plutonic rocks. The Miocene epoch began 23 Ma, long before La Gomera was formed, and lasted until 5.3 Ma. These plutonic rocks are mainly gabbro, which are intruded by basalt dykes.

Fig. 3. View from near Imada, looking southeast down the Barranco de Santiago.

Gabbro

Gabbro is a coarse-grained rock in which individual grains can be seen with the naked eye. They have the same chemical composition as basalt – plagioclase feldspar, pyroxene and amphibole, and have a silica content of between 45% and 52%. They are coarse grained because the crystals have cooled slowly at depth.

They are exposed in the north of the island between Arguamul in the west and Hermigua in the east, and are shown on Fig. 2 as the Submarine Edifice. It is the oldest part of the island and is dated as being between 11.5 Ma and 9.1 Ma.

After the erosion of the basal complex, a shield volcano – centred on the Vallehermoso area – was formed over the region. This volcano, known as the Old Edifice, was active between 10.5 Ma and 6.4 Ma and represents the main structure of the island. It had two phases: the Lower Old Edifice and the Upper Old Edifice. It has been estimated that the Lower Old Edifice had a diameter of 22km and stretched 5km north of the present coastline and reached a height of between 1,300m and 1,900m (Anchochea et al., 2006). It was dated as being between 10.5 Ma and 8.7 Ma and is composed of basalts.

Basalts

Basalts are dark-coloured, igneous rocks of mafic composition with a fine-grained groundmass and a silica (SiO₂) content of between 45 wt.% and 52 wt.%. They are composed of plagioclase feldspar, pyroxene and amphibole, and have less than 5 wt.% of sodium and potassium (Fig. 4). Basalt is fine grained because the crystals have cooled quickly at or near the surface.

Fig. 4. Plot of total alkalis (Na2O + K2O) against SiO2 showing widely used subdivisions of
volcanic rocks (Appropedia, 1989).

Due to weathering of the iron oxides, most of the basalts seen on the island now have a more reddish-brown colour (Fig. 5). The best outcrops of this age are found in the northwest of the island, along the coast between Alojera and Tazo.

Fig. 5. View from Agulo town just north of Hermigua, looking north towards La Palmita. Note the reddish-brown colour of the rocks in the centre. These Pliocene basalts were laid down almost horizontally.

The Upper Old Edifice was larger than the current size of the island. It had a diameter of 25km and reached a height of between 2,000m and 2,600m, and was centred near Alto de Garajonay (Anchochea et al., 2006). It was active between 8.6 Ma and 6.4 Ma and erupted basalts and more felsic lavas of trachytic-phonolitic composition. (Trachytic is a rock texture in which crystals show parallel alignment due to liquid flow and phonolite is a fine-grained volcanic rock composed mostly of alkali feldspar). These lavas have a higher sodium and potassium content than basalts, normally in the range of 6% to over 15%, and are of intermediate or felsic composition, that is having a SiO₂ content of more than 52%. Intermediate lavas are normally in the range of 52% to 63% and felsic lavas are in the range of 63% to 77% (Fig. 4). The more complex acidic compositions are typical of the later stages of volcanic activity.

The Young Edifice lasted between 5.7 Ma and 4.0 Ma, and is mainly Pliocene in age. It was a smaller event and was limited to the island’s central and southern parts, and also the deep barrancos of the north. It was made up of lavas of basaltic, trachy-basalt and trachy-andesite composition (Fig. 4). These lavas show a slight enrichment of sodium and potassium (between 5% and 9%) and, with the trachy-andesite, an enrichment of SiO₂ content in the range of 57% to 63%.

The Young Edifice is divided into two episodes: the Young Edifice 1 and the Young Edifice 2. The Young Edifice 1 filled the canyons in the south of the island, such as the Barrancos de Santiago (Fig. 3). Often, these barrancos are less than one kilometre apart. The Young Edifice 1 was composed of basalts and trachy-basalts, and cut by a few dykes. The Young Edifice 2 had a short period of emission between 4.6 Ma and 3.9 Ma and consists of basalts, trachy-basalts and trachy-andesites laid horizontally or cross-cutting as dykes. Extensive Pliocene basalts are exposed in the south of the island from Valle Gran Rey in the west, south to Playa Sanitago and round to San Sebastián in the east.

Andesite

Andesite is a dark, fine-grained, brown or greyish intermediate volcanic rock, which is la common constituent of certain and is the intermediate type between basalt and rhyolite (ranging from 57% to 63% silicon dioxide).

Igneous intrusions: sills and dykes

Fig. 6. A view of a south-easterly dipping dyke in the Santa Catalina valley near Hermigua.

Dykes

Dykes are sheet-like igneous bodies that cut across pre-existing rock layers and are discordant. Sills are also sheet-like intrusions but are intruded between the pre-existing rock layers and are concordant.

Sills and dykes are amongst the easiest geological features to observe on La Gomera. Two main types of dykes are visible in the subaerial units: vertical and inclined dykes. Unlike the vertical dykes, which are visible in rock units of all ages, the inclined dykes are only visible in the Lower Old Edifice, in the rock exposures in the northwest and northeast of the island. Ancochea et al. (2008) provide a detailed description of the dykes on la Gomera. In describing the inclined dykes, they note that they intrude more or less conformably to the Lower Old Edifice lavas and can be considered as sills (p. 209). Fig. 6 shows a photograph of these dykes near Hermigua, which are composed of basalt and dip seawards, generally at less than 30⁰. In Alojera, they dip west to southwest; and in the Valle de Gran Rey they dip south-southwest. These inclined dykes are dated at between 10 Ma and 9 Ma.

Vertical dykes are much more common throughout the island and vary greatly in terms of age and composition. For example, those seen near Imada (Fig. 7) are of basic or intermediate composition (trachyte) and aged about 4.2 Ma +/- 0.3 Ma.

Fig. 7. A vertical trachyte dyke intruding basaltic rocks at Azadoe near Imada, looking north. The dyke is about two metres wide.

Fig. 8 shows a close-up shot of a dyke intruding the metamorphosed red agglomerate (tosca or toba roja). This rock was formed from a crystal-rich scoria (that is, basaltic lava ejected as fragments from a volcano, typically with a frothy texture) and lapilli (that is, rock fragments ejected from a volcano) sequence that was buried under Miocene basalt flows and experienced subsequent burial metamorphism. It is used in the construction of many sacred and municipal buildings, including the Iglesia de San Sebastián (Fig. 9).

Fig. 8. A 10cm-wide dyke intruding the red agglomerate in an abandoned quarry on La Lomada Road, just outside El Molinito, near San Sebastián.

Fig. 9. Iglesia de San Sebastián. Note the doorway and tower composed of red agglomerate.

Ring dykes

A ring dyke is an intrusive body that is circular, oval or arcuate (curved) in plan. It is formed by the upwelling of magma in a conical or cylindrical fracture system.

A good example of a ring dyke can be seen just south of Benchijigua on the west side of the valley, leading down to Pastrana and Playa Santiago (Fig. 10). This trachyte ring dyke has intruded through the Miocene basalts.

Fig. 10. Looking west – an example of a ring dyke located above Lo de Gato, east of Imada between Benchijigua and Pastrana. Note the small white house for scale.

Lava domes and plugs

Domes of Miocene age are found in the east of the island and include Lomo Majona aged 7.8 Ma, located to the north of San Sebastián, and Risco Grande aged 8.2 Ma, located to the west of San Sebastián.

Domes of Pliocene age are found mainly in the centre of the island and to the southwest. They include the lava plug of Roque Agando aged 5.1 Ma (Fig. 11) and the lava dome of Fortaleza de Chipude aged 4.36 Ma (Fig. 12).

Fig. 11. Looking south from the road to Roque Agando near Garajonay. The rock rises 160m above the road.

Fig. 12. The lava dome of Fortaleza de Chipude, just east of Valle Gran Rey.

These felsic domes are often associated with explosive volcanism, which involves the violent expulsion of pumice and ash. Examples of ash deposits can be seen in the central part of the island near Roque Agando (Fig. 13). The basaltic deposits filled the old caldera and formed a broad plateau that occupied the centre of the island. It is now heavily dissected by fluvial erosion and weathering. This can be seen in Fig. 14, which shows the valley below Roque Agando stretching south towards Playa de Sanitago.

Fig. 13. Looking southwest towards the horizontal beds of ash, viewed from near Roque Agando. The rock face is about 500m high.

Fig. 14. View of the valley below Roque Agando, looking south towards Playa de Santiago.

Examples of volcanic ash deposits and lava flows

In the west and south of the island, examples of volcanic ash deposits intermingled with lava flows can be seen in the cliffs at Valle Gran Rey and Cala Cantera, near Playa Santiago. Those at Valle Gran Rey (Fig. 15) are felsic in composition and were laid down during the Upper Old Edifice about 7 Ma. Examples of ash deposits laid down about 4 Ma during the Young Edifice can be seen in Fig. 16.

Fig. 15. Examples of ash deposits and lava flows above Playa del Ingles, Valle Gran Rey. The photograph was taken from the beach looking east. The top of the ridge is 600m in height.

Fig. 16. Horizontal deposits of volcanic ash, laid down during the Young Edifice in the cliffs below La Cantera, near Playa Santiago. The cliffs rise to a height of about 100m above sea level.

La Caldera

There is only one recognisable volcanic cone remaining on the island: La Caldera. It is situated in the south of the island to the west of Playa Santiago. The cone was formed during the Young Edifice about 4.2 Ma. The best views of La Caldera can be seen along a narrow footpath leading from Quise down to Cala Cantera. Much of the landscape near La Caldera is covered by red coloured, highly weathered scoria (Fig. 17).

Fig. 17. View of one of the sides of the caldera, showing the covering of red scoria.

Teide Eggs

Teide Eggs are accretionary lava balls that have rolled down the front or slope of a lava flow. They are formed like a snowball grows as it rolls down a snow-covered slope.

Fig. 18 shows the section through a Teide Egg seen on the GM3 near Jerdune, close to the junction with the GM2. This Teide Egg is enclosed in a lava flow – the flow may have been the original flow which carried the egg or a later flow that then enclosed the egg.

Fig. 18. A Teide Egg in section view near Jerdune, on the main road between San Sebastián and Playa Santiago. The egg is about three metres in diameter.

Fig. 19. Another Teide Egg enclosed in a lava flow, on the paved road near El Cedro just south of Hermigua. This is a smaller egg, with a diameter of about one metre.

Los Organos

Los Organos (the Organ Pipes) is one of the most spectacular geological features on La Gomera (Fig. 20). It is located on the northwest tip of the island, 12 nautical miles from Valle Gran Rey. It can only be viewed from the sea and consists of giant basalt columns that look like organ pipes. The trachyte dome, Punta de las Salinas, has been partly eroded by the sea to expose the columns, which were formed by the cooling of molten basalt from the exterior of the dome towards its interior. As the basalt cooled, vertical cracks developed in a hexagonal pattern and formed the columnar jointing (as is clearly seen in the more famous example of the Giant’s Causeway in Northern Ireland).

Zeolites and other minerals

Walking along many of the tracks on the island, such as the one descending from Benchijigua to Pastrana, small pyroxene crystals can be seen in many of the rocks. For something more unusual, zeolites and augite crystals make even more exciting discoveries.

Zeolites

Zeolites comprise a group of silicate minerals with crystalline structures. They are formed by regional metamorphism when sediments are chemically changed by increases in temperature and pressure. The zeolites formed when levels of temperature would have been between (200⁰C to 300^oC) and pressure was between 50 and 400 MPa.

Good examples of zeolites can be seen east of the ruined banana plantations at Playa de Hermigua. Fig. 21 shows basaltic lavas with cavities in-filled with calcite and zeolites, such Thomsonite crystals and needles.

Fig. 21. Calcite overgrowth and cavities containing zeolites (white). The rock outcrop is above the old cafe and swimming pool at Playa de Hermigua.

About two kilometres northeast of Fortaleza, a car track descends to Barranco der Erque. Exposures of yellow-brown tuff can be seen. Fig. 22 shows a section of the yellow tuff, which is rich in augite crystals.

Fig. 22. Augite crystals in yellow tuff, along the road to Barranco der Erque.

Conclusions

This guide has attempted to provide a brief geological history and overview of the island of La Gomera and identify some of the major geological features that visitors can see at first hand. For those wishing to travel further afield, the nearby island of La Palma forms an excellent contrast. A short ferry trip from San Sebastián will take you to an island formed by very recent volcanic activity, with the opportunity to observe fresh lava flows and lava tunnels, and to walk around recently formed craters, such as Teneguia, which last erupted in 1971.

About the author

Ken Madrell is a retired educational adviser. He has recently completed the post graduate course in the Geology of Yorkshire and northern England at York University.

All the photographs were taken by the author.

Acknowledgements

My thanks go to Dr Annette McGrath for developing and encouraging my interest in geology through my engagement in The Geology of Yorkshire and northern England, an online postgraduate course delivered through the University of York. I would also like to thank Annette for suggesting the publication of this article, and for her guidance and support with its content.

References

Ancochea, E., Hernán, F., Huertas, M.J., Brändle, J.L. and Herrera, R., 2006. A new chronostratigraphical and evolutionary model for La Gomera: implications for the overall evolution of the Canarian Archipelago. Journal of Volcanology and Geothermal Research, 157(4), 271-293.

Ancochea, E., Brändle, J.L., Huertas, M.J., Hernán, F. and Herrera, R., (2008). Dike-swarms, key to the reconstruction of major volcanic edifices: The basic dikes of La Gomera (Canary Islands). Journal of volcanology and geothermal research, 173(3), 207-216.

Appropedia (1989). A classification of igneous rocks and glossary of terms. [Online] Available at: https://commons.wikimedia.org/wiki/File:Classification_extrusive_rocks_EN.svg. [Accessed 9 July 2018].

Appropedia (2004). Los Organos. [Online] Available at: https://commons.wikimedia.org/wiki/File:Los_Organos_La_Gomera.JPG. [Accessed 9 July 2018].

Carracedo J .C and Troll V.R. (2016). The Geology of the Canary Islands. Elsevier.

La Gomera: A short geological guide