La Gomera: a short geological guide

Introduction

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:

(1) A Miocene basaltic shield, including a basal plutonic complex (that is igneous rock formed by solidification at considerable depth beneath the earth’s surface);

(2) 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); and

(3) The youngest Pliocene volcanism.

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

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.

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

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.


READ MORE...To view the rest of this article, you need A subscription. FROM JUST £2.95.

If you are already a subscriber, login here.


Fig. 20. Los Organos (Appropedia, 2004).
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.
Fig. 21. Calcite overgrowth and cavities containing zeolites (white). The rock outcrop is above the
old cafe and swimming pool at Playa de Hermigua.
Fig. 22. Augite crystals in yellow tuff, along the road to Barranco der Erque.


Buy Fossils, Crystals, Tools
Subscribe to Deposits
Join Fossil Hunts
UK Fossil Locations