Geology and Tectonics of La Palma
Updated: Jan 10, 2022
La Palma, historically known as San Miguel de La Palma, is the most north-westerly island of the Canary Islands, Spain. La Palma has an area of 708 km², making it the fifth largest of the eight main islands of the archipelago. Like the others, it is a volcanic ocean island located on the African Plate. La Palma is currently, along with Tenerife, the most volcanically active of the Canary Islands.
The northern part of La Palma is dominated by the Caldera de Taburiente, with a width of 9 km and a depth of 1,500 m. It is surrounded by a ring of mountains ranging from 1,600 m to 2,400 m in height. Erosion has exposed part of the seamount in the caldera. Only the deep Barranco de las Angustias ("Ravine of Anxiety") ravine leads into the inner area of the caldera. The outer slopes are cut by numerous gorges which run from 2,000 m down to the sea. From the Caldera de Taburiente to the south runs the ridge Cumbre Nueva ('New Summit', which despite its name is older than the Cumbre Vieja, 'Old Summit.') The southern part of La Palma consists of the Cumbre Vieja, a volcanic ridge formed by numerous volcanic cones built of lava and scoria.
Geology and tectonics
La Palma's geography is a result of the volcanic formation of the island. Like all the Canary Islands, La Palma originally formed as a seamount through submarine volcanic activity. It rises almost 7 km above the floor of the Atlantic Ocean; the highest peak reaches 2,400 m above sea level, and the base of the island is located almost 4,000 m below sea level. On the island, two volcanic domains can be distinguished: the rounded northern third, or Taburiente domain, and the elongated dorsal domain from Taburiente to the southern tip of the island.
The Taburiente domain is an enormous truncated cone-shaped relief with a semicircular plant that currently rises to 2,426 m (Roque de los Muchachos), with a large central depression of erosive origin: the Caldera de Taburiente. It has its origin in the superposition of large central volcanoes (stratovolcanoes) whose craters have followed one another by progressive reactivation, always in the same place. Since the island's emergence, successive subaerial volcanic stages have piled up materials vertically to heights of over 3,000 m. The result of these constructive processes is the circular form that presents the domain, as a single huge central volcano. The entire subaerial phase rested discordantly on underwater volcanic materials that were heavily traversed by basic dikes that constitute the so-called Basal Complex (4 Ma).
The rise of this formation exceeded sea level and was later instructed by gabbros and dikes which are interpreted as the magmatic chambers and emission ducts of the subsequent eruptions. The first subaerial emissions constitute the so-called Garafía volcano (1.77 Ma), which was very hidden by the later volcanic phases and composed of mostly basaltic flows, crossed by basic vertical-subvertical dikes. The Taburiente volcano was built on top of it, with a thick lower section (1.20 Ma) of agglomerates and basic slide breccias that covered almost the entire emerged island. A large pile of basaltic flows was formed above it. Subsequently, the upper section (1.1-0.80 Ma) was emitted, with a similar composition to the previous one and crowned by frequent strombolian cones, also basaltic, some of them hydromagnetic. Finally, a subrecent phase (0.80-0.71 Ma) of phonolytic and tephritic cones was produced.
About half a million years ago, at the end of the construction of the Taburiente complex, the accumulation of material on the southern flank must have been so great that, due to gravitational instability or increasing lithostatic pressure, a great collapse occurred, known as the Aridane slide. The volume of the mobilized material has been estimated between 180-200 km³ and it is located at the bottom of the sea covering the slope of the island. The resulting depression created the Aridane Valley, which was occupied by the Bejenado volcano (1,854 m) emitted 0.530 Ma ago, which was sustained by the materials that were left uncovered from the substrate of the Basal Complex. Phonolithic lava flows were emitted. Bejenado has steep slopes, the center of which must have been located further north, inside the Caldera. The rapid erosive widening of the caldera and the possible landslides it suffered have completely dismantled the northern flank of this building. The maximum thickness on the underwater substrate of the Basal Complex is about 600 m at Pico Bejenado.
On the sides of the mouth of the Las Angustias ravine there are thick deposits of sand and gravel that form two large walls. They extend upstream to the area of La Viña, and have a maximum thickness of about 300 m. The resulting sedimentation on reaching the edge of the sea forms a fan delta. In the lower parts of the deposit there are phonolytic lava flows interspersed, with a composition similar to that of the Bejenado building, which leads us to believe that both events were, in part, simultaneous.
The entire Taburiente Domain is, at present, totally inactive. Volcanism migrated southwards through the eruptive fissure of the Dorsal or Cumbre Vieja.
The dorsal domain corresponds to the southern half of the island and is structured by an eruptive axis in a north-south direction, in which two sectors with well-defined reliefs can be clearly distinguished: Cumbre Nueva, or northern sector, with a very regular height of around 1,400 m, a length of about 7 km and an arched shape. Cumbre Vieja, or southern sector, which constitutes a volcanic structure that reactivates towards 0.123 Ma through the 21.5 km long N-S rift that builds what is called the Cumbre Vieja Ridge. The six historical eruptions (last 500 years) on the island have taken place on this ridge, the last two in the 20th century (1949 and 1971). It is mainly formed by the concentration of fissures and the rift zone, which individualize two very pronounced slopes, eastern and western. At all times, the lava flows ran along the sides and reached the coast most of the time, occupying the seabed and fossilizing the previous cliff which is now seen as a paleo-cliff. The materials that were emitted are mainly basic alkaline lava flows (alkaline basalts, basanites, trachyba-salts and tephrites) and strombolian-type pyroclastic deposits. There are also a significant number of phonolytic domes scattered over the rift, such as the Teneguía dome that gave its name to the forelast eruption on the island. The current seismicity of the island is clearly associated with this Cumbre Vieja ridge.
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