How La Palma was born? Geological overview (part I)
La Palma has a surface of 706 km ² and its highest point reaches 2,426 m above sea level. It is the fifth largest island, and the second highest of Canary Islands archipelago. Geologically the island is composed of two volcanic groups, located in the North and South with an impressive mountain pass between them: In the North, we find the old and dominant volcanic shield (formed by the lava-flow creating multiple overlapping basaltic washes), together with the Caldera (Taburiente’s Boiler) located in the heart of the island. In the South, geologically young and still active, is volcanic chain of the Cumbre Vieja. In between the Caldera and the Cumbre Vieja, we find the Cumbre Nueva with a shape like a crescent moon. In the west, the oceanic floor falls down to some 4.000 meters of depth, which means that this volcanic complex, with more than 6.400 meters of height is one of the highest of the planet.
The sub-aquatic stage
La Palma has experienced two important stages of geological development - typical of any volcanic oceanic island-: the first sub-aquatic stage which took place from the bottom of the Atlantic Ocean up to the surface, and then the “sub-air” stage above the surface.
The volcanic structure of La Palma began to be formed from 5,3 to 2,6 million years ago, in the geological age of the Pliocene, with a sub-aquatic bulge of more than 1.000 meters of height, which was probably caused by faults in the African oceanic plate, in combination with a “Hotspot”.
La Palma, with a thickness of only 10 kms or less, is relatively thin. Drifting towards the East it pressed (and is still pressing today) against the African Oceanic plate, which presses on the 150 km (or more) thickness of the African Continental plate. This produced violent tensions in the basalts of the ocean ground. When these tensions exceeded the resistance of the rock, cracks were formed - and according to Carracedo and other authors (1999), there was a system of cracks with a shape of a star- that allowed the ascent of magma fused with, and sprouting from the living red mantle of the Earth.
When magma emerged below the level of the water, the so called pillows of lava were formed. This is a sure indicator of a sub-aquatic formation. Since the above mentioned pillows are today found at heights of even more than 500 m above sea level (in the Barranco de las Angustias and at the back of the Caldera), we can deduce that a large part of the elevation process of the islands, by some thousands of meters, occurred under water.
The most ancient rocks of La Palma are these pillow lavas, and the intrusions of magma that have penetrated them. The pillows are only discovered in the deepest zone of the Barranco de las Angustias (Ravine of Distresses) and at the back of the Caldera de Taburiente.
When the volcano finally, some two million years ago, entered in contact with the waters of the oceans surface, the hot lava of at least 1.000 °C created steam which expanded rapidly, with a volume 1000 bigger than that of the liquid water. This produced the so called phreatomagmatic eruptions, which disintegrated the rock into tiny particles.
These then formed conglomerates of angular fragments of rock, called pyroclastic sediments (from the ancient Greek: pyr = fire and klastós = fractures) are also called magmatic breccia (or rubble), and these are located on the pillow lavas. Pillow lavas and magmatic breccia together are called ‘Basal Complex’.
Sub-air stage On this basal complex, four volcanic structures were formed successively above the marine surface:
1. Between 1,77 and 1,20 million years, the volcano of Garafía was the first one to arise. Some of these early and principally basalt rocks from this volcano, can be seen today only in the steep walls of the Caldera de Taburiente, and at the back of some ravines that cut deeply in the north side of the island: Barrancos los Hombres, Franceces, Galllegos, El Agua and Jieque. Otherwise, they are mainly covered by following volcanic eruptions. The volcano of Garafía became unstable due to magmatic intrusions that penetrated its volcanic chambers, and also due to its own height and weight, it finally collapsed some 1,20 million years ago.
2. Approximately in the same place of the volcano Garafía, between 1.08 and 0.78 million years ago there formed the old volcano Taburiente,
3. The old volcano was followed by a more recent Taburiente volcano, between 0.78 and 0.41 million years later. To distinguish between the deposits of the Garafían volcano, and the rocks of the following Taburiente volcanos, is only possible by comparing the variation of angles of the different lava flows-those which occurred during the collapse of the Garafían, and the formation of the Taburiente volcano in the crater of the landslide. On the basis of these results, there can differentiated the more ancient volcano (Lower or Taburiente I) and the other younger (Upper or Taburiente II).
The volcanic activity migrated towards the South in the last stage of the younger Taburiente’s formation, changing the initially conical form of the volcanic complex, and spreading and lengthening the cone towards the south. When the volcanic activity in the North of the island ended, approximately 500.000 years ago, it had formed a massive volcano of 25 km diameter and approximately 3.000 m high. The western side of the volcanic complex then collapsed in a massive landslide, creating the crater we know today as the Caldera de taburiente, with its steep flanks in the northwest and the north, and also Aridane’s valley (valle de Aridane) and the Cumbre Nueva in the East.
4. Then the volcano Bejenado was formed in the Southern centre of the Caldera, this fourth and last volcano of the North shield, occurred between 0.56 and 0.49 million years ago.