Teide’s Restless Depths
The depths of Mount Teide have been restless for almost a month. It began with imperceptible seismic pulses, followed by swarms of tremors, and during this period, the occasional small, isolated earthquake. University of La Laguna geologist, Ramón Casillas, rules out that the recent volcanic activity recorded in Tenerife will lead to an eruption in the short term. He explains some of the possible scenarios should a magmatic ascent occur on the island, as well as the lessons the La Palma volcano has provided for science.
A Gradual Acceleration of Activity
What is happening is something that, from 2016 onwards, has been accelerating and which intensified from 2023. Since then, there has been an increase in seismic activity, a small accumulated ground deformation of about two centimetres has been detected northeast of Teide, and there have been minor increases in carbon dioxide emissions in water galleries to the east of the volcano.
Movement of Fluids, Not Necessarily Magma
Due to the characteristics of the detected seismic movements, they appear to be due to the movement of fluids—meaning gases or supercritical fluids—but not necessarily magma at this stage. The base of the crust is about 12 kilometres down, so these seismic events are occurring slightly above that. These fluids, which seem to be causing the seismic events, appear to come from the degassing of basic magmas that could be accumulating in the mantle or at the base of the crust.
Two Magma Types and a Potential Trigger
The latest seismic tomography studies confirm the existence, beneath the Teide-Pico Viejo complex, of small, residual, more superficial magma chambers of differentiated, phonolitic magmas at various depths. In principle, the degassing is more likely related to basaltic magma, which is more basic—similar to what caused the La Palma eruption. What could be more dangerous is if this basaltic magma rises and comes into contact with one of those phonolitic magma chambers. This possible contact would add heat and gases to the more differentiated magma, potentially causing its reactivation and subsequent ascent towards the surface.
Possible Eruption Scenarios for Tenerife
From there, we have two possible extreme scenarios. On one hand, the reactivated phonolitic magma could generate an effusive eruption, forming very viscous lavas with slow and short flows, as was the case with the Coladas Negras of Teide (1,300 years ago). Alternatively, it could cause a more explosive eruption of phonolitic magma, like the Montaña Blanca eruption (2,000 years ago). Everything would depend on the initial gas content of the phonolitic magma and the depth of the magma chamber. If the chamber were initially above sea level and had more than 2.5% gas content, the eruption would likely be explosive.
On the other hand, if the eruption were of basic magma, we could have a scenario like La Palma or El Hierro, on one of Tenerife’s active ridges, with mainly effusive and Strombolian activity. For now, it is difficult to lean towards any of the described scenarios. Once we have evidence of magma movement in the crust or within the island’s structure, we can assess which magmas are involved and their location, and therefore try to predict possible outcomes.
No Direct Signs of Magma Movement Yet
We are in a phase where we do not detect direct signals of magma movement. It seems there may be an accumulation of basic magma in the mantle or at the base of the crust, which would explain the increased degassing. But we have no evidence that this magma is moving, as happened on 11 September 2021 in La Palma and in El Hierro on 19 July 2011. We are not in that scenario. Could it change in two hours? That we cannot foresee. In any case, between eruptions, we will always be in a pre-eruptive phase.
The Role of Gases and Silent Magma Movement
Magma contains a series of dissolved gases, primarily water vapour, carbon dioxide, and sulphur dioxide. The solubility of gases in magma depends fundamentally on the pressure it is subjected to, its chemical composition, and, to a lesser extent, its temperature. Here in the Canaries, the most primitive basic magma is generated by the partial melting of mantle rocks at a depth of between about 80 and 110 kilometres. That magma rises slowly.
An increase in gas emission indicates that either a basic magma in continuous degassing is slowly ascending in the mantle, or that basaltic magma is accumulating at the base of the crust and degassing. Apparently, what seems to be moving or accumulating is a rather basaltic magma. It must be made clear that if it were indeed moving, its movement would not necessarily be producing seismicity. The problem is that if the rocky material it is passing through responds plastically, earthquakes are not generated. That happened in the La Palma eruption, for example.
Lessons from La Palma: Deformation as a Key Signal
We saw that the seismicity was concentrated at three depths, but between those depths the magma was moving and there were hardly any earthquakes because the material the magma passed through behaved in a plastic or ductile manner. In fact, a very remote scenario is that basic magma ascends directly from the mantle to the island’s surface through an already open fissure, without producing any earthquake. In this case, we could detect it through ground deformation, increased gas emissions, or changes in other geophysical parameters.
This was clearly demonstrated, for example, in La Palma. There was an IGN GPS station, LP03, located in Tajuya, that showed tremendous jumps. From the beginning, we saw that the magma, in its ascent towards the eruptive focus, experienced a significant change of route in this area. During the eruption, every time there was an uplift at LP03, there was some disruption in the eruption: a very high lava fountain at the effusive vent, new fissures opened, or existing ones lengthened.
The Importance of Knowing the Island’s Internal Structure
These more plastic materials are often associated with hydrothermal alteration zones, where rock minerals transform into iron oxides and clays that behave plastically. This means that when magma encounters one of these zones, it struggles to pass through and break it, and seeks another path. Therefore, it is essential to know the internal structure of the island and understand which areas are more rigid and which are more plastic, as well as the location of faults and fractures as potential ascent pathways.
Many geophysical studies have already been carried out: magnetotelluric, gravimetry, magnetometry, and especially seismic tomography studies that have helped us understand the interior of the island. These studies have already been done on almost all the islands and provide very interesting data for volcanic monitoring, yielding valuable information on possible magma escape routes.
Last-Minute Warnings and Historical Precedents
It is true that a few hours before an eruption occurs, there are some phenomena that the population detects, such as noises, smells, and other strange phenomena that can relatively determine, with some precision a few hours in advance, where an eruption is going to occur. But that only happens when the magma is already very close to the surface. For example, in La Palma, there is the case of a woman in El Paraíso who the night before the eruption saw the toilet water boiling. She called the Guardia Civil but they didn’t have much information either. In the end, she, who was a foreigner, packed her bags and left.
In the same way, several people told me they heard underground noises in Todoque the night before the eruption. And another example: in the 1949 La Palma eruption, in the eruption area, pine trees previously burned from the roots. In the 2021 La Palma eruption, once the eruption began, this phenomenon occurred both north and south of the eruptive centre.
Not All Magma Movement Ends in Eruption
I believe that the 2004 seismic-volcanic crisis was different. On that occasion, there seems to have been some magma movement; currently, for now, only fluid movements have been confirmed. In 2004 there were even earthquakes felt by the population and even more intense deformation than there is now. At that time, there seems to have been magma movement. This means a seismic-volcanic crisis occurred. The thing is, magma can move many kilometres in the crust and the island’s structure without reaching the surface and generating an eruption.
This is the case, for example, of the island of São Jorge in the Azores, where in 2022 the movement of magma generated many strongly felt earthquakes, very close to the surface. There were even evacuations, but in the end, the eruption did not occur. In the Canaries, there is a documented case. Some historians record that in 1914 there were earthquakes, and even visible gas emissions in Pájara, Fuerteventura. But suddenly it all stopped. So not all deep magma movement ends up producing an eruption.
The Influence of Tides and Assessing Hazard
In La Palma, during the pre-eruptive week, each day that passed the probability of an eruption increased, but the magma could also have stopped before reaching the surface. During this pre-eruptive week, the frequency of earthquakes seems to have been influenced by Earth tides. Tides, produced by the gravitational attraction of the Moon and Sun on Earth, not only affect the hydrosphere or atmosphere but also the geosphere. It seems that when maximum tidal attraction occurs, the Earth suffers a deformation in the direction of the Sun and Moon. If magma is trying to ascend to the surface, the tidal attraction aids that ascent, and therefore earthquakes occur.
Hazard maps are made by applying a basic principle of Geology: uniformitarianism. This principle tells us that geological processes always occur in the same way and generate very similar effects. Under this concept, one can interpret the rocks formed in the past, even if the process was not seen, as well as foresee the future. Hazard indicates the probability that a specific geological phenomenon capable of causing damage will occur in a place, at a specific time, and with a particular intensity and characteristics. In this way, the probability of an eruption recurring in a specific area can be evaluated.
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