Under the volcano: here’s what happens before the eruption


Mauna-Loa in Hawaii, Semeru in the island of Java and Stromboli in the Aeolian Islands. Three large volcanoes fired 2022 shaking the earth.

In May 2022 Etna was the protagonist and in September 2021 it coincided in eruption with the Tajogaite volcano on the island of La Palma. A year has now passed since the end of this eruption and it may seem that a page has closed. But the Earth is still alive and large volcanic eruptions are always probable, both for the spectacle they represent and for the tragedies they bring with them.

El Tajogaite has caused the evacuation of more than 7,000 people, many of whom have lost their homes and even livelihoods, the destruction of roads and basic infrastructure, and the formation of lava flows that have devastated more than 1,200 hectares with a thickness which in some places reached 70 m. Without forgetting other consequences that are now manifesting themselves, such as continuous gas emissions, respiratory problems, skin diseases, mental problems and anxiety in the population that will take a few years to be resolved. Therefore, the search to detect the precursors of these volcanic eruptions is, if possible, even more important.

under the volcano

To get ahead of an eruption, we need to understand what’s happening underneath a volcano before it happens. The heat and pressure are so high within the mantle that it causes rocks and gases to melt, forming magma. These high pressures and temperatures cause the magma to try to escape through the earth’s crust, looking for fractures or weak areas to progress in its ascent. When it finds one of these structurally weaker areas, it builds up there, forming reservoirs we call magma chambers.

The rise of magma can be very fast or so slow as to invest up to decades. On this journey, successive magma chambers are formed. When there are only a few kilometers left to reach the outside, the signs appear on the surface in the form of gases they give off, earthquakes produced by the breaking of the crust in the rising of the magma and deformation of the ground. The latter caused by the positive pressure of the magma and by the horizontal and vertical cracks of the crust.

Detect magma before it reaches the surface

During the volcanic eruption of La Palma in 2021, researchers from the Institute of Geosciences (IGEO-CSIC), together with other researchers from the Complutense University, the Polytechnic University of Madrid and researchers from several foreign centers in Italy, Canada and the United States Together we started working on a technique that was able to detect the accumulation of magma in an area close to the surface, months before the eruption occurs.

This technique is based on the study of those ground deformations produced by the ascent of the magma before the eruption and by the ruptures of the crust, recorded with the latest generation satellite radar interferometry (InSAR) observation techniques, combined with a new technique for the interpretation of these deformations.

Precursor signs of the rash

The eruption of the Tajogaite volcano on La Palma began on 19 September 2021 on the western slope of Cumbre Vieja and was active for 85 days, generating lava flows that add to those of previous eruptions on this slope.

Simplified geological diagram of the island of La Palma, with the historical eruptions of which we have references, located on the slopes of the Cumbre Vieja, in the south of the island.
author provided

Our results show that about three and a half months before the eruption an increase of magma appears, which grows over time, thus forming a magma reservoir at a depth of about 2.5 km in a structurally weak, fractured and porous area.

In the image that follows this paragraph, we have represented in red the strong positive pressure under the earth’s surface probably caused by the intrusion of magma. This is suggested by this path of points, which would occur below the city of Jedey, being about 5 km south of the main volcanic cone of the eruption. Two months before the eruption, at a depth of about three thousand metres, sources of vertical fracturing begin to appear (represented in yellow), suggesting a fragile crustal response due to the increased upwelling of magma.

Inflow of magma from the mantle between May and September 2021 before the eruption. In red the positive pressures related to the intrusion. In yellow, the internal fractures that cause the magma to rise. Surface evolution of magmatic intrusion under La Palma before and during the 2021 eruption, Scientific Reports, 2022, Author provided

In conjunction with the seismic activity before the eruption, the appearance of these sources of fracturing and stress increases, which we associate with the magma dike that used that fissure to come to the surface. In these structures of pressure and tension, in addition to the main branch associated with the eruption, two other ascending branches are activated, also represented in red in the image below. These two branches are located one under the ocean south of Puerto Naos and the other west of the city of jedywithout reaching the surface as failed magma intrusions.

Location of the magma chamber under the surface of the island, with its three routes of ascent. Middle: plan view. Right: view in North-South and East-West profiles. Surface evolution of magmatic intrusion under La Palma before and during the 2021 eruption, Scientific Reports, 2022, Author provided

The model of magma reservoir, magma upwelling and associated ground fractures obtained in this study would also help explain other phenomena, such as the still continuous emission of gas in the areas of Puerto Naos and La Bombilla.

Are we any closer to being able to anticipate volcanic eruptions?

The results of this methodology, which were published in the journal Scientific reports, show that this technique can aid in the detection and monitoring of new volcanic reactivation episodes. Also to determine potential areas of magma accumulation, aid in the determination of possible eruption paths and advance in the prediction of the start of a next eruption on La Palma, Canary Islands, as well as other active volcanic islands.

Are we now closer to being able to anticipate volcanic eruptions? We cannot answer with a resounding yes, unfortunately there is still much research to be developed in this field. The results that have been obtained in the study of this eruptive process seem useful for moving in this direction, it would also be very useful in the design and planning of infrastructure and urban development in the reconstruction of the island after the eruption. We can know a lot about what happens under the volcano, and this can allow us to anticipate the catastrophe.The conversation

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