Etna volcano: an explosive start for 2021

15/04/2021

Etna, on the Italian island of Sicily, is Europe's most active volcano and it is famous for the incandescent lava flows that glide down its flanks, forming a spectacular backdrop for the city of Catania. Since December 2020, volcanic activity has intensified markedly and eruptions have become increasingly violent. In this article, volcanologists from Belgium describe and explain the causes of Etna's latest rumbles.

Authors: Dr. Sam Poppe, Lorenzo Cappelli, Dr. Paul Wallace, Prof. Karen Fontijn - Volcano scientists at G-Time, Department of Geosciences Environment and Society, Université Libre de Bruxelles. Editing: Kathelijne Bonne.

During sixteen episodes of intense volcanic activity, ash plumes have sometimes risen more than 10 km into the atmosphere. Ashes have showered on nearby towns, and the Catania International Airport has had to close a couple of times. Volcanologists think this recent uptick in explosive activity may be caused by unusual movements of the magma located deep below the volcano: it is ascending more rapidly than normal. 

Paroxysms vs. normal behavior of Etna

To understand Etna's intense behavior of the last couple of months, we must understand what is normal for Etna, because this volcano always shows some kind of activity, and in the end, this is what volcanoes do. The most common type of activity at Etna are "effusive eruptions" that produce gentle lava flows, or "Strombolian eruptions" that are weak explosions spraying molten lava around the crater. But sometimes, Etna's activity is more explosive, generating more intense lava fountains and ash columns that can rise up to several kilometers high into the atmosphere. These more intense phases are what volcanologists call "paroxysms".

Etna in 2021: Ash-laden skies

Since becoming more active in December 2020, a few weeks of "normal" Strombolian activity passed, but then the explosive intensity gradually intensified and reached its climax during the third week of January. From that time onwards, a crater at the summit, the so-called "New South-East Crater" continued to produce explosions (see title picture). 

A lot of molten rock overflowed the crater towards the east, feeding large lava flows. Small, diluted plumes were either dispersed in the atmosphere or directed (south-)eastward along the main winds, covering the nearby villages with thin layers of volcanic ash. Clouds of sulfur dioxide gas, detected by the Sentinel-5P satellite, sometimes reached as far as Sardinia, Corsica and the Italian mainland, as shown on the image below.

During the first half of February, Etna became quieter again. But then, on the 16th of February, a sudden release of energy marked a drastic change in volcanic activity. Intense lava fountains shot up to an impressive 500 m high, feeding lava flows of several kilometers long. At the same time, explosive ash columns carried volcanic ash particles, gas and steam several kilometers high into the atmosphere. Volcanic ash fell as far as Catania and even on the city of Siracusa, located 80 km to the south. The airport of Catania closed its gates. By the end of March, at least sixteen paroxysms were witnessed: some lasted up to 3 hours and sent ash columns up to 10 km high. During the last week of March, several paroxysms, lava fountains and lava flows occurred.

So, what is next? To find clues about future behavior of a volcano, volcanologists look at its past.

Etna from Catania International Airport (2021).
Etna from Catania International Airport (2021).

Young Etna

The Etna we know today started to form about 15,000 years ago on top of the remnants of a massive ancient volcano called Ellittico. Although eruptions along the volcano flanks formed many different craters, over time, eruptions progressively clustered near the summit and gave the volcano its steep-sided shape. Around 10,000 years ago, the eastern flank of young Etna collapsed in a huge landslide that formed the large valley of "Valle del Bove" where most of the volcanic activity has been concentrated since then.

Etna in the 20th century

During the last century, the intensity of eruptions and volume of erupted magma progressively increased. The first half of the 20th century mainly saw sporadic but intense lava flows and mildly explosive eruptions, the typical, 'normal' type of activity for Etna. Most of this activity took place at the four craters that formed the volcano's summit, and only sometimes lower on its flanks. After 1971, the volcano experienced an increase in activity and sometimes the lava flows reached the inhabited settlements on the higher flanks of the volcano.

Etna's "New South-East Crater"

In 2007, intense eruptions started to occur along a fracture along the flanks of the already existing South-East Crater at the summit. By 2011, enough material (ash and rocks) had accumulated along this fracture to form a new crater, aptly named the New South-East Crater. A total of 25 paroxysms produced vigorous lava fountains and explosive ash columns. The New South-East Crater has been especially active throughout the last decade: between 2011 and 2015, it produced more than 40 paroxysmal eruptions, and since 2015 it has often produced lava flows and small explosions.

Inquisitive minds will now ask: what is the cause of the occurrence of the more violent behavior in 2021?

Evidence from the depths

The answers so far have come from studying the volcanic ash and lava that erupted during the first paroxysm of 16th February 2021. These rocks have provided preliminary evidence of the cause behind the violent explosive eruptions at Etna. Scientists from the University of Geneva (Switzerland), in collaboration with the Italian National Institute of Geophysics and Volcanology (INGV), looked at the crystals in the rocks and their chemical compositions. In the image below, a microscope image of a pyroxene crystal, used in the analysis, is shown. This revealed that the new magma feeding the explosive eruptions came from great depths beneath the volcano (~25 km). This magma rose and then stalled, only briefly, within a magma reservoir at 5-6 km below the surface, before it arrived at the surface, causing eruptions. The studied rocks also suggest that gas bubbles that were trapped in the magma did not have sufficient time to escape as they rose to the surface. As a result, they made the magma more explosive than 'normal', obeying the same laws of physics as a shaken bottle of sparkling water that is opened. The cause of the explosivity of recent eruptions at Etna might thus be an unusually rapid ascent of magma from deep below the volcano.

Etna in the future?

Paroxysms of this kind have occurred in the past and are thus not new to Etna. Yet, the introduction of new magma into any volcanic system requires careful monitoring for the future. It is too early to tell if activity might continue for a while, or slowly decline, with less explosive episodes. Our Italian colleagues at the National Institute for Geophysics and Volcanoes (INGV) will continue to monitor the situation, because they know things can change quickly at Etna. Using a large set of modern equipment to look at ground movement, earthquake activity, evolution in gas output and magma chemistry, they will continue to provide the best possible forecasts of near-future activity at Europe's most busy volcano.

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Quiet Etna, towering above the city of Catania.
Quiet Etna, towering above the city of Catania.

Picture credits:

  • Title picture: Etna's New South-East Crater: Angelo T. La Spina / CC BY-SA 4.0 on Wikimedia Commons.
  • Etna from Catania International Airport (2021): Official U.S. Navy Page / CC BY-SA 2.0 on Wikimedia Commons.
  • Quiet Etna, towering above the city of Catania: BenAveling / CC BY-SA 4.0 on Wikimedia Commons.

Follow @INGVulcano on Twitter and the webpages of the Etna Observatory and the Global Volcanism Program page for Etna for the most recent news, and the account of local INGV volcanologist Dr. Boris Behncke @etnaboris on Twitter for spectacular views from a local volcanologist's home.

This article is a translation of a blogpost which appeared in Dutch on the website noodweerbenelux.com: https://www.noodweer.be/explosieve-start-van-de-vulkaan-etna-in-2021/

Read also our article on Campi Flegrei, Italy's unknown, but most dangerous volcano.