What is a paroxysm?


Eruption column produced during the paroxysmal eruptive episode of 9 July 2011 at the New Southeast Crater, photographed from the toll house of Giarre, on the lower eastern flank of the volcano. Photo taken by Antonio De Luca and published here with the generous permission of the author

In these days there has been much talk about "paroxysms" or "paroxysmal episodes", obviously referring to the latest eruptive events at the Southeast Crater (or more precisely, the New Southeast Crater, whose cone is now growing around what until early 2011 was known as the "pit crater"). But, what exactly is a paroxysm, and is the application of this term in the volcanological language correct?

The general sense of the word "paroxysm" is "a sudden convulsion, the instantaneous increase of a symptom or a sudden emotion or action"; in the Dictionary of the Italian Language of Sabatini Coletti also the volcanological significance is given as "during an eruption, [a paroxysm is] the whole of the explosive phenomena that constitute the most violent and dangerous phase". It seems that the first application of this term to a volcanic event was made by George Poulett Scrope in his classic 1825 book "Considerations on Volcanos: The Probable Causes of Their Phenomena, the Laws Which Determine Their March, the Disposition of Their Products, and Their Connexion with the Present State and Past History of the Globe". In the case of Etna, the use of "paroxysm" seems to have become more common during the past couple of decades, when speaking of one of the numerous episodes of lava fountaining, lava flow emission, and tephra columns, which occur at the summit craters of the volcano. However, it is not clear wheather this denotes the whole episode from the first signs of activity to its total cessation, or if it refers only to its culminating phase. The term "paroxysm" is furthermore used for the more powerfully explosive events at Stromboli.

A typical Etnean "paroxysmal" eruptive episode, like the 38 episodes of 2011-2013 at the New Southeast Crater, consists of three main phases: (1) prelude and waxing, (2) climax, (3) diminution and cessation. The true paroxysm is in fact the phase (2) of each of these events, whereas the three phases together should rather be called "paroxysmal [eruptive] episode" or "eruptive episode".

Phase (1) begins with a resumption of the eruptive activity, often with small Strombolian explosions (sometimes preceded by ash puffs), at times with lava emission (this has happened mainly during the numerous paroxysmal episodes of 2000-2001). The activity then gradually increases, with more and more intense and frequent explosions, and an increase in the lava output rate. This increase is accompanied, on the instrumental level, by a rapid increase in the volcanic tremor amplitude; during the events of 2011-2013, also a shift of the volcanic tremor source toward the New Southeast Crater is typically observed. At the passage from phase (2) to the next phase, the Strombolian activity become almost continuous. The duration of phase (1) can be from a few tens of minutes to several days.

Phase (2) is marked by a strong incrase in the intensity of the eruptive activity, which sometimes passes from vigorous Strombolian activity into lava fountains hundreds of meters tall in just a few minutes. Often the onset of fountaining is accompanied by the generation of a column rich in ash and lapilli, which depending on the meterological conditions will either rise more or less vertically to several kilometers height, or when strong wind is blowing, be bent downwind and form a plume several tens of kilometers long. Ash and lapilli falls occur downwind, sometimes also up to several tens of kilometers away. Nearly all paroxysmal episodes produce lava flows, with effusion rates being highest during the culminating phase; the longest lava flow emitted during a recent paroxysm was that of 10 May 2008, from the same New Southeast Crater, with a length of 6.2 km. The duration of phase (2) can be very short (many paroxysms in the year 2000 lasted only 10-15 minutes), but can also be many hours, such as the 10-hours-long paroxysm of 4-5 September 2007. Toward the end of the paroxysmal phase, the activity begins to diminish, and sometimes ceases altogether in a few minutes.

Finally, phase (3), which can last from a few minutes to several hours, is characterized by the rapid diminution of all eruptive phenomena: the lava fountain returns to Strombolian explosions and/or ash emission, lava effusion diminishes rapidly and soon ceases completely. Isolated explosions can still occur many hours after the end of a paroxysm, as happened on the afternoon of 13 January 2011, 14 hours after the cessation of lava fountaining. In some cases in early 2013, the final activity of a paroxysmal episode blended smoothly into the prelude of the next episode, making it difficult to state when exactly the earlier episode ended and the next one started.

At Etna, paroxysmal eruptive episodes almost always occur in sequences of more or less numerous events - 20 episodes at the Northeast Crater between July 1977 and March 1978, some fifteen episodes at the Southeast Crater in September 1989, 23 more episodes at the same crater between September 1998 and February 1999, and 64 episodes between January and June 2000 (plus two more at the end of August 2000), and finally about 15 episodes between early June and mid-July 2001. In 2006, a new series of about 20 paroxysmal episodes occurred at the Southeast Crater, followed by four more between March and May 2007. The New Southeast Crater (which essentially exists since May 2007, and will be the subject another article on this site to be posted soon) produced three paroxysmal eruptive episodes between September 2007 and May 2008, and 38 episodes between January 2011 and April 2013.

Joomla! è un Software Libero rilasciato sotto licenza GNU/GPL.
 Valid XHTML 1.0 Transitional CSS Valido! [Valid RSS]