Small ash emissions, steam rings, summit access for tours reopened
Update Fri 14 Jul 2017 08:59
The volcano remains restless although no one knows what will come next. After a few weeks of "total silence", tiny ash emissions started again to occur from the SE crater since 9 July, volcanologist Dr Boris Behncke reported on facebook. This activity, which could represent small but deep-seated explosive activity inside the SE crater's conduits, has been continuing over the past days, with a slight increase yesterday (see video below).
Steam ring blown from Etna's Voragine crater this morning (image:Radiostudio7 webcam)
Access to the summit craters for tours guided by authorized mountain guides has been allowed again.
Bocca Nuova with its collapsing crater floor on 13 July 2017 (image: Gio Giusa / facebook)
Small ash puff from the SE crater on 9 July (image: Boris Behncke / facebook)
Other than that, there is intense degassing from the Voragine summit crater, which sometimes also produces beautiful smoke rings (steam ring vortexes), while the Bocca Nouva's crater floor continues to undergo slow concentric collapse (see images on facebook)
Intermittent ash emissions from SE crater
Update Mon 26 Jun 2017 08:24
Ash puff from Etna's SE crater this morning (image: RadioStudio7)
After a period of calm during the past weeks, the SE crater started again to show signs of unrest. Since yesterday morning, small to moderate ash emissions have been occurring intermittently from the summit vent of the SE crater complex.
Stronger ash emission yesterday morning (image: RadioStudio7)
Thermal imagery indicates that the ashes are relatively cold, suggesting that the magma column at the moment resides still low in the conduit.
Whether the observed ash puffs are caused by deep-seated explosive activity (=degassing of magma) or by internal collapses is difficult to say.
Other than from the SE crater, there is strong degassing from the Voragine as well as weak incandescence in the Voragine and the crater open on the eastern side of the SE crater.
Regular "mini-eruptions" at SE crater - an interpretation
Update Fri 19 May 2017 08:48
Since 11 May, Etna has been in an unusual state of activity. At remarkably regular intervals of approx. 8 hours, the "saddle vent", located between Etna's old and new Southeast Crater, has been producing more than 20 brief episodes of mild strombolian activity, each accompanied by a short-lived increase in volcanic tremor amplitude. The episodes are further characterized by the absence of strong degassing.
Strombolian activity at the SE crater's saddle vent and the peaks of tremor accompanying each short-lived episode (image: Boris Behncke / INGV Catania)
What causes this unusual and certainly transient behavior is not exactly known. Researcher Dr Marco Neri tries to give an explanation in an article on La Gazzetta Siracusana(loosely translated):
"In fact, Etna volcano produces "mini-eruptions" that last a few tens of minutes, located at the top of the New South-East Crater, at about 3300 meters. There would be nothing strange about this, except that these events occur with an impressive regularity, about 3 a day, and at fairly constant intervals of about eight hours. A phenomenon that closely resembles that of geysers, although in this case it is magma to erupt at the surface and not water. ...
These small eruptive events are always accompanied by an increase in the amplitude of volcanic tremor, which corresponds to a vibration produced by movements of volcanic fluids circulating within the eruptive duct. When the magma approaches the surface and erupts, tremor increases sharply. ... Therefore, the tremor graph shows a series of almost regular peaks for amplitude and intervals. It's like volcano's hiccups!
"Why do these mini-eruptions happen? What makes them so regular and frequent? Without claiming to always understand everything about a volcano, especially Etna which is full of surprises, we can try to give an answer:
Etna's feeding system, and in particular the eruptive ducts that feeds these eruptions, can be imagined like a narrow and long pressure pot. The heat of the magma is similar to the fire lit under the pot, while the gases separating from the magma and accumulating in the top of the duct correspond to the water vapor that forms inside the pot when the water is heated and bubbles.
Finally, the eruptive vent from which the mini-eruptions occur can be compared to the valve of our pressure cooker. Every time the gas pressure inside the duct exceeds a certain critical value, the "valve" opens and some magma erupts on the surface. And to complete the similitude, the characteristic "whistle" of the pot valve that opens can be compared to the hissing sounds that people nearby perceive during these mini-eruptions.
"The extraordinary nature of the phenomenon that we observe at the moment lies, however, in the fact of the remarkable regularity of the mini-eruptions, showing that the system has reached an excellent balance between the magma that is being fed from below into the upper ducts and the magma that is lost (expelled) from the system during these mini-eruptions.
What will happen?
How can the hiccups of Etna cease? The phenomenon may last for a long time, at least until this balance between the rising magma and the eruptions stays in place. Things will eventually change when the available magma volume will decrease, or, if the volume of fresh magma rising from below increases significantly, leading probably to eruptive episodes such as observed during the past months.
Also the geometry of the upper portion of the the vent, or, to stay in the image, our "volcano" vent valve, plays an important role: should this vent change, e.g. collapse, or become obstructed, it would certainly also change the course followed by magma to reach the surface, as well as the energy needed to produce an eruption.
In this case, it would change the eruptive style of the volcano and perhaps also the position of the eruptive vent." (Marco Neri / INGV Catania)
New effusive episode from SE crater during 26-28 April 2017
The two lava flows from the SE crater during the night 26-27 April 2017
The first lava flow on the southern side of the SE crater and lava fountaining at the vent in the early hours of 27 April
Another mainly effusive eruptive episode occurred from the SE crater during 26-28 April.
Steam from a melt-water avalanche on the northern lava flow in the morning of 27 April
Starting from the evening of 26 April at around 18:00 local time, a lava flow started to emerge slowly from the summit vent of the Southeast Cone and descended to the south and then bended eastwards at the foot of the SE crater complex.
There was with no significant explosive activity in the beginning: this was likely because the first lava to be erupted was the remaining, degassed lava still in the conduit from the previous episode being pushed out first by a new batch of rising magma.
SE cone collapses, new lava flow to the north
During the night, after the new magma arrived at the surface and mild strombolian activity (or low pulsating lava fountains) started to occur as well, as effective degassing of the fresher magma. At around 01 am in the night, the SE cone collapsed partially, probably because its edifice, already fractured, could no longer withstand the pressure of magma in its conduit. A second lava flow emerged and traveled north from the vent.
The eruption with two lava flows continued all night and during the morning of 27 April.
Starting from noon of 27 April, the lava flow to the southern flank of the SE crater started to decrease and ceased by around 13:30. The northern lava flow continued during the evening, but also ceased during the night 27-28 April.
During the morning of 27 April, the advance of the new lava flow on the northern side over fresh (not covered by recent flows) snow-covered ground triggered a series of small phreatomagmatic explosions.
The most significant such explosion took place between 08:46 and 08:48 local time. It begins with the emission of steam from the center of the lava flow, shortly followed by the ejection of fragments from the flow itself. This event also triggered a rapid melt-water hot avalanche that traveled about 700 m in less than 2 minutes.
A smaller melt-water flow was also observed earlier at around 06:52 local time.