Hekla, a brief description
Hekla is probably Iceland’s most well know volcano. It is a fissure volcano, but during its history it has started more to resemble an elongated stratovolcano. It shares together with Grimsvötn the infamy of being the two most active volcanoes on Iceland.
Hekla is the least reliable volcano on the planet. It normally gives very little as a warning, during the last eruption the volcano started to have small quakes just a couple of hours before erupting. And those earthquakes were below the human threshold to notice. Even though there were a few small earthquakes a couple of hours before, it is more correct to say that the time from anything was noticeable until a column of ash started to form was 62 minutes. The same unpredictability goes for all recorded eruptions.
There have been reports that during the hours before an eruption there is a sulphuric smell to the northwest of the volcano, but I would not trust that as a definite sign. All it would take is a bit of wind and you would not notice it.
Hekla is the ashiest of Iceland’s volcanoes. Analysis of tephra finds in Scottish, Norwegian and Swedish peat bogs has lead to a calculation that two thirds of all ash in northern Europe comes from Hekla alone. This ashiness has helped researchers to narrow down when other volcanoes on Iceland has erupted. If you are looking at the Global Volcanism Program and see the word tephrochronology you can assume that it has been dated after comparing to the Hekla tephralayers. Hekla is normally producing andesitic basalts, not the normal tholeiitic basalts common to the other Icelandic rift volcanoes.
The tephras are normally very high in fluorine, and the eruptions emit large quantities of fluorine gas. This often causes large scale problems for the sheep industry since the grazing sheep gets severely affected or killed by acute fluoridosis.
Hekla has one of the world’s most complex innards. 27 different researchers using different methods have found 13 different magma chambers ranging from 14 kilometers depth all the way to the boundary where the volcano starts to jut up from the surrounding ground. It is probably this intricate maze of chambers, dykes, tubes and whatnots that makes the volcano so explosive and quick to erupt.
Hekla is normally running in eruptive cycles. The current cycle started in 1947 with a VEI-4 eruption that caused large scale impact and fatalities. From 1970 onwards it has erupted about every 10 years with the exception of the 1980 and 1981 twin eruption.
Hekla’s eruptions normally has two phases, first a highly explosive phase producing a high ash column, then often follows a lava phase where either the entire fissure opens up, or parts of the flanking fissures open up. The length of the fissure that opens is normally a sign of how large the output of lava will be. The size of the lava output can range from anything from 0.1 cubic kilometer to several cubic kilometers. Hekla can show any of all known eruptive behaviors ranging from lahars to pyroclastic flows. Lava bombs are common during the initial explosive phase, pumice is not uncommon. Pretty much nothing can be ruled out during an eruption.
An eruption normally starts with a few earthquakes ranging between 1.7 and 2.0M, and then a large borehole transient is registered as the fissure is pulled apart. Harmonic tremoring then skyrocket as magma is explosively pushed up. During the last eruption magma travelled 3500 meters in less than 15 minutes. In 2000 the time from the recorded 1.7M earthquake to formation of the ash column was 62 minutes. This makes Hekla into the most dangerous volcano on the planet to be on top of. If you are on top and feel even the slightest earthquake, or minute tremoring, then you are dead.
Signs of the upcoming eruption
In 2006 the inflation of Hekla passed the level of inflation from before the 2000 eruption. This prompted IMO to send out a first warning stating that Hekla would erupt during the next 2 years. The rate of inflation has since been constant, and it has been calculated that the amount of magma in the system is now twice as large as before the 2000 eruption. The inflation first happened directly under Hekla in at least 3 different magma chambers. Since 2007 inflation also has occurred in either a cryptodome or a laterally removed chamber under Isakot/Búrfell.
During 2010 Hekla started to show a new feature, when earthquakes happened directly to the west of the volcano it started to show contractive borehole strainmeter transients, the same as recorded during the opening phase of the 2000 eruption, but on a smaller scale. This has been interpreted as being a sign of the volcano having reached critical systemic pressure.
During late 2010 Hekla started to have small earthquakes. So far they have been rather small, but they have consistently been followed by transients. This is rather worrisome since Hekla up until now has been considered to be rather seismically inert. On the seventh of July a 2+M quake was recorded together with aftershocks. This was followed with a very large transient reaching up the 1/3 of the energy recorded during the 2000 eruption. This prompted IMO to release a public warning that Hekla was about to erupt. Hekla though quieted down after that.
Other signs that have been reported during the last two years are reports of sulphuric smell; this was reported last during the July seven event. Another sign is reports of wells drying up around the volcano. This so far has been temporary only. But it is regarded as a sign of hydrothermal changes in the Hekla system.
There has also been recorded harmonic tremoring and magmatic signatures in all of the earthquakes during the last year. The last recorded episode of harmonic tremoring was during the last 24 hours.
What would happen if an eruption occurred now?
If an eruption starts it would most likely follow the same behavior as before. A couple of hours of very week earthquakes ranging up towards 1M, then followed by one or two earthquakes ranging between 1.7 and 2.3M, a very large transient, harmonic tremor skyrocketing, and then within the end of the first couple of hours a massive ash column would form.
The probable size would be a large VEI-3 up to a sturdy VEI-4. The amount of magma inflation gives a high probability for the entire fissure opening up, with a slight likelihood for a fissure opening up towards Isakot/ Búrfell. Due to the larger than normal accumulation of magma in the system it is not unlikely that the effusive phase would produce about 1 cubic kilometer of lava, together with 0,5 cubic kilometer of tephra. But this is guesswork, and the eruption could be either smaller or larger. This is to be seen as a statistical average based on inflation rates.
When could an eruption occur?
An eruption should actually already have happened, so most likely from a statistical standpoint, and with the signs of an upcoming eruption vectored in, we are anything from an hour, to some weeks away from an eruption. One should though recognize that predicting volcanoes is an impossible task. Icelandic Met Office has the best volcanologists on the planet, and they are very open with information (thank you!). During the 2000 eruption they actually hit the eruption on the minute, but it is rumored that it was a combination of tremendous skill and pure luck that made it possible. On the other hand, the skilled are normally lucky. But then, both of their last predictions for Hekla did not work. Why? Because Hekla is one of the most complex and unpredictable volcanoes in the world. But, I would be surprised if we got into June without an eruption having occurred.
Sadly the Hekluvöktun is not fully operational right now, but it is still the best source of information. I though hope that IMO will fix the page soon. It is not good that the borehole strainmeter plots have been down for months. It is after all the best way to see that an eruption is starting. Currently we have a larger chance by watching the cams.
And a stark warning!
Do not, for any reason climb Hekla. If you do you are playing with your own life. I would also strongly suggest that you should stay at least 5 km away from the volcano. If you are within that distance you would probably not have time to get to safety. This is not a joke.