Thanks to today’s bathymetry we now know that pretty much all we thought we knew about the volcano known as Bob south of La Restinga is wrong. To be quite honest it is not odd that so many professionals wanted to say that the eruption was over, and where wrong every single time, the reason for this being that all signs point to an end of activity. Question is more why Bob does not stop when she should?
Bob is not a volcano
It is rather obvious, but we tend to treat the volcanic vent of Bob as a volcano. But it is not. It all has to do with what we perceive as a volcano. And normally we perceive the cone and the vent opening as the volcano. But from a volcanological standpoint it is the innards that are important. And if we follow the feeder tube back from Bob we end up under Tanganasoga and its magma chamber(s).
Results of the new Bathymetry
Bob itself has only grown with about ten meters since the last bathymetry. The depth from the surface to the top of Bob is 120 meters; it now has a base (foundation) of 350 meters. The reason for the limited growth is of course that Bob has grown a 75 meter high parasitic cone.
The effect of this is that Bob has by now clogged up and will most likely not be active any longer. Instead the parasitic side cone has taken over as the main vent, and will given enough time devour the old Bob.
During the last bathymetry the total ejected volume was 0.14 cubic kilometer. Today it is around 2 cubic kilometer of lava that has been ejected. That is an impressive number for such an unassuming little volcano. If it continues at the same pace for another couple of weeks it will surpass the total ejected volume of Eyjafjallajökull (2010) and Grimsvötn (2011), and those two are volcanic brutes compared to El Hierro.
Data from GPS:s are hard to get, and even harder to interpret since IGN for reasons unknown only give out summed up displacement figures, and that is not the up or down values that makes a volcanologist happy.
Regardless of IGNs oddities we know that the GPS values have been steady during the entire eruption. And that means that the amount ejected is the same as magma arriving into the system.
The other data, from the University of Nagoya, has been forbidden to be used by INVOLCANs so called director, aptly named Nemesio. But I do not really care about him, those GPSs also show stable values.
Gas values have been rather constant; yes they have gone up and down, but not in any spectacular fashion really. There is though one measurement value that would be very good to have, and I hope they are taking, and that is gas values inside the cave systems on the island. I will get back to this.
Earthquakes are now far in between, very low in amplitude (strength), and actually point to an equilibrium state in the internal pressure. So, as long as Bob putters on, not much will happen regarding the earthquakes. One should also note that the earthquake are fairly stable in depth, and stay well within the boundary of the original earthquake wad.
I would here like to seriously warn against falling into the almost hysteria that has been the last few days regarding the earthquakes. I can understand that the Hierreños got scared and felt betrayed when they saw equipment and aid personnel leave the island, and that is making them more afraid of the normal earthquakes. But it does not help if we give fuel to their fears. The earthquakes are of no importance at the current state.
Before worrying look at the earthquake wad above, and also at the scale-plot down below. Remember that the volcano could take heavy 4M earthquakes without breaking apart. Look very closely at the lack of motion on the plot below that shows that the energy levels have been minute for a long time.
What we are seeing in the tremor is rather impossible really and makes Bob a rather intriguing event. We should have seen high tremoring as we did before. Why? Well, Bob is still ejecting as much as before and thusly we should see no shift in tremor. Instead we now have ultra low values of tremor.
What I think has happened with the tremor is due to several factors. First of all I think that the new vent of Bob is wider, I also think that the eruption has smoothened out the walls of the conduit giving a clear and nice ride for the magma to the surface. I also think that the composition of the magma has changed, more about that below.
Regardless, I was fooled by the drop in tremor, and so was almost everyone else. I at least admit to having been fooled by it.
What one should look at is the 0.59Hz tremor given by EOSO and EGOM stations. That is probably the tremor from the deep feeding tube. It might even be from the mantle plume.
In the beginning we saw pretty much reactivated old evolved magmas and semi solid rocks being erupted, after that followed a long phase of what probably was reactivated rhyolitic mush. That would explain all the ash in the water (suspended tephra) that we saw back then. Now all of that is probably cleared away and we have an unevolved stream of basalts comparatively low on gasses being ejected. This basalt is of deep origin as evidenced by the UrTh (Uranium/Thorium) content in the samples that has been collected from the surface.
The future of Bob
Time for me to look deep into the magic chrystal eight ball.
Given what we now know there is no reason for Bob to shut down for at least the next couple of weeks. As long as tremor levels are low, there is no rapid increase in earthquake rate and amplitude, and the 0.59Hz is going, then it should just be ticking away. I would not even rule out that Bob pops up from the surface in about a year.
What could happen in a couple of weeks is that Bobs new vent clogs up. And it is then it could get interesting. Most likely a new vent would open up close to Bob again, but that is not sure. If a new vent does not open up the Volcano no longer has an open pressure relief valve. And then systemic pressure will start to rise. In that case we will see earthquake rate and amplitude start to rise. And sooner or later the magma would go somewhere. The most likely thing is still not an eruption of Tanganasoga central volcano.
The most likely thing to happen is that a vent opens up somewhere upstream of Bob’s feeder tube from the magma chamber(s). The other likely thing is that the magma will find a new easy route out and that would be one of the old lava tubes that already exist all over the island, namely the cave systems. And that is why they should really have gas measurements taken inside of the caves.
Right now there is no apparent danger of the eruption changing into something more dangerous. As long as Bob is puttering away, there is just not enough pressure for anything else happening.