El Hierro – Survival of a stubborn Volcano

Image by IEO. Here we can clearly see the vents of Bob.

Life normally is filled with patterns. El Hierro has a pattern of its own, IGN declares the volcanic vent affectionately named Bob as dead, and a couple of days later the scientific crew of the ship Ramon Margalef publishes a report proving them wrong.

This time in addition to the bathymetric data we also got acidity values and electrical conductivity curves. The interesting thing with these is that they prove that during the measurement period there was a point source feeding acidic compounds into the water. The most likely culprits for the acidity are sulphuric acid and CO2. The values are still roughly the same as during the last measurements was taken, and this leads to the obvious conclusion that the vent is open and active. Also the electrical conductivity is pointing to an active source changing the conductivity.

The bathymetry gives at hand that the cone has now reached 88 meters from the surface. This gives that the growth rate during the last month has been 36 meters. At the going rate we should start to see surface disturbances in about two to four week as hydro magmatic explosions start.

Also noteworthy is that the shape of the volcanic main vent has changed from the stratovolcanic type with steep angle of slopes, to a shallower angled shield volcano type (26 to 33 degrees). One should though remember that a sub-aquatic volcano always has steeper angles compared to a sub-aerial one. This due to the water cooling the lava faster than the air does.

Image by IEO. Here we can see how shallow the angles of Bob have become during the last couple of months. The true angle is now between 26 and 33 degrees, normal angle for a sub-aquatic shield volcano constructed from unevolved basalts.

Another tidbit given is that the lava now is more fluid then before. This goes well with the less steep angles that we see now.  It all points towards that the first phase of eruption was containing large parts of reactivated rhyolitic material. This gave a high discoloring of the stain due to the high amount of ash content suspended in the water. Now the old rhyolitic mush is cleared out from the system, and the volcano is emitting predominantly unevolved basalt of deep origin.

This last part explains the low harmonic tremor since basalts travel easily in the conduits compared to the semi-solid rhyolitic mush, especially in a well worn feeder tube like the one leading to Bob. Another thing is that rhyolites are famously explosive and creates a lot of noise when erupting.

As I have now said time and again. Bob is not a volcano. The volcano is Tanganasoga, with the feeder system leading to Bob functioning as a pressure relief valve at the far end of the fissure swarm leading south from Tanganasoga. During the eruption there have been long periods of very low tremor associated with possible deep magmatic movement. This low frequency tremor is most easy to spot at the stations named EOSO and EGOM, and are found as clear red lines at the 0.59Hz and the 0.3Hz bands.

This gives that as the pressure has gotten into equilibrium the noisiness falls even more. Bob is now ejecting magma at the same rate as it is introduced into the system by the hotspot. With this in mind we can rule out any large changes in the eruption as long as the vents are open. And the eruption can be a very long one.

Thanks to the regular commentators Judith and CarlosB for digging out the newly released reports in a very quick fashion.

http://www.ieo.es/apartar/ieoprensa/hierro/hierro_leg11.pdf

http://www.ieo.es/apartar/ieoprensa/hierro/hierro_leg12.pdf

CARL