As time goes by we understand more and more about the activity under El Hierro. The Canarian Hotspot has several plumeheads reaching up under the volcanically active islands in the Island chain. It seems that for unknown reasons the activity has changed at depth so that El Hierro has become far more active than previously.
The island has been active previously, last time in 1947 when a merchant ship captain reported activity beyond where the eruption outside of La Restinga took place a couple of years ago. This eruption is though uncertain; the same goes for the 1793 eruption at Lomo Negro to the Northwest.
There are no sources pointing towards the island having suffered anything even remotely close to what we are seeing now during historical times. As the activity has continued a pattern has started to emerge were magma pulses travel up via the plumehead and causes intrusions at depth under the island. So far every intrusion has occurred in different places under the island. Why the intrusions favor different locations is anyone’s guess really, it might though have to do with layering at depth causing the intrusions to occur in more brittle layers and that when they have filled up with hot material they stop being brittle and new magma goes somewhere else.
The pattern seems to be that an intrusion happen roughly every 90 or 180 days apart. They contain anything from a couple of hundred earthquakes up to a couple of thousand. The earthquakes form around what are most likely sill and dyke emplacements.
Explanation of the plot above: This is a view of the very recent quake swarm which began on March 14th. Update up to 16/03 7h14 AM. I have kept only the quakes of the present swarm and have supressed all the quakes prior to the 14th. For the first 2 views, the title bar shows count, datespan, date and hour of the events as well as magnitude. The color bar shows as usual, date on the left and terrain elevation on the right. The first view is a fixed view from the south, the second view is a fixed view from the east , both with an event by event animation. On each view there is a forward tilt of 10°. The following views are rotations with all events since the beginning of march, and views from the top and back. Data from IGN and NOAA, made on Gnu Octave 3.2.4 (Linux). Plot and explanation by DFM
The latest intrusion is to the northeastern side of the island, an area previously believed to be volcanically dead, which was the reason it was selected as the evacuation zone on the island. It is unlikely that an eruption will occur in this area even after this intrusion, the area was last active roughly two million years ago.
As I am writing this there has been 276 earthquakes and uplift seems to be delayed by two days before becoming visible on the GPS. Todays uplift at the HI00 station was 40mm and we can probably expect to see uplift to continue for 2 to 5 days more if the intrusion finishes today, otherwise it will continue for a while longer.
Explanation for the plot above: El Hierro 3D plot with the Gorbatikov microseismic inversion model. 2011 to 15 March 2014. Plot by Cryphia.
I do not at this time expect any eruption to follow this intrusion; it seems to be fairly small compared to what has happened previously. And even if an eruption occurs it will not be in the same area, instead the most likely spot is somewhere along the line from the Tanganasoga volcano to the previously erupting vent out in the ocean from La Restinga. There is also a smaller chance that Tanganasoga or Lomo Negro will erupt.
I would be highly surprised if this was the last intrusion we will see at El Hierro. And as the intrusions continue they will sooner or later start to happen in the same place and a true magma reservoir will start to form under the island. There might be a few small scale eruptions during this stage, but nothing major. As time goes by we will get to see how large the chamber will be, and if the previous intrusions are indicative it could in the end become pretty big. Remember that the average uplift is 7.5cm per year for the island as a whole.
So, expect years to decades of continued activity if the pattern holds true. El Hierro seems to be walking down the path from a fairly insignificant fissure eruption to the rebirth of a major central volcano. But, volcanoes are unpredictably so everything might just calm down without anything happening.
Another small danger of the activity is that the recurring intrusions will create increased mountain tension on the island and that a part will slide down. So far this does not seem likely, but one should not discount the risk of it happening. Thankfully the IGN has monitored the island for this for a long time so they should be able to stay on top of it.
Volcano of the Month
It is time to vote for volcano of the month for February. This time there was two nominated volcanoes that was active during February and they are Kelud and Nishinoshima.
CARL (text), CRYPHIA & DFM (plotting)