IGN and Pevolca have given us the final good bye of the volcanic vent affectionately known as Bob. It is the fifth time that they declare that the activity is dead. I also note that they claim that they “have put an end to the eruption”. I for one did not know that humanity could put an end to any eruption.
In the same missive they say that there is still tremoring and inflation going on, and that the earthquakes are ongoing. And I who thought this was signs that you should not state that an eruption had ceased.
On the contrary the volcano seems to have been rather active, and has by now reached a distance of only 88 meters from the surface. This with the above given signs make the Pevolca statement rather iffy in my opinion.
Another thing is that the low level harmonic tremor has steadily increased for a week; it is best seen on the EOSO and EGOM stations. This tremor at 0.59 and 0.3 respectively is a sign that most likely is associated with deep magmatic movement. Due to the wave-length a wave of such a low frequency cannot build in a narrow tube or small magmatic chamber. The increase probably means that new magma is moving upwards.
Either we should see new activity at the old vents around Bob, or a new vent formation will take place in the coming week or so.
Iceland
Photograph by Ragnar Palmason. Showing red Brimstone in a volcanic cave of Brennisteinsfjöll.
After the medium sized earthquake swarm that took place at the MAR 20 km to the SSW of Reykjavik there have been reports of HS2 gas smell in the vicinity. The first one to report that was our own commenter Irpsit, who went trekking in the area of Lake Thingvellir. He reported strong smell on both sides of the lake, and that it numbed his nose within minutes. This tells us that the concentration was higher than is technically good for you. The danger with HS2 is that numbs the nose and then you can get into an area with poisonous levels of gas.
It is interesting that the gas emission rate in that area has increased after the 4.2M earthquake swarm. It probably means that in some way one of the volcanic systems in the area was affected since the area is rather large and requires more than a little gas puffing up to affect the nose in such away on both sides of the lake.
In reality there could only have been two volcanoes responsible for this emission, one of them is of course Hengill. Hengill is the largest of the volcanoes in that part of Iceland. But, I would like to make the culprit into Brennisteinsfjöll.
Photograph by Freysteinn Sigurðsson. The photograph shows weathered brimstone (sulphuric oxides) at Brennisteinsfjöll.
It erupted at 1000AD, 1200AD and at 1341AD. What makes into a likely culprit is that it is known to stink. The actual meaning of the name of the volcano says it all. Etymology is sometimes a useful science for volcanological purposes. Brennisteinsfjöll quite simply means Burnstone Mountain, or even more precisely, the Mountain of Brimstone.
Brennisteinsfjöll normally erupts in an effusive fissure style yielding between 0.2 to 3 cubic kilometers of lava. Two of the eruptions have VEI-2 numbers, the latest during 950AD. So if it erupts it should not be an ashy affair. At least at long as the lake does not get involved.
Picture from sandandreas.org Picture shows the perhaps most famous faul line on the planet, the San Andreas Fault. It is the cause of large earthquakes in California.
What is an earthquake?
Perhaps the question should have been; when does an earthquake start? For me as a physicist what we normally perceive as an earthquake is just the boring business end of a rather fascinating process, because the earthquake actually starts far back in time.
What we normally see and feel as an earthquake is nothing else than rock breaking. During a small earthquake it is not a larger piece than what you might find in your back yard, and when it is a large earthquake it can be a breakage running for hundreds of kilometers in a fault-line. To simplify it we say that the amount of energy released is the same as the area of the rock breaking in a fault line. There are other factors of course, but this is not the time for that.
Back to when an earthquake starts. To answer that well we have to understand that there are 3 types of earthquakes (that we need to mind today, there are more types of course).
Tectonic earthquakes
This kind of earthquake is driven by the movement of the continental plates; the motion is rather slow from a human standpoint. The speed of the continental plates is normally ranging from 1 to 5 centimeters per year. This type comes in 3 common subtypes; rifting (Mid Atlantic Rift) is when two plates are being pulled apart, clearly visible as the MAR slowly pulls Iceland apart. The second is subduction earthquakes as one continent plate is pushed down under another. Japan is a good example of this. Then we have shearing rifts, which is when two continental plates are sliding past each other.
In general the rifting earthquakes are the weakest, followed by the medium of shearing rift quakes, and then the subduction earthquakes as the strongest (Japan and Chile). The reason for this being the relative crust thickness, a rift zone has to thin crust to create the largest earthquakes, a shearing quake only affects crustal parts that are rubbing together, and the old crusts that are involved in subduction earthquakes has had the time to grow thick enough.
Math is simple. If the fracture is ten km long in a rift zone with ten km thick average crust, then a full faulting would affect 100 square kilometer. The same for 50 km thick subduction earthquake would be 500 square kilometer. Also we have the fact that the old rock in a subduction zone is generally more brittle and have a longer faulting.
Tingvellir on Iceland, the site of many fault lines after earthquakes. The entire area has sunk down due to all rifts.
Magmatectonic
A magmatectonic earthquake is when magma pushes upwards and releases pent up energy that already exists in a rift, shear or subduction zone. Here the movement of magma works as a trigger mechanism, but is rarely the cause of the earthquake. It works more like the small stone that topples the wagon.
Magmatic earthquakes
These are caused by inflow of magma into dykes or magma chambers. The pressure of the magma pushes the rock until it flexes and finally breaks. I will come back to this one.
Age of earthquakes
Here comes what I find to be the interesting part of earthquakes. The age and the processes that make some places able to contain more energy (the last part of which won’t be covered here). I am going to use MAR earthquakes as an example here. As the MAR is spreading with an average of 5cm a year it starts to stretch an area of newly formed crust, our part of the crust is when it starts to stretch 10km thick (just an example). It is rather ductile, so it can take quite a bit of stretching, how much of it we know fairly well.
How do we know it? Well, as an earthquake happens it makes a crack, these cracks normally ranges from 1 to 10 meters or sometimes even more. Our earthquake will give a 5 meter crack. Simple mathematics would give that it would take about a hundred years for our earthquake to form. Problem is just that normally only half of the energy pent up is released in one event. So, it would be more like two hundred years since our earthquake got under way.
The size of our quake will now be determined by the area affected, and effected via the length and depth of the faulting line.
I have here made enormous simplifications, so anybody who wants to explain more in detail in a comment is most welcome to do so.
A magmatectonic earthquakes age is harder to determine, it can be a small amount of magma that hits an old pre-earthquake fault line that is close to faulting. Or it can be a lot of magma hitting a juvenile fault line. The effect will be pretty much the same. With one difference, the first one will most likely not cause an eruption; the second is likely to do so. And, if you have a mature fault line hit by a lot of magma, well that would be your basic Laki eruption.
Magmatic earthquakes are normally fast, anything from a few years, to a few days. In the case of Theistareykjarbunga it took about a year from inflation started to the first earthquake, at El Hierro it took weeks. This is normally governed by the size of the magma chamber, which in Theistareykjarbungas case is huge, compared to El Hierros chambers.
Picture copyright by Magnus Lundgren. Ever been dreaming about diving from one continent to another? Thanks to Magnus stunning picture we can see how it is can be done on Iceland. On one side we see America, on the other Europe. Only in Icelands Thingvellir Fault Zone is it this easy.
Earthquakes and Volcanoes
Magmatic earthquakes are caused solely by pressure from magma and the gases released by the magma. The exception is of course rift zone volcanoes like Krafla. It can either be pressure from below as new magma comes up through the mantle, it can be dyke intrusions, feeder tubes opening up, or inflation of magma chambers, and of course as the vent is opening up as the last stage of an eruption.
Here comes the thing, size does matter. A small 1M earthquake is cracking your average garden stone (well, a large one). A 3M quake is producing a fault that is roughly 100 times larger, and releasing 1000 more times the energy counted in destructive force. I am not going into the math of that here, it is fairly complex. But if someone wants to do it in a comment, please feel free.
So it takes 100 1M earthquakes to fault as large an area as one 3M earthquake, and it takes a thousand of them to crush as much stone (this would be the important number when talking about magma chamber formation). You should think mining blasts here. It is one thing to crack a fault line the length of the detonation on the video below, it is something completely else to blow all that rock to smithereens.
So please, if you see one earthquake of 1M strength, it is not important. If you see a hundred it might be interesting as a sign of something. But to actually have any significance at all, you need a lot more than that. Before a normal eruption you have hundreds or thousands of earthquakes ranging from M1 to M3 or more. And the 1Ms are not even important really. Only exception to this rule that I know of is of course Hekla. That one just needs a few below 1M, and then one around 2M, and then it goes off. But that is the known exception.
And let me reiterate this, I am not a geologist, nor a geophysicist, I am just a normal physicist trying to explain something really hard, and by doing so I have simplified things a lot. If you find that it is too simplified, or simplified so much that it is useless, feel free to compliment and explain further in the comment field. I will only appreciate it.
And once again, small earthquakes are really small, don’t be fooled.
And now over to a huge detonation courtesy of New Boliden Mining AB, I just like big detonations. Sorry, it is probably genetic among Swedes, blame Nobel. 1100 ton explosives used, 2Kton destructive nuclear device equivalent. And now to the real candy, this a 4M earthquake equivalent.
Photograph by Greg Headley. Reykjanes is quite literaly the spot where you can see how the world is dividing. This out-cropping of the MAR is where the Mid Atlantic Rift comes up out of the ocean.
Six o’clock yesterday (tuesday) a small swarm of earthquakes started at Reykjanes Ridge, it started with a 3.8M at 05.48 in the morning. That quake was followed by a limited number of quakes that trended downwards in strength.
Earlier today (wednesday) the swarm returned and picked up strength and amount of quakes quite considerably.
Image by Icelandic Met Office. The telltale signs of 3M+ earthquakes. The green stars of Iceland.
Image by Icelandic Met Office.
These earthquakes are visible on all SILs in iceland. I have here though chosen to show what it had as effect on the Hekla Borehole strainmeter plot. Look at how it produces first two large spikes, and then a bell-curve as energy increases, and then decreases in the swarm.
Image by Icelandic Met Office. Bell curve of seismic energy.
Reykjanes Ridge is a sub-aquatic volcano as well as a part of the MAR. It has had numerous eruptions, sometimes producing ephemereal islands, and some islands that has stayed above surface.
There are though no sign of this being an eruption, at least yet there is not telltale harmonic tremoring. So, it is most likely a normal tectonic earthquake episode.
Bathymetric map of the Reykjanes Ridge.
Just to put this into perspective. The current swarm of earthquakes at Reykjanes Ridge has during the last couple of hours (as I am writing this) released more energy than Katla has done in the last half a year, and the amount of energy released from the last 3 months of activity at El Hierro, put together. Awesome come to think about it.
A very inspiring view from the ocean, Mount Fako rising from the ocean as it sits on the coastline.
For those of you who have missed this giant of a Volcano, it is the highest mountain in West Africa with its imposing 4 100 meters. The mountain is also called Mount Fako, and I will use that name since it is the original local name.
Mount Fako is one of the most active volcanoes in Africa, and for that matter on the planet. During the last 350 years it has erupted 18 times. In a way it is a rather odd volcano since it is neither a subduction volcano, nor a rift volcano. For all points and purposes it is now in the middle of a large tectonic plate. But, that has not always been true. In reality this volcano is a remnant of an ancient volcano that started its life at the Mid Atlantic Rift. Some geologists even believe that it is a remnant of a super-volcano that might, or might not, has started the rifting of the South American plate, and the African plate.
Be that as it may, it is a very ancient volcano that has had an enormous eruption at the probable time of the division. If it was caused by the division, or helped to cause it is a moot point really. All we need to know is that this 4 100 meter remnant probably is the oldest active volcano on the planet, and that makes it interesting enough.
Lava flow during the 2000 eruption.
Tombel Graben
Technically Mount Fako is a strato volcano on the coastal end of the much larger (800 square kilometer) Tombel Graben. It is rather misnamed since it is not a Graben really; it is a large caldera with a Graben inside. The caldera floor of the Tombel volcano has been filled in by lava floods. The Tombel volcano has not erupted during historical times, but there was one eruption at Le Djungo about 200 years ago. Tombel has two large strato-volcanoes, Mount Fako at the coast, and Mount Manengouba with its large calderas. There is a young crater row at Mount Manengouba that is believed to have erupted during the last 1 000 years. The Tombel Graben has one of the largest lava reservoirs that have been mapped.
Mount Fako erupting, aerial photograph from the distance.
Cameroon Volcanic Line
The Tombel volcano is situated on the Cameroon Volcanic Line. A distinction shared with Oku Volcanic Field, mostly famous for the Lake Nyos disaster. In the other end sits the active volcanic Island of Bioko. The volcanic line is trending from north-east to south-west. It is most likely a fault line that is a remnant from the separation of the continents that trended from the MAR as the Tombel Graben moved away from the centerline of the Atlantic Ridge. It is now cut off from the MAR completely.
Photograph by Tom Humphrey 1982 (Gulf Oil). Lava flow taken during the 1982 eruption.
Mount Fako today
On Tuesday an earthquake swarm started with epicenters ranging from Mount Fako to the magma reservoirs under the mountain. There is also a trending line of quakes going from the magma reservoirs of the mountain towards the center of the Tombel Graben. At the same time low frequency harmonic tremor started at Tombel Graben, and normal magmatic harmonic tremoring at Mount Fako. The earthquake swarms at Mount Fako before an eruption are normally strong to very strong for being at a volcano, with almost continuous quakes ranging from 2M to 5M before and during an eruption
Before onset of current activities an episode of rapid GPS movement started less than 70 days ago in the Tombel Graben caldera and GPS-movement consistent with inflation at Mount Fako.
On the Friday a set of smaller explosions happened on the flank of the volcano known close to the Hut 2 tourist lodge. The explosions where small and most likely caused by hydrothermal vents blowing out, as pressure increased inside the volcano. Two tourists where lightly injured during one of the explosions.
This has lead to increased surveillance of the volcano. It is the best monitored of the African volcanoes, with permanent local staff, additional French experts, and also equipment from a Power Production Company in place.
If an eruption occurs, it will most likely be on the south west flank. It would be a rather explosive affair ranging from VEI1 to VEI3. After and during the explosive phase there will be a lava flow moving towards the coast.
Picture by Cameroon Post. The greatest danger for the population is houses falling apart due to strong earthquakes.
The authorities in the area are used to the volcano, and the same goes for the residents. If needs be there will be an evacuation of threatened villages, but not much more.
The biggest danger of the volcano is that the earthquakes before and during the eruptions will make houses collapse on top of people. Otherwise the volcano is generally harmless.
