The Usual Suspects and Friday’s NtV Riddle

Photograph by Eggert Norddahl under exclusive right to Volcano Café. Hekla 1980 eruption.

We are minutely studying any hiccup from the Icelandic Usual Suspects. In the case of earthquakes we are quite used in getting all sorts of useful stuff out of them. One of my favorite ways of studying earthquakes in volcanoes is the cumulative seismic release (CSR). In the case of Grimsvötn it might even be the perfect way of predicting how close he is to erupting. Except for Hekla the Icelandic volcanoes are rather noisy, but how noisy is a volcano in the making?

I had this question ringing in my head yesterday so I tallied up the sums for Iceland’s only known proto-volcano, the Gódabunga cryptodome. A cryptodome is a magmatic emplacement that is unerupted; it is a magmatic system in the making. Normally it suffers several magmatic emplacements before erupting, but most often a cryptodome never erupts. The reason for them not erupting is that it takes a tremendous amount of energy and pressure for a volcanic opening to form. These virgin territory emplacements are hideously noisy compared to a regular volcano.

I had handy data for Eyjafjallajökull, Gódabunga, Grimsvötn, Hekla, Katla and Torfajökull. I discarded Hekla since it would not even show as a speed bump compared to the others. In the end the results got interesting.

The data is from July 1991 up to February 2013 except for Grimsvötn (1996). Energy is in Joules, as a comparison one could use a Big Mac; at 540 kilocalories it has 2.2 million Joules of energy (roughly equivalent to a 1.1M earthquake).

Eyjafjallajökull

Photograph by Eggert Norddahl under exclusive agreement to Volcano Café. Eyjafjallajökull eruption 2010.

Eyjafjallajökull was rather active before the eruption in 2010, the total CSR was 6.8e+9 and otherwise Eyjafjallajökull was rather quiet during the period. The total tally including the VEI-4 eruption during the period was 1.06e+10. As you will notice this is surprisingly little. Let us now walk over to the quiet neighbor to the north. After the eruption this volcano has been very quiet.

Torfajökull

Torfajökull, the forgotten and quietly playing volcano.

This volcano latest erupted in 1477 and has since then not erupted, but it is still active. It suffers from two opposite forms of earthquakes. It simultaneously has earthquakes associated with cooling magma, and earthquakes associated with magmatic emplacement. It is probably safe to assume that this large volcano has more than one chamber, and that the most likely spot for an eruption will not be at the same place as the last one.

It would therefore have been interesting to have these earthquakes separated, so we could compare. But alas, we do not have that. The total sum during the same time period is 4.8e+9. Except for 2010 Torfajökull had higher CSR then Eyjafjallajökull during the entire period.

Grimsvötn

Photograph by Eggert Norddahl under exclusive agreement to Volcano Café. Grimsvötn 2011 eruption, sun over ash cloud.

For being a very large volcano Grimsvötn is surprisingly quiet. The earthquakes during the period from 1996 are rather even over the years, with the only exception being
the period after the 2011 eruption where the volcano has been very quiet.

Grimsvötn seems to erupt as soon as it has reached a certain value of CSR. At least it did that the last two times in 2004 and 2011. That value is slightly more than 3.8e+9. The total strain release between 1996 and now is 7.7+9e, or slightly more than Eyjafjalla released in the 3 months before erupting up to the end of the eruption. Who thought Grimsvötn would be the shy quiet one?

Katla

I guess nobody will be surprised that Katla is a noisy volcano. It suffers from numerous small to medium sized earthquake swarms. Many of those are signs of magma entering the system. The noisiest year was 2011-2012 when the volcano suffered from a CSR of 8.9e+9. During the entire time the CSR was 3.1e+10. Now, let us go and visit the noisy unborn baby volcano next door.

Gódabunga

Gódabunga Núnatak next to the SIL station.

Gódabunga was a quiet unassuming little Núnatak (cliff protruding above a glacier) just on the outside of the caldera wall of the massive Katla volcano when, to the surprise of everyone, it started kicking around. In 96-97 it had a CSR of 1.8e+9 and most thought it was Katla having a side emplacement. The next year it got a CSR of 6.5e+9 and it was noticed that the hypocenter of the earthquake ball was outside the Katla system. It caused a bit of a stir; remember that all of a sudden out of the blue there was more CSR than Grimsvötn produces during the entire run up to an eruption.

For two years it was showing signs of calming down after the initial emplacement. Gódabunga seemed destined to dwindle into the mist of magmatic emplacement history, the same way 99 percent of all emplacements go.

After the two years Gódabunga went on a massive spree of large emplacements that really knocked on the roof of the teenage room.  During the next five years it had at least two more even larger emplacements than the initial one. The CSR for the period was 6.03e+10 with the record year (2002-2003) blasting an impressive CSR of 1.83e+10.

After those five years Gódabunga has calmed down, but the CSR is still 100 times higher than before the commotion started. The total release during the period is a whopping 9.02e+10.

What will happen to Gódabunga is written in the stars so far. Remember that science have never seen a large new volcano blast into life, especially never with instrumentation like this. The ones science has seen have been rather small, and they have been un-instrumented. If Gódabunga ever is born we only know one thing, it will be very noisy.

Ruminations

For those who are surprised over how quiet Grimsvötn is should remember that this volcano is erupting often. The system is permanently heated and filled with magma, so the roof over the chambers cannot withstand a lot of pressure. One should also note that the earthquakes in a volcanic system are a sign of pressure increase (or in some cases of pressure decrease as magma cools down and contracts). So, Grimsvötn will show comparatively few earthquakes before erupting since it cannot take a lot of pounding.

The fact that Grimsvötn has a weak top is what makes CSR into a good predictive tool. The amount of pressure increase the roof can take will be about the same between the eruptions. At least for as long as the eruptive cycles have roughly the same time spans.

As the roofs above volcanoes cool down it will take more and more pressure for a volcano to break through. Also size and depth of the magma chambers are factors that affect the amount of pressure needed for an eruption to occur.

Eyjafjallajökull had a rather small magmatic system that was fairly close to the surface (after an emplacement just before the period my data covers), so it had a surprisingly small conflagration of earthquakes before erupting.

Katla has not erupted since 1918, and has a very large magma system. So the roof has had time to solidify and can due to the size flex quite a lot over time. So, it is not that surprising that it can withstand a lot of CSR over time.

Gódabunga on the other hand has an unknown size of the magmatic system, but one thing is clear, it is fairly deep down, and the roof is (no pun) rock hard. Here the magma has to quite literally pound its way through layer after layer of hard old rock so naturally there is a lot of music being played. How much more pressure will it take before an eruption occurs? The answer is that nobody knows. All we can do is waiting for a new emplacement and then try to track the progress of the earthquake ball hypocenter upwards. For all we know it could withstand anywhere up to ten times as much CSR, especially if it is temporally well spaced.

Numbers taken from Icelandic Met Office and treated by Carl, then made into a plot by GeoLurking.

If the CSR is a representation of the systemic pressure inside an unborn volcano, the Gódabunga is potentially a bad one. If we compare with El Hierro that had an eruption after a long repose time and had a very noisy eruption the energy released there was still 1 000 times less. In the end we are faced with the small thing that the CSR are caused by a magma emplacement, and the amount of activity gives a hint of the amount of magma emplaced.

CARL

Name those Volcanoes Riddle

 1 point for each Volcano …

No 1 - Legendary lost home of the hairy eared dwarves? SOLVED Mount Shasta 1 point Sa’ke

http://en.wikipedia.org/wiki/Mount_Shasta

http://en.wikipedia.org/wiki/List_of_lemur_species

No 2 - 4295 eponymous deposit. SOLVED Katla / Vedde Ash 

http://en.wikipedia.org/wiki/Katla_volcano

http://en.wikipedia.org/wiki/Vedde

No 3 - Thoroughly well bred parent of the US Ambassador? SOLVED Acatenango 1 point KarenZ

http://www.sportinglife.com/racing/profiles/sire/98444/acatenango/progeny

http://en.wikipedia.org/wiki/Acatenango_(horse)

http://en.wikipedia.org/wiki/Acatenango

No 4 - Slightly under 50 miles north of a pillar of salt. SOLVED Aukland volcanic field 1 point KarenZ

http://en.wikipedia.org/wiki/Auckland_volcanic_field

No 5 - Its location and summit hold a global distance record.  SOLVED Chimborazo 1 point Sa’ke

http://en.wikipedia.org/wiki/Chimborazo_(volcano)

No 6 - Site of the earth’s nastiest outside loo. SOLVED Mount Elbrus 1 point Alison

http://en.wikipedia.org/wiki/Elbrus

KILGHARRAH

How to read the Icelandic borehole strain and seismicity plots and NtV Riddle

In this post I will elaborate on how to understand the Icelandic borehole strain and seismicity graphs. For the experts I might just be stating the obvious, but for the more general public (like myself) this might be a guide on how to understand all these enigmatic waves and ripples.

This map shows the locations of three kinds of instrument that monitor earthquake and volcanic activity around Hekla volcano. SIL stations (of the South Iceland Lowland automatic earthquake data acquisition and evaluation system; black triangles), GPS stations (yellow) and volumetric borehole strainmeters (green squares).

Location of the SIL and GPS stations and borehole strainmeters.
Image courtesy of IMO
http://hraun.vedur.is/ja/hekla/Stadsetning_stodva_31052011.jpg

Strainmeters can be of various design. In Iceland we are dealing with Sacks-Everton volumetric strainmeters. Wikipedia reveals: “a design that uses specially shaped volumes to measure the strain tensor.” In other words, changes to the volume of a fluid filled chamber anchored in the borehole.

Image courtesy of IMO
http://hraun.vedur.is/ja/hekla/thensla_burfell.html

The sample rate of the volumetric strainmeter data is one second (1 sps = samples per second, i.e. 1 Hz). The unit “strain counts” on the vertical axis is arbitrary, because a gain is manually set to determine what amount of relative change in strain or stress is one count. Strainmeters indicate ground velocity (displacement per time). Positive strain values mean volume increase in the bedrock (extension due to tension force, i.e. strain), negative values decrease of volume (contraction due to compressive force, i.e. stress). If you think of driving a vehicle, this plot shows your velocity relative to the starting velocity, since the start of the trace is always set to zero. A massive drop or rise might for example indicate you came to full stop at a tree or reached escape velocity for space travel.

Whether a strainmeter shows extension or contraction during an eruption depends on its relative position to the conduit/rift, see the opposite reactions during the Hekla 2000 eruption.

Búrfell darkblue,
Saurbær blue, Skálholt red, Geldingaá yellow, Stórólfhvoll violet, Hella light blue. Image courtesy of IMO
http://hraun.vedur.is/ja/englishweb/heklafigure1.html

Besides the Hekla strainmeter Búrfell is the second closest to Hekla, roughly 15 km at a perpendicular angle to the rift direction. The huge strain drop (i.e. massive stress increase) at Búrfell was interpreted as magma forcing it’s way up, opening a conduit. On the other hand, the simultaneous strain increase (decreased stress) at the other stations was due to emptying of the magma chamber. Here is further (paywalled) read on the strain during the 1991 Hekla eruption. The unit nanostrain indicates a change by a billionth part of the volume, i.e. 10^-9. Earthtides are known to have an amplitude of about 50 nanostrains. The 2000 eruption caused a sudden drop about an order of magnitude larger.

A seismometer literally measures shaking, i.e. motion of the ground, which can be recorded as a seismogram.
The seismometers of the SIL array can both measure ground displacement (unit is meters per second, m s^-1) or be used as accelerometers (unit meters per square second, m s^-2).
Most Icelandic seismometers are 5 sec (0.2 Hz resonant frequency, limiting the frequency range) Lennarz seismometers. The sampling frequency is 100 Hz. The Haukadalur seismometer (63°58´08.4´´ N / 19°57´54.0´´ W, appr. 10 km West of Hekla) is a LE-3D/5s, measures oscillations in three dimensions (“transverse”, North-South; “radial”, East-West; “vertical”, Up-Down).

Tremor amplitude time series with different frequency bands. Vertical axis: One-minute averages of the vertical component of the tremor amplitude, x micro meters s^-1. Image courtesy of IMO
http://hraun.vedur.is/ja/hekla/oroi_hau.html

First of all, this graph does not show the raw seismogram, but is a spectral analysis. You remember the colorful spectrograms from the El Hierro stations? A spectral analysis is performed on the waves of the seismogram to extract oscillations of different frequencies. Several algorithms can be used to create a spectrogram, for example STFT, short-time Fourier transformation, or CWT, continuous wavelet transform. For El Hierro the amplitudes are given over the whole frequency range while in Iceland they show averages of three frequency bands.

This example is a tremor amplitude time series showing averages of the frequency bands 0.5–1.0 Hz (red line), 1.0–2.0 Hz (green line) and 2.0–4.0 Hz (blue line), of the vertical component (Z) for the station HAU. Unfortunately the vertical axis is not labelled, but is presumably representing the amount of bedrock displacement in micro meters per second multiplied by a variable scaling factor (x). The values are presumably one-minute averages. An example for this analysis is described e.g. in this thesis, see p. 564 ff.

The blue trace (high frequency band, fast shaking) mainly represents earthquakes and the green and red traces (low frequency bands, slow shaking, harmonic tremor) tremor from magma movement, which for Hekla is usually in a well-defined spectral band at 0.5–1.5 Hz (see the thesis).
Based on previous observations, the  following scenario might occur when the next eruption is about to happen: First there will be more earthquakes opening a fissure, showing as an increase of the blue earthquake trace amplitude by an order of magnitude. When the fissure is opened earthquake activity seizes and the blue trace will go back to normal. Meanwhile the magma starts spilling out and a sudden increase in the red and green tremor trace amplitude by at least an order or magnitude will be seen, which gradually decays with decreasing pressure. What we should actually look for in this graph is not the width of the traces, which only indicates how much the shaking amplitudes vary, but a really really strong rise of the curves as seen in 2000:

Tremor amplitude time series with different frequency bands. Vertical axis:  One-minute averages of the vertical component of the tremor amplitude, x micro meters s^-1. Image courtesy of IMO
http://hraun.vedur.is/ja/hekla/hau20000226.gif

Lastly, the following graph is a composite of data derived from the volumetric borehole strainmeters and from the Haukadalur seismometer, plus information on local earthquakes determined by the SIL system.

Upper panel: Volumetric strain rate.
Lower panel: earthquake magnitude (left), horizontal components of tremor amplitudes (right)
Image courtesy of http://hraun.vedur.is/ja/hekla/borholu_thensla.html

The upper part shows the “two-minute median from one-second data” of borehole strain rate (strain counts per second) measured by the four stations Búrfell, Hekla, Hella and Stórólfhvoll. See the green squares on the map. A change of the strain rate means the bedrock is compressed or extended faster or slower than before. The cause of this is a change in the pushing or pulling forces. Think of it as your vehicle being accelerated or decelerated when pushing your gas or brake pedal. This graph shows what your feet do. When Hekla erupted in 2000 the strain rate looked like this:

The rate of strain changes in Búrfell (blue, 15 km from Hekla) and Skálholt (red, 45 km from Hekla) (nanostrain per hour)
Image courtesy of IMO http://hraun.vedur.is/ja/englishweb/heklafigure3.html

The minimum in the strain rate indicates the time of the surface breakout of the magma, along with the visual observation of the eruption at 18:17.

Because the ground is moved by several variable sources, mainly earth tides (very slow change in strain counts rate) and microseismicity (very fast change in strain counts rate) the above mentioned two-minute time range is chosen by which these events are filtered out. Then the median, the mean value separating the higher half of a data sample from the lower half, is plotted.

The left axis in the lower part shows the magnitude (in Ml) of local earthquakes. Since most of the time there are no earthquakes (counted in the lower right corner) no trace appears.

The right vertical axis in the lower part indicates the bedrock displacement, i.e. velocity in micro meters per second. The data is derived from the horizontal components (North and East) of the Haukadalur tremor amplitude time series data, which are 60-sec averages. Short-lasting shaking, for example caused by single earthquakes or a sledge hammer, are averaged out by plotting the the three-minute median. When an eruption is imminent, the blue (high frequency) trace will rise first indicating fissure opening and the green and red traces will follow when the eruption starts.

Standard VolcanoCafé disclaimer: I am not an expert on this topic, just read a few papers while researching for the post. Please excuse me if I jumped to false conclusions and feel free to post corrections!

chryphia

Many thanks to the dragons who read the draft and special thanks to Geolurking for helpful comments!

Other links:
-The SIL seismological data acquisition system – As operated in Iceland and in Sweden. Abstract only (2003)
-How a seismometer works from Sep 25, 2012 by Geolurking
-Summary about long and short period and broadband seismometers in this blog post
-ON THE USE OF VOLUMETRIC STRAIN METERS TO INFER ADDITIONAL CHARACTERISTICS OF SHORT-PERIOD SEISMIC RADIATION
-Seismometers of the SIL used as accelerometers
- Earthquake engineering research center, University of Iceland operating the Icelandic strong-motion network since 1984.
-Sturkell et al., 2005, Volcano geodesy and magma dynamics in Iceland
-Description by IMO of the Hekla 2000 eruption.
-Visualizing Stress is a good site, even if you are not into the math aspects of it, it has some really good narative data in the tutorials.

Name those Volcanoes Riddle

1 point for each volcano … enjoy!

No 1 - Did it crash in the Gobi Desert during CE3K? SOLVED COTOPAXI

No 2 - Volcanic group associated with siblings and satelites. SOLVED LES PLEIADES

No 3 - In English it can be added to seal, crow and mantis. SOLVED HEKLA

No 4 - Bruce and Nigel’s buddy studies this one. SOLVED Axial Seamount

KILGHARRAH

Name that Lava, Herdubreid Scientific Challenge and Volcano Riddle

Photograph courtesy of Dr Carmen Morataya.

Having the brain rebuilt from top down every Friday never get’s old. I do not know how you guys are doing since I am bad at both riddles and recognizing lavas and volcanoes.

But it is fascinating to witness how the blood hounds of the Volcano Café unerringly zeroes in on the pray. This week I want the name of the lava, the volcano and the name of the local fast food. So, 3 points to be had all in all. I will of course ding when someone has scored.

Tomorrow, or the day after I will post you an explanatory article.

Herðubreið

Photograph by Eggert Nordahl under permission.

Herðubreið is a tabletop volcano (tuya) in Iceland that has been active during the last few years with repeated earthquake swarms and magmatic emplecements. Supposedly the volcano is a defunct part of the Askja volcano. Evidently it is not as defunct as previously believed, and there is quite a lot pointing to it not being a part of Askja.

So, last night I proposed that we should do something with the volcano since it is rather poorly investigated. Ontop of the two questions above we can also have a bit of fun with the youngish looking strato volcano on top of the tabletop mountain. Here is the question, have it erupted after the last glacial period?

I thought we could do this as a case of crowd science. We have access to some equipment, we have access to a lot of people with various expertise, so we could actually collectively make a fairly nice piece of scientific work on Herðubreið.

So, in another word, I propose that Volcano Café officially adopt Herðubreið as our project. Now and then we should post status reports on our progress, if anyone feel up to it, being the collector and editor of what has been said please do not hesitate to come forward! Herðubreið and Volcano Café needs your services.

Have a nice weekend everyone!

CARL

Name Those Volcanoes Riddle

The riddle contained the mixed up clues to 2 volcanoes … 1 point awarded for each …

At the intersection of Rts 36 and 119 Peter was totally over come by the glowing clouds; then Bill, Andie and Chris arrived to cover the celebrations!

1 point Sa’ke for Mount Pelee … “Peter was totally over come by the glowing clouds”

Glowing clouds was the English translation of a French term nuée ardente first used to describe the pyroclastic flows of Mount Pelee that engulfed the town of St Pierre in 1902 killing 30,000 people.

http://en.wikipedia.org/wiki/Pyroclastic_flow

http://en.wikipedia.org/wiki/Mount_Pel%C3%A9e

1 point KarenZ for Candlemass Island … “At the intersection of Rts 36 and 119; then Bill, Andie and Chris arrived to cover the celebrations!”

The clue pointed to the movie Groundhog day (1993) set in Punxsutawney, starring Bill (Murray), Andie (MacDowell) and Chris (Elliott). Groundhog Day is the US term for Candlemass Day.

http://www.fisheaters.com/customstimeafterepiphany3.html

http://www.volcanolive.com/candlemas.html

KILGHARRAH

Countdown to Hekla

It has been a long time since I wrote about Hekla. But, I guess nobody is surprised at what I am about to write.

Everyone with a genuine interest in volcanoes have their favorite volcano. As many in here know Hekla is my favorite volcano to bang my head against. Few volcanoes are as intricate as Hekla, and few have such a short run up before an eruption as Hekla. Normal run up time to an eruption is between 30 – 80 minutes from the first sign.

This time around has been different. But let us first recapitulate what has happened since the last eruption. For those who are curious about how Hekla works I would like to recommend my own post about her innards:

http://volcanocafe.wordpress.com/2012/01/10/deconstructing-hekla-hells-gate-revisited/

Background

In 2004 Hekla had received as much new magma as was discharged during the 2000 eruption and sometime during late 2008 to early 2009 that figure had doubled. After that the inflation stagnated and no real uplift was measured at the GPS-stations with the exception of what was most likely magma moving between the different magma chambers.

During the summer of 2011 earthquakes was registered and a public safety alert was issued stating that Hekla was close to erupting. From then on Hekla has had earthquakes ranging from miniscule to 2M+ without erupting. For those who are not familiar with Hekla one should notice that she normally is aseismic, or in other words, that she does not have a lot of earthquakes.

From 2010 and onwards Hekla started to show a new feature that I dubbed “transients”. The transients are sudden rapid drops in the strain measured at the borehole strainmeters. These transients have only been seen before as Hekla erupted. They had before 2010 never been seen without an eruption occurring. A transient is in short happening as the mountain strains to open up.

On the 13^th of March and onwards Hekla had a swarm of earthquakes and once again transients were noticed on the strain-meters. There was also harmonic tremor measured indicating rapid movement of magma. This caused IMO to issue a public safety warning, and the London VAAC issued a flight code warning Orange. This followed the exact pattern of how all the previously instrumentally monitored eruptions had started so far.

As we all know nothing happened in the end. We can now safely say that we know even less than we did before about how Hekla acts before an eruption. Because now we have to figure out why Hekla did not erupt when she should have. I guess someone will have a research career out of it in the end.

Present

After this Hekla entered into a new phase never seen before, this time a phase of very rapid and unbroken inflation started. What happened is most likely that the earthquake swarm removed blockages inside the deep feeder tubes of Hekla enabling fresh magma to flow into the volcanic system.

The rate of inflation varies a lot depending on where the GPS station is placed. The big exception is Mjóaskard situated to the west of Hekla. It has only suffered an uplift of 5mm in the last 5 weeks. For the other stations the rate of inflation is between 15mm in Hestáalda and 32mm at ISAK. Average uplift is 16mm, and 21mm if MJSK is not counted. This type of rapid inflation has so far never been measured at Hekla.

http://strokkur.raunvis.hi.is/~sigrun/HEKLA.html

During the entire inflation phase there have been scattered earthquakes and micro-quakes.

If the current rapid inflation continues there is a very low chance of Hekla not erupting. Yes, we do not know what is happening with Hekla now since we have never seen this type of behavior. But Hekla is constructed in such a way that she can’t take a huge increase in pressure without erupting.

Image by GeoLurking based on data by the University of Iceland and Professor Sigrún Hréinsdóttir. All areas are showing uplift on this image covering the period from 4th of April up untill now. The area with the highest uplift are due north of Hekla.

If the inflation continues at the current rate Hekla will erupt. When? Well I am not going to make any bets, but any time from 1 hour from when you read this to 4 weeks. Remember that 4 weeks into the future the combined uplift in 2 months will have exceeded 50mm at many GPS stations. As seen on the image above the largest uplift is happening on the northern slopes. This is a known site for one of Heklas primary magma chambers. The area to the northeast are not showing correctly, there is uplift there too, but due to lack of a GPS station there the model get scewed.

Image courtesy of the University of Reykjavik and Professor Sigrún Hréinsdóttir.

I personally would not at any cost get closer to Hekla then 10 km from now on. And then I would stay in the car on the road. If you are closer the chance of you surviving is not good and 5 km the chance of you surviving the initial blast is pretty much nill.

What will the eruption be like? Here I will be guessing since Hekla has changed her behavior compared to the last eruptions. I would say that Hekla has remobilized old evolved magma during all that moving of magma, and this latest inflation phase seems to fill up a lot of old magma chambers. This causes me to fear a rather explosive start of the eruption. I would also say that there is quite a high likelihood of there being more lava erupted then was seen during the last 3 eruptions. I will hedge my bet by saying that I would expect it to be anything between a VEI2 and a VEI4 on the volcanic explosivity index, and that Hekla will effuse between 0.1 to 2 cubic kilometers of lava. Based on the GPS plot above my best judgement is that the eruption will start at the top of Hekla proper and then open up the fissure both to the south, but mainly to the north. Most probably the Hekla fissure will open over all of Hekla proper with a fissure extending to the Northeast.

For those who wish to follow the eruption, here is the Hekluvöktun page:

http://hraun.vedur.is/ja/hekla/

CARL

Lazy sunday afternoon ( evening)

Webcam screenshot by Graniya

El Popo / Popocatepetl showed some action and distrubuted a cloud of ash.

http://www.foxnews.com/world/2013/04/13/popocatepetl-volcano-spews-ash-over-central-mexico-state/

Sunset at Puebla with erupting Popocatépetl:

http://www.webcamsdemexico.com/webcam-popocatepetl.html

Commenters speculated what the “second” plume could be. A forest fire? A industrial plume from a factory? Gas emissions? I still dont know for sure.
Geolurking provided a helpful link: http://www.ssd.noaa.gov/VAAC/messages.html
Parent page is at http://www.ssd.noaa.gov/VAAC/messages.html
———————————————————————————————————–
Etnas “paroxysmic eruption” ( very nice word Newby)  was most active very late in the night of April 12th.

Boris Behncke opinion: Paroxysm or no paroxysm?
http://www.flickr.com/photos/etnaboris/8642730164/
and
http://www.wired.com/wiredscience/2013/04/brief-explosive-eruption-from-etna/#disqus_thread

Some activity  ( glow) can still be seen on these cams: http://www.lave-volcans.eu/webcams_etna.php?numero=2
http://www.etnaweb.net/nunziata/webcam.php
( Thanks KarenZ)

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Alan C´s Evil riddle: The Answer was Ulexit:

Graniya: “Gigantic hunks of Ulexite are found in the form of fibrous, compact veins. When polished, these specimens become the well-known “Television Stone” or “TV Stone” sold to amateur collectors. The optical effect exhibited by Television Stone is caused by each of its individual fibers acting as fiber-optic cables, transmitting light from one surface to the other. Since all the fibers are parallel and compacted together, any image at below is transmitted through each crystal fiber to the top surface. For this effect to be seen, the specimen must be polished with smooth surfaces. Fibrous Ulexite bundles can also be carved into cabochons that display strong chatoyancy. However, due to its low hardness, it is unsuitable for gem use.“

Alan C: “Well, ’tis obvious that our Granyia is well read-up on AA Milne!
Winnie the Pooh fell into a gorse bush honey hunting
gorse is genus Ulex – the prckly subject – so we have Ulexite!! DING!!!
Ulexite (NaCa hydroxyborate) – nickname TV Mineral from optics c/f fibre optic cables“

Graniya had not read AA Milne but still got the right answer.

http://www.mindat.org/photo-237267.html

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El Hierro

dmf: “Here is the update on the density plotting for El Hierro. Up to the 12th.
In this plot the first part shows daily earthquake density with a mesh of 90 x 90 m.
For the last days the plot shows the limitation of the system. Too few earthquakes and too much dispersion (see the scale on the right blocked at 0.9)
The second part shows a cumulated earthquake density, with a day by day cumulation.
Data is from IGN & NOAA, made on Gnu Octave.
The second part shows very well the most active areas and the evolution of the quakes focus.“

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And in Iceland:
The TFZ earthquake swarm is progressing north and now active around the island of Kolbeinsey, about 75 km north of Grímsey.http://en.vedur.is/earthquakes-and-volcanism/earthquakes/atlantic/ ( Thanks IngeB)

Spica