Edinburgh – Volcanic heart of Scotland

 

Image from Wikipedia. Arthurs Seat, Edinburgh.

Edinburgh – home of the Scottish Parliament, Military Tattoo, Princes Street and gardens, Scott memorial, Murrayfield, Valvona and Crolla’s food emporium, sundry pubs (!), volcanoes…Eh, volcanoes?

Surprising as it may be to some people, Edinburgh plays host to a great variety of igneous rocks. The most obvious and in our case, the most interesting, are the volcano remnants of Arthur’s Seat, and the Castle Rock. These are long extinct and date from the Lower Carboniferous, about 350 million years ago. Arthur’s Seat, the larger of these, is assumed to be the main structure, the other a subsidiary, or satellite vent. These are central pipe vents from pipe conduits, cf linear conduits as associated with fissure eruptions.

The rock of Arthur’s Seat is mainly a vent agglomerate with several crystal phyric microgabbro eruption pipes (Lions Head and Lions Haunch vents so called as at a distance the remnants resemble a lion lying down) – agglomerate being a mix of explosive block and vent collapse debris, ash and lava, whilst crystal phyric refers to the presence of some minerals present with larger crystals than the background rock. The presence of gabbro in the conduit pipes indicates these volcanoes erupted a basaltic lava, gabbro being the intrusive coarse grained equivalent to basalt being of the same mineralogical composition. (Dolerite, or Diabase to some authors, again has the same composition, but has a grain size intermediate between basalt and gabbro and is usually assigned to these rocks when found in dyke and sill intrusions). The combined vent material as mapped, gives an irregular vent of 750-1000 metres across.

Picture of pipe brecciated St Austell granite to illustrate vent agglomerate appearance; a true vent agglomerate has a much larger fragment range in both size and composition however.

Photograph by Alan C who gratiously has bestowed the rights to the Volcano café. Pipe brecciated St Austell granite

The opening picture shows the main vents of the Lions Head (left) and Lions Haunch (right) in the background, with Salisbury Crags in front; these are the quarried remains of a post volcanic episode Teschenite (olivine analcime microgabbro – analcime is a hydrous sodic zeolite mineral comparable to feldspathoids; zeolite minerals usually being associated with vesicle-filling in amygdaloidal basalt flows) sill intruded into sub-volcanic sediments of Lower Carboniferous age.

Salisbury Crags are part of a site dedicated to James Hutton, who has been called the ‘Father of Geology’. It was here that Hutton part formulated his theory of Uniformitarianism, that confounded the existing Neptunism movement that insisted the Earth dated from the biblical Great Flood, by siting the presence of a rock of obvious molten origin being intruded into sediments/volcanic rock, the sediments being just visible below the sill.

Distal from the volcano, mapping and borehole evidence shows there to be up to 250metres thickness of tuffs and lavas adjacent to the vent and in the Midlothian borehole, some 10km to the south east, approximately 70m of volcanic material was found at the horizon of the Arthurs Seat volcanics.

Castle Rock is the erosional core of a relatively small and assumed satellite volcano off Arthurs Seat, composed of microgabbro approximately 150 metres in diameter. The vent again cuts through shallow water marine sediments of Lower Carboniferous – Dinantian – age. These sediments comprise predominantly sandstones and minor shale horizons; but it should be noted that in some texts the sediments erroneously are noted as limestones, a confusion arising from the rocks of this age being assigned to the Carboniferous Limestone division.

Picture from rampantscotland.com showing Castle Rock and vent.

The reference cited here below gives an artists impression of Edinburgh with the two volcanoes superimposed along with an imaginary sea level – with a lot of imagination this image could well have been a Carboniferous equivalent of El Hierro (Arthurs Seat complete with aerial cone) and our Bob (Castle Rock)!

http://www.geo.ed.ac.uk/arthurseat/geology/overlay.html

Image of how Edinburgh would have looked like during the "interesting times".

More recently, during the last Ice Age, ice sheet movement has produced a classic example of a ‘crag-and-tail’ with the Castle Rock – the crag – protecting the bedded Carboniferous sediments of the Royal Mile – the tail – from ice erosion, indicating mass ice movement from the west. More recently, the ice-deepened gouge channel on the north side (Princes Street gardens) has been utilised by the railway as an ideal route through the city!

Image from Google Earth.

And finally;

Image from from http://www.irocks.com/db_pics/pics/d06-238a.jpg Mystery stone...

ALAN C

Sheepy Dalek – Nemesio learns to Vaporize!

Very happy man celebrating his discovery.

Involcans Presidente for Life, Nemesio Péréz, has discovered the Final Solution to the problem with Bob the Volcano. In a stroke of genius, he has discovered that you can use everything coming out of Bob in a Volcano Vaporizer to get magmatically stoned.

After the discovery he is believed to have said; “I see things in a new light, holy basalt dude!”

Volcano Vaporizer instructor Zaphod Beeblebrox in the process of instructing Nemesio at an unknown dormitory conference at Harvard. The teacher later said; "Basalt is heavy shit Dude!"

Nemesio discovered the Volcanic Vaporizer while attending a conference in an unknown dormitory at Harvard. He emediatly saw the possibility to solve all volcanic problems (and baldness) in the world with this final solution. Together with his teacher Zaphod Beeblebrox (above) he found that all samples taken from Bob was very vaporizable. In the testing all know samples from Bob was used up. It is believed that El Hierro will get an economic upswing from selling Bob material for vaporizing.

“I have found Bob to be very enjoyable. Rock on dude!” said the very happy volcanologist.

Involcans new press material with a very happy supermodel.

CARL

P.S. Volcan Café is against all forms of volcano imbibing, it is bad for health, may cause excessive weight-problems, harmonic tremoring and loss of GPS-data. D.S.

Hekla – GPS variance

I just could not stay away from this picture. It is just to stunning.

For those who need a brusher up on Hekla I recommend reading the two posts that is linked below if you need a refresher. The first one is a rather traditional write up about Hekla; the second is a revision of how I interpret Hekla based on Papers by Sturkell and Carmichael respectively.

http://volcanocafe.wordpress.com/2012/01/02/hekla-general-alert/

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

In short one can say that even if Hekla good at being hard to interpret she is rather consistent in her uplift between the eruptions. So far it has been smooth going from the 2000 eruption up until 2012-01-30.

Back then we had a period of activity in and around the Saurbaer area with small scale tremor, a few minor earthquakes ranging from 0.2M to 0.7M and some microseismic activity. That was then interpreted as hydrothermal activity in the area.

As that calmed down we then got an earthquake of 1.2M at shallow depth in the topmost part of the fissure at 0.1km depth. The focal point of the earthquake was exactly at the 1947 eruption center.

For being Hekla that is a lot of activity in a short time.

Deflation?

A few days ago I looked through the GPS-plots of the network that the Institute of Earth Sciences runs under the supervision of Sigrún Hreinsdóttir. Without the gracious release of that data publicly it would not have been possible to do the plot below.  All copyright to the data used is held by the Institute of Earth Sciences and Sigrún Hreinsdóttir respectively.

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

On the various GPS-plots I noticed that around the first of February a marked deflation started at several of the stations around Hekla. Due to the complexity and sheer number of stations I could not get a clear picture, so I enlisted the help of the master of plotting, GeoLurking. Any mistakes in the interpretation of the plot are entirely mine.  GeoLurkings plot is a daily average trend line for the period 02-01 to 02-18.

“This is the daily linear trend based on the interpolated available data from 1 February to 18 February. It was then re-grided with a quadratic poly sheet at 255 x 255 in order to obtain the fill    between stations points. Purple is -1.2 mm/day, red is 0.4 mm/day. Green is -0.26 mm/day.” (GeoLurking, 2012)

Result

Image by GeoLurking. Please click on the image to get a bigger version.

There is due west of Hekla a clear center of hard deflation (purple), there is also a marked deflation NNW that is deflating rapidly (dark blue). From the last spot there is a pale blue band running over the northern part of Hekla proper.

On the southern part of Hekla central volcano (green) there is a small deflation going on.

Regarding the parts with inflation (red 0,4mm daily) there is no surprise that the area up towards Búrfell is inflating, it has been doing that for a long time. What is though surprising is that this trend continuous even though all of northern Hekla is deflating. This inflation has been interpreted as being a dyke emplacement running from Hekla. That is starting to look a bit less likely with this data.

What is brand new is the well defined inflation hypocenter west of Hekla. This is an unusual spot for activity at Hekla since it would be outside of the regular fissure of Hekla.

The red part seen towards Vatnafjöll is equally surprising since Hekla and Vatnafjöll has never during the last 10 000 years erupted at the same time. When Vatnsfell is active, Hekla goes dormant. And do not make the mistake of believing them being the same volcano, or even volcanic field. Vatnafjöll is erupting magma normal for Iceland, and Hekla is erupting lava associated with subduction volcanoes. So the link is mechanical only, probably some kind of pressure release function. The inflation at Vatnafjöll might therefore be a sign that Hekla is nearing the end of its 1 000 year long cycle of high eruptive pace, but that is highly speculative at this stage.

Conclusion

Hekla is known to have more than one chamber, as many as nine major chambers has been suggested in various papers. Be that as it may, Hekla has complicated innards, and it seems like magma can run freely between the chambers. In any other volcano on Iceland a displacement of this large amount of magma would be associated by continuous large scale harmonic tremor and numerous seismic events and here almost nothing of that has been evident.

From the looks of it a possibility is that Hekla would be more likely to have a flank eruption this time, or even a new fissure opening up in parallel to Hekla. If so, it would be the first time that has been observed by humans. One should though note that the Litla Hekla Fissure is in that direction, and that the inflation center is right on top of yet another fissure. So I guess it is not totally out of the question.

All of this should be read with a large caveat in the back of the head, any interpretation of a volcano is most likely wrong in parts. This is the least reliable volcano on earth, so all of it might end up being wrong.

CARL

Earthquake – Focal beach balls

Picture by Associate Press: Earthquake in Japan 2011.

Stand in the place where you live

Now face North

Think about direction

Wonder why you haven’t before

Now stand in the place where you work

Now face West

Think about the place where you live

Wonder why you haven’t before

If you are confused check with the sun

Carry a compass to help you along

Your feet are going to be on the ground

Your head is there to move you around

From        “Stand” by R.E.M.

 

This is a bit of a follow up on Carl’s excellent “Earthquakes – What’s the fuzz?” post. I was asked to elaborate more on the “beach balls” that I mentioned in a response on that thread.

I’m not going to give you the details on that… I’m not capable of doing the math myself or quoting esoteric concepts about how to process it. I am however, going to give you enough geospatial background to understand what they are all about and what they mean.

The first thing I need to cover, mainly in order to bring every one up to speed… is the compass rose. The compass rose appears on many navigational charts and a method of calculating direction from one point to another. When coupled with an actual compass, you can find your way from one point on the chart to another.

The rose is noted in degrees from 000° to 359° (actually 359.99….) which is a full circle. North is 000°, due South is 180°. Many features having to do with the earth are discussed in the bearing that the feature lies in or has moved.

Image by GeoLurking: Compass rose.

Which brings us to:

Lineament n (often plural)

3. (Earth Sciences / Physical Geography) Geology any long natural feature on the surface of the earth, such as a fault, esp as revealed by aerial photography.

On Carl’s “Earthquakes – What’s the fuzz?” post, that stunning picture of the San Andreas is a Lineament, or a linear feature. It’s a surface manifestation of the thousands of years of slip along that section of the San Andreas. In some places, you can find creeks that have had their channels offset by several meters… in other words, they don’t line up across the fault. It has shifted that much since the creek bed was formed.

San Andreas fault.

Not all lineaments are formed this way, remember, a lineament is just an odd linear feature. In the New Madrid Seismic Zone (NMSZ) there are several lineaments that were un-explained until modern research revealed them for what they are. (well, at least to the point that we can talk intelligently about them) Crowley’s ridge is one of them.

Crowley’s ridge is a linear structure sitting right in the middle of the flood plain of the Mississippi river. It’s a raised structure made up of loess, which is wind blown silt.  A lot of loess deposits are ultimately of volcanic origin, being the fall-out of some of the larger eruptions that North America has had over the millennia. So… how is a 170 meter high ridge of silt able to exist in the middle the flood plain of one of the more powerful rivers on the planet? In all likelihood, it’s from uplift due to the mechanics of the NMSZ. (If you wonder what this has to do with volcanoes, the NMSZ has several emplaced plutonic structures scattered along its extent… those are “failed” volcanoes along the “failed” rift structure)

Now to bring this into something more on the subject. (that was all lead in)

Generally, when a quake occurs, it is along a fault plane that is oriented in relation to the stress on the rock.  In the case of the San Andreas, it’s from the westward moving North American plate and the northward moving Pacific plate (relative motions).  When the quakes occur, they usually are oriented along the trend of the fault. The focal solutions usually show a fault plane oriented on a line from the Northwest to the Southeast.

Faults are not two dimensional structures. Most are a diagonal break in the rock that angles down as you go deeper. This is called the “dip” of the fault. Dip angles that are less than 180° are normal faults, those that are angled greater than 180° are reverse faults. Which side the fault is measured from depends on which side is the headwall, or the part that rises relative to the other side. Those that principally slide past  one another without one side lifting, are transform faults. The dip angle is usually pretty close to vertical (near 180°)

This is all represented in a neat little graphical construct called a “Focal Mechanism.” A common name for them are beach balls because that’s what they looks like.

Here is one for a recent quake in California.

Image by GeoLurking. Focal Beach Ball, the name is really self evident.

For this graphic, the extensional part of the quake is shown by the shaded region.  Seismic stations in that area would have shown the “first motion” on the traces to be going up as the wave arrived. Stations in the unshaded quadrant would have seen the “first motion” as going down. The best way to read this, is to think of the two unshaded regions moving towards the middle of the ball, and the two shaded regions being pushed away from the ball. The orientation of it illustrated the fault orientation. In this case, the dip angle is 23° from vertical. The NP1 solution (mathematically derived) shows the fault plane to be oriented along a bearing of 11°. The alternate solution, NP2, shows 226° NP1 being the most likely explanation of how it happened. (Note, this is subject to correction since that is the best that I can find… but could be wrong)

Now… I mentioned lineaments earlier. Here’s why. A fault that is oriented at 11° is also oriented along 191° (11+180). If you draw a line along those two bearings from a point on a map, say where the epicenter is at, that would describe the fault plane as represented by the focal mechanism.

You can  explore this more at the USGS explanation of it here: http://earthquake.usgs.gov/learn/topics/beachball.php  or at http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/faults.html        .   For an authoritative discussion about it from an actual geologist, I recommend http://scienceblogs.com/highlyallochthonous/2009/12/5_focal_mechanisms.php

Now that I’ve covered that, a word about volcanic quakes. Magma forcing its way into a segment of rock usually will not generate “double couple” solutions, or if they are there, they will be either too small or too chaotic in order to pull this sort of data off of a lot of seismographs. Magmaintrusion causes most of the first motion traces to be upward as the overall volume increases and the rock splits apart. This means that pretty much all of the waves in all directions will be compressive waves. That was why that 1996 Bardabunga quake was so weird… the math and the motions pointed at no net volume change, and no double couple that matched something tectonic.

Remember, not all quakes will have a focal mechanism solution in the quake report, only the larger ones. This is mainly due to the large number of reporting stations that have to be used to derive it. Smaller quakes just don’t register on as many stations. Usually you will find the ‘beach balls’ in the technical details for the quake.

Well, I hope this helped. In order to get anything more detailed than this you’re gonna have to poke at a real geologist to pony up more info. This is just a layman’s understanding of it.

GEOLURKING

Activity in Iceland’s Dead Zone

Photograph by Skúli Thor Magnusson. Stunning view of the Veidivötn Lakes.

The Dead Zone

In the area going from Vatnajökull down to Torfajökull and Myrdalsjökull there is normally almost no seismic activity. This has given the area the not so scientific name of the Dead Zone. It contains the fissure swarms of Bárdarbunga, Grimsvötn and Katla. Grimsvötns fissure swarm is called Laki and Katlas is called Éldgja.

Veidivötn

Bárdarbungas fissure swarm is called Veidivötn, and has had the largest lava-floods on the planet in the last 10 000 years. The largest of those is called the Thjorsahraun putting out an estimated 20 cubic kilometers of lava.

Photograph by Örn Óskarsson. Anybody who wishes to rent a cabin?

Rifting Fissure Eruption

When Katla, Bardarbunga or Grimsvötn has a major and sudden influx of magma from the hotspot, and the fissure swarm of the volcano in question suffers a rifting of the Eastern Icelandic Seismic Zone opening it up. Then what happens is a VEI-6 eruption at the central volcano, and a minimum of 10 cubic kilometers of lava erupts down a minimum of 100km fissure. It is the most destructive type of eruption likely to happen in the northern hemisphere.

The reason of them being that problematic is that it at the same time it releases a lot of ash into the atmosphere, about 100 times the amount from Eyjafjallajökull and also the fissure releases a lot of sulphuric gasses. This phenomenon is called dry fog, and was reported in most of northern Europe and the USA during the eruption of Laki in 1783.

The Laki eruption is the most deadly in European modern history due to the famine that followed. 1783 is also known as the year without summer.

Dead Zone activity of today

Image by IMO. Look at the line running up from Torfajökull/Myrdalsjökull, that is Veidivötn.

During the last 12 hours a series of small (1 to 1,8M) earthquakes happened inside the Dead Zone. They trend from Torfajökulls Landmannalaugar up along the Veidivötn fissure towards Skrokkalda.

What is remarkable that this phenomenon has never been recorded before.

At the same time there where harmonic tremoring in the SIL-station of Skrokkalda, Vatnsfell and also Snaebyli, shown here is the activity at Vatnsfell.

Image by IMO. Vatnsfell SIL-station showing harmonic tremor.

There has also been activity at the Dyngjufjöll, the principal SIL-station of the Bárdarbunga volcanic complex.

http://hraun.vedur.is/ja/oroi/dyn.gif

And here is a webcam of sorts for the Veidivötn area:

http://helgi.dk/?page_id=352

Caveat for the press

Please dear all, it is still far from any possible eruption in the area. At most this is a signal of what can come, nothing more.

CARL