Chile Volcanoes 2015 – Quetrupillán, Villarrica, and Llaima   Leave a comment

Summary

I had a delightful month in Chile earlier this year (February-March 2015) with the main focus being a two week expedition to Volcán Quetrupillán to complete my project on volcano-ice interactions.

Whilst at Quetrupillán our nearest volcanic neighbour (Villarrica) erupted and showered the tent and landscape in ash. And for the final few days of my trip I helped US colleagues do a recce of Volcán Llaima as they wanted my help to look for examples of lava-ice and lava-water interactions (two of my research specialisms).

Villarrica (left) after the 3 March 2015 eruption and Quetrupillán (right) with a dusting of ash from the eruption. Lake is Laguna Blanca.

Villarrica (left) after the 3 March 2015 eruption and Quetrupillán (right) with a dusting of ash from the eruption. Lake is Laguna Blanca.


Villarrica

So let’s deal with the excitement of Villarrica’s big eruption first! Prior to the big eruption there had been a number of small eruptions – a bit of throat clearing as well as small amounts of new magma exploding. I saw a few of these – see below.

Small eruption from Villarrica volcano, a few days prior to the major eruption of 3 March 2015. Model is Jonathan Moles, PhD student at The Open University.

Small eruption from Villarrica volcano, a few days prior to the major eruption of 3 March 2015. Male model is Jonathan Moles, PhD student at The Open University.

Villarrica had its big eruption early in the morning of 3 March 2015, in clear and cloudless weather. http://www.bbc.co.uk/news/in-pictures-31726286 This meant that many people took great images and videos. Not us unfortunately. At our campsite c.25 km away we were in thick cloud and heavy rain, with a c.300 m high ridge between us and Villarrica. We awoke to a bright and sunny morning, and immediately noted something was odd: there was ash on the soap, in the cooking pots, and on the tent, and when we brushed against the trees we had the rare experience of ‘ash showers’.

Villarrica ash on the tent on the morning of 4 March 2015.

Villarrica ash on the tent on the morning of 4 March 2015.

Two of the largest clasts from Villarrica's 3 March eruption. Collected from the tent flysheet.

Two of the largest clasts from Villarrica’s 3 March eruption. Collected from the tent flysheet.

On examining the ash shards it was clear that it wasn’t just a big throat clearing event – this was an eruption of fresh and gas-rich magma. It had to be Villarrica! After a hasty breakfast we ascended the c.300 m high ridge to see what had happened. What a transformation – the summit cone was coated in thick black pyroclastic deposits, with clear evidence of material being mobilised and flowing down towards the base of the summit cone. The day on the ridge (4 March) as we travelled north the ash layer became thicker and the shards became bigger, so we were clearly walking towards the main axis of dispersal (i.e. the direction-line along which wind had blown the ash cloud).

Villarrica on 4 March after the big eruption. From the ridge above and west of Laguna Azul, Quetrupillán.

Villarrica on 4 March after the big eruption. From the ridge above and west of Laguna Azul, Quetrupillán.

We saw the unusual sight of pumice floating in the lake below us (Laguna Azul), and saw that the summit ice cap of Quetrupillán was dusted in ash. A dusting of ash is known to increase ice melting rates, so the Quetrupillán ice cap may melt at a higher rate until the next big snowfall covers the new ash.

Pumice raft from the 3 March eruption of Villarrica floating in Laguna Azul. Ash-covered Quetrupillán summit in background.

Pumice raft from the 3 March eruption of Villarrica floating in Laguna Azul. Ash-covered Quetrupillán summit in background.

We still had no idea how big the eruption was, and we only found out how serious it had been when we got back to civilisation and hearing that over 3,000 people had been evacuated from the town of Pucón. So we were there, but we missed the big one.


Quetrupillán

Although Quetrupillán has the potential to produce decent eruptions that could affect nearby towns and communities – and there is evidence of such eruptions in the past few thousand years – what most interests me is that Quetrupillán has an unusual number of well-preserved examples of volcano-ice interactions. In 2014 I had a couple of weeks discovering and working on some, and the aim of 2015 trip was to discover new examples and do some detailed sampling and work on selected examples. So the images below and their captions describe some of the examples of volcano-ice interactions that exist at Quetrupillán.

One of the chief aims of this work is to provide insight into the longer timescale over which a volcano has been growing and erupting.

Basically, for volcanic systems that have been active for hundreds of thousands of years, the past 9-12 thousand years is a mere snapshot and is unlikely to be representative. Having a longer timescale to play with allows insight into cycles of behaviour and activity (e.g. surges of fresh material rising into the volcano over centuries to millennia) and this can improve predictions of future eruptive activity and the associated hazards.

The day we sneaked into Argentina without a visa, because volcanoes pay no attention to nation boundaries. This is an example of a small lava flow that was flowing along a ridge in a channel melted in the ice, and that decided to send a lobe into the flanking ice on the side of the ridge.

The day we sneaked into Argentina without a visa, because volcanoes pay no attention to nation boundaries. This is an example of a small lava flow that was flowing along a ridge in a channel melted in the ice, and that decided to send a lobe into the flanking ice on the side of the ridge.

A sub-ice fissure eruption formed the ridge above the green lake, with lava sheets and lobes flowing beneath the ice away from the ridge. The distinct snow-capped twin peaks of Mochu-Choshuenco volcano are in the background.

A sub-ice fissure eruption formed the ridge above and to the right of the green lake, with lava sheets and lobes flowing beneath the ice away from the ridge. The distinct snow-capped twin peaks of Mochu-Choshuenco volcano are in the background.

Pyroclastic deposits (pale) erupted into a vault within the ice at the start of the eruption. Subsequently capped by agglutinate (welded pyroclasts) and lava-forming effusions as explosive activity waned.

Vent area of the ridge in the above image. Pyroclastic deposits (pale) erupted into a vault within the ice at the start of the eruption. Subsequently capped by agglutinate (welded pyroclasts) and lava-forming effusions as explosive activity waned.

Example of a fissure eruption into ice, with the lava confined near to the vent area by thick flanking ice. This produces an overthickened lava flow that lies close to vent. This emphasises the important role of ice in confining lava flows, especially the more viscous and lower-temperature compositions in the dacite-rhyolite range.

Example of a fissure eruption into ice, with the lava confined near to the vent area by thick flanking ice. This produces an overthickened lava flow that lies close to vent. This emphasises the important role of ice in confining lava flows, especially the more viscous and lower-temperature compositions in the dacite-rhyolite range.

We only have good knowledge of the eruptive history of volcanoes spanning the last 9-12 thousand years – basically since the end of the last glacial period when the ice sheets melted and left the land ice-free. So the carpets of explosive pyroclastic deposits, and the lava flows, have not been destroyed and are therefore available for study. And we have techniques such as carbon dating to tell us when eruptions happened. Some example of recent eruptions at Quetrupillán are below.

Pyroclastic deposits (some reworked) from recent Quetrupillán eruptions. (Holocene for the pedants.)

Pyroclastic deposits (some reworked) from recent Quetrupillán eruptions. (Holocene for the pedants.)

Young lava flows (dacite if you're interested) flowing from a fissure. The lava lobe that flowed into Laguna Azul raised the lake level by a few metres. Note the ice-capped summit of the beheaded Quetrupillán stratocone.

Young lava flows (dacite if you’re interested) flowing from a fissure. The lava lobe that flowed into Laguna Azul raised the lake level by a few metres. Note the ice-capped summit of the beheaded Quetrupillán stratocone.

A young tuff ring (circular feature in middle ground), formed during an explosive eruption sometime in the past few thousand years. No lava effused from this one, but others have produced lava flows.

A young tuff ring (circular feature in middle ground), formed during an explosive eruption sometime in the past few thousand years. No lava effused from this one, but others have produced lava flows.


Llaima

A US colleague from Boise (Brittany Brand http://earth.boisestate.edu/people/brittany-brand/) asked me to help out on a recce of Llaima volcano with the aim of looking for lava-ice and/or lava-water interactions, as she was running a field trip afterwards. It’s tough finding such evidence in terrain covered with abundant young lava flows, especially as Llaima is one of Chile’s most frequently erupting volcanoes. But I found a few (subtle) examples where lavas had flowed into an old river valley. One is below.

Young lava flow from Llaima showing lava-water interactions. They are a bit subtle, but the curving fractures, the glassy and compact texture, and the block-column structures require additional cooland to form. And the coolant has invaed the lava from above.

Young lava flow from Llaima showing lava-water interactions. They are a bit subtle, but the curving fractures, the glassy and compact texture, and the block-column structures require additional cooland to form. And the coolant has invaed the lava from above.

A particularly spectacular eruptive unit we also looked at is the Curacautín Ignimbrite, which erupted c.13,200 years ago with a volume estimated at 24 km3. What’s unusual is that this ignimbrite is basaltic to andesitic in composition (50-59% SiO2 for those who are interested). It’s unusual because basaltic-andesitic ignimbrites are very rare – usually it’s the more evolved rocks (dacites and rhyolites) that produce large ignimbrite-forming eruptions. However there are few in this part of Chile, and good old Villarrica has also produced ignimbrites of basalt-andesite composition. It’s fair to say that why these volcanoes have produced ignimbrites of this composition is not fully understood.

The Curacautín Ignimbrite from Llaima volcano. Note the pale-coloured clasts - these are xenoliths of old granite from the basement.

The Curacautín Ignimbrite from Llaima volcano. Note the pale-coloured clasts – these are xenoliths of old granite from the basement.

Other beauties of being around Llaima are the lava-dammed lakes, and the ethereal Araucaria woods and forests (commonly known as Monkey Puzzle trees). These trees are highland trees and only feel comfortable growing naturally at elevations above c.1000 metres. There are some beautiful examples growing on the slopes of Llaima, and there’s also areas where the Araucaria forests have been invaded by lava flows.

Young lava flom Llaima (twin-peaked volcano in background) dammed a stream and created this beautiful clear pool, filled with stumps and trunks of dead trees.

Young lava flom Llaima (twin-peaked volcano in background) dammed a stream and created this beautiful clear pool, filled with stumps and trunks of dead trees.

Araucaria forest on slopes of Llaima volcano.

Araucaria forest on slopes of Llaima volcano. Pale granite forms the mountains beyond the lava plains.

Araucaria bark. Amazing stuff.

Araucaria bark. Amazing stuff.


Looking forward

To push back the timeline of Quetrupillán’s geological history I’ll be working with colleagues to obtain dates of selected volcano-ice eruptives. When I have some reliable dates and have worked what it all means, I’ll do another blog entry. More about my research and publications can be found at http://www.open.ac.uk/people/dwm4

Acknowledgements

The fieldwork was partly funded by a Santander mobility fund, which is gratefully acknowledged http://www.santander.co.uk/uk/business. As usual, the ever-reliable Rancho de Caballos provided the horses and expert guides to take us up to the volcano with our food and equipment, and came to collect us at the end with our many kilograms of rock samples. http://www.rancho-de-caballos.com/

The fabulous Lanín volcano, with an eroded subglacial-emergent flank eruption from Quetrupillán in the foreground (red material - oxidised).

The fabulous Lanín volcano, with an eroded subglacial-emergent flank eruption from Quetrupillán in the foreground (red material – oxidised).

Posted April 2, 2015 by davemcgarvie in Uncategorized

So what if there was an eruption at the Bárðarbunga caldera…?   1 comment

Health warning! I am heading out soon to explore a relatively unknown volcano with a new PhD student but I wanted to put some thoughts down before heading off. It’s a stream of consciousness account. So what you’re going to get here is a lot of text and some links plus some images. There will be a rambling preamble providing underpinning information before I get to what Bárðarbunga might actually do.

I will try and add links and images later. Thought it best to get the text posted first in case I don’t manage to do any more work on this post.

 

Introduction

The c.15 m subsidence measured at the Bárðarbunga caldera is the largest that has been measured reliably in modern times at any Icelandic caldera IMAGE. And it has been accompanied by a massive release of seismic energy via a series of large earthquakes triggered by the downward movement (along faults) of the inner blocks against the stationary outer rim of the caldera.

And quite rightly there is concern that this large downward movement and major disturbance of the structural integrity of the caldera may lead to an eruption. The problem is that because such an eruption has never been witnessed in the modern era, we really don’t know what will happen. We can take some clues from activity at other volcanoes such as Krafla which has a major volcano-tectonic event in 1975-84 involving dyke intrusion, but at Krafla the caldera did not subside as it is now doing at Bárðarbunga LINK. Askja has a well preserved caldera that formed during a major eruption in 1875 IMAGE, but the amount of subsidence prior to the major eruption is not known with certainty. However the subsidence afterwards was measured reliably and so we know that formation of the present Askja caldera took nearly 40 years. And some subsidence is still happening. (LINK) The amount of subsidence at Askja was considerable, and at 220 m deep the lake formed in the caldera is Iceland’s second largest. If you want to read more about the Askja 1875 caldera formation check out LINK

 

The Bárðarbunga caldera and other Icelandic calderas

But let’s get back to Bárðarbunga. It has a c.10 km diameter caldera filled with c.700 m of ice. The caldera rims are also covered in ice, so we don’t even get some clues about the compositions of the volcanic rocks erupted here. And this is fairly important, because these rocks could provide clues on how the caldera formed. For example, there’s also a massive ice-filled caldera at Hofsjökull IMAGE, where all of the nunataks (rocky outcrops sticking above the ice surface) on the caldera rim are of rhyolite. From work done by colleagues and myself we know that the effusive phases of rhyolite eruptions into ice form tall towers and ridges because the erupting lava finds it mechanically easier to grow upwards through thinning ice than to try and melt its way sideways through endless ice IMAGES. In essence the ice confines the erupting rhyolite. We also know from studies of well-exposed calderas in other parts of the world that the ‘ring faults’ that are integral to calderas can be leaky, and that it’s not uncommon for lava effusions to escape from these and form domes, which are often of an evolved composition such as rhyolite. Put this process into a subglacial context and hey presto you can create really impressive and tall caldera walls by erupting only a modest amount of lava.

There are other examples in Iceland that corroborate this such as the basalt-dominated caldera walls of the Askja volcano, and the elliptical chains of subglacial rhyolite domes at the Torfajökull volcano. And I’ll finish this list with mention of a most impressive example – the tallest volcano in Iceland – Öraefajökull. IMAGE At Öraefajökull there is a c.8 km diameter caldera filled with up to 500 m of ice, and fortunately there are nunataks on the caldera rim to give us clues. These have not been studied properly, but accounts I have read suggest that most are rhyolite and a few are basalt. The highest point in Iceland – a land dominated by basalt – is ironically the rhyolite dome of Hvannadalshnúkur. The largest explosive rhyolite eruption since Iceland was settled took place from this volcano in 1362 AD IMAGE and the devastation caused to the rich farmland to led to the unusual event of a volcano changing its name, from Hnappafellsjökull to Öraefajökull. Öraefi means ‘wasteland’.

 

Bárðarbunga caldera and a major eruption

So back to Bárðarbunga (at last, I hear you groan). What it will share with other large Icelandic central volcanoes possessing calderas is a set of sub-circular faults on which upwards and downwards movements takes place to accommodate subsidence and inflation of the underlying plexus of magma bodies (or a large single chamber). Think of a cafetiere of coffee with a leak at the bottom – push it down (subsidence) and the lid goes down and the coffee moves out. Now reverse the process by replenishing the cafetiere via pumping coffee via the leak and the lid will rise up. The cafetiere is the magma chamber, and the lid is the caldera roof. Coffee = magma.

A crucial point is the composition of the magma sitting near the top of the Bárðarbunga magma system. It could be rhyolite,  but as the tephra layers representing explosive eruptions from Bárðarbunga are all basaltic let’s assume a large explosive basaltic eruption occurs. Well now we are on reassuringly familiar ground, because back in 2011 we had Iceland’s most powerful basaltic explosive eruption in over a century. Yes, good old Grímsvötn. IMAGE From this we learned a great deal and even though twice as much ash was injected into the atmosphere that Eyjafjallajökull did the year before, the disruption during 2011 was a fraction of that during 2010. If you want to know more and understand more, I can strongly recommend John Stevenson’s blog entries, as he is a top expert on how explosive Icelandic volcanoes may affect northern Europe. LINK So, a worst-case scenario of a large basaltic explosive eruption from Bárðarbunga is something we are fairly well prepared for given the lessons learned in 2010 and 2011. Yes there will probably be disruption to commercial air travel, but for the simple reason that all planes were grounded by law in 2010 and this law has changed means it will never again be as bad as 2010.

 

What if it’s not a major eruption?

If an eruption at Bárðarbunga happens and it’s not a major 2011 Grímsvötn-type eruption, then what’s likely to happen is one or more eruptions along the ring fractures as magma leaks from below, and depending on the rate and amount of magma erupting, it may or may not reach the ice surface. What is fairly certain though is that any magma that does not escape the ice confines of the glacier will form a tall tower or ridge that will contribute to the caldera structure within the ice.

So there you have it in a nutshell – one worst-case scenario that is less threatening because of we had a similar one in 2011 from Grímsvötn. And other scenarios that are even less threatening to international air travel.

 

Flooding in Iceland

However any subglacial eruption will produce a lot of meltwater as basaltic magma can (under ideal conditions) melt up to 14 times its own volume, though perhaps 5-10 times is more realistic. This will cause flooding in Iceland which may do damage to the road and power infrastructure. Iceland usually bears the brunt and cost of its volcanic eruptions – 2010 was a rare exception.

 

Final thoughts – keep an open mind

I have focused only on an eruption taking place at the Bárðarbunga caldera itself. There are other possibilities such as a major eruption in the fissure system to the south-west. But as there is absolutely no current indication of this it’s best ignored. The caldera is in turmoil, and that’s why it’s best focusing on this for now.

 

Final thoughts 2 – the oddness of subglacial calderas?

I have also revealed via a few tweets and in the above that I consider an important mechanism in the evolution of subglacial calderas in Iceland to be due to a combination of ‘normal’ flexing of the caldera floor in response to subsidence plus upwards growth of caldera rim zones from the icy confinement of effusive eruptives. Other scientists may disagree. That’s not to say that Icelandic calderas cannot also form via major explosive eruptions as happens elsewhere in the world. I am merely pointing out that there’s something special about subglacial calderas that can make them seem much more impressive in their vertical extent without them being formed solely during a single major explosive eruption. After all, there is as yet no evidence that has convinced me that the formation of any major Icelandic caldera is linked to a single cataclysmic eruption….

 

(Aside. Rhyolite is a rock type that is much more viscous that the basalt being erupted at Holuhraun, and usually contains more gas so it has a higher potential to erupt explosively and produce higher proportions of fine ash relative to basalt.)

Posted September 7, 2014 by davemcgarvie in Uncategorized

Part 2 of Bárðarbunga – everything you need to know (well, almost)   Leave a comment

Edinburgh has lots of great pubs, and we went to one of my favourites, which is Teuchters in the west end. It always has Jarl which is (as we say in Scotland) a good ‘session ale’. I mention this purely in the shameless hope that the bar staff in Teuchters read this and provide me with a free ale or two.

After that sublime first sip of Jarl the Q&A session continued. I have decided to illustrate some of what we chatted about with a few relevant images. You had to be there to appreciate the artistry on the beer mats and scraps of paper….

Q. So why does this dyke intrusion you told me about not just force its way to the surface and erupt? Is something stopping it?

A. Well the magma in the dyke intrusion is quite ‘happy’ where it is. The magma doesn’t actually have a lot of energy to expend in breaking apart the crust. After all isn’t easier to break apart a rock with a sledgehammer than with a bag filled with gloopy concrete?

Q. I don’t know, I’ve never tried it.

A. Ah, sarcasm. Sup your pint and listen quietly.

The tip of a dyke at Askja volcano that was propagating laterally towards you (i.e. out of the page). Note the start of a 'split' in the tip of the dyke. As this widens the two cooler leaves of the dyke move apart and fresh magma squeezes out. It's likely to be an episodic process (stop-start).

The tip of a dyke at Askja volcano that was propagating laterally towards you (i.e. out of the page). Note the start of a ‘split’ in the tip of the dyke. As this widens the two cooler leaves of the dyke move apart and fresh magma squeezes out. It’s likely to be an episodic process (stop-start).

I like to think of magma as being a lazy beast as it will always move to where it’s easiest to do so. So all things being equal, if there’s no easy pathway to the surface the magma will just sit and stew and solidify within the crust. Nobody knows exactly what the crust above the dyke intrusion is like, but if for example it’s a stack of lava flows, then these horizontal slabs of solid rock form a formidable barrier (or lid) on top of the magma.

An example of rifted Iceland crust at Thingvellir. Notice that there's no single fault but rather a set of faults linked to one or more major faults.

An example of rifted Iceland crust at Thingvellir. Notice that there’s no single fault but rather a set of faults linked to one or more major faults.

But in Iceland we have rifting and this means that the crust splits in a preferred direction. So there is a pervasive weakness in the Icelandic crust that is especially well developed in the active rift zones, and when a weakness develops and a crack happens to connect underlying magma to the surface, then you get an eruption. Driven by gas expanding and accelerating as the magma ascends to shallower crust and lower pressures. Remember bubbles?

 

Q. Sigh, you and the bubbles again. Why can’t the magma in your dyke intrusion just go where it wants?

A. If the crust near the dyke is weaker in one direction then this is where the magma will go. That’s why the dyke has been moving towards the NE so far. Now what actually happens around dykes in stress and strain terms is a tad complex, but let’s just say that the dyke wouldn’t have been able to form and propagate unless the crust was already weak in this area. And that the presence of the magma-filled dyke will influence the local stress field and favour some further weakening in the vicinity of the dyke. So although the regional stress field will largely dictate where the magma in the dyke intrusion can go, the dyke itself will have some influence in this.

 

Q. In simple terms now please? You know how garrulous and nerdy you get when you mix ale and enthusiasm.

A. Where the magma in the dyke intrusion goes is largely dependent on weaknesses in the local crust which are either there already and ready to part, or will appear as this event develops. The magma itself has a say in this, arguably a minor one.

Two dykes intruding fragmented basalt at Askja. The dykes have 'wavy' margins because the fragmented basalt was a bit 'sloppy' rather than being a brittle solid. Note the prominent chilled margins on the outside of the dykes - a sign that the fragmented basalt was also a tad wet.

Two dykes intruding fragmented basalt at Askja. The dykes have ‘wavy’ margins because the fragmented basalt was a bit ‘sloppy’ rather than being a brittle solid. Note the prominent chilled margins on the outside of the dykes – a sign that the fragmented basalt was also a tad wet.

 Near where the pair of dykes in adjacent image are exposed. High up on Askja's south caldera wall, looking westwards.

Near where the pair of dykes in adjacent image are exposed. High up on Askja’s south caldera wall, looking westwards.

 

Q. So did you learn about Bárðarbunga from a hurried swotting-up as this event kicked off?

A. I already knew a lot about Bárðarbunga because I did my PhD on the rhyolite-dominated volcano c.100 km to the SW (Torfajökull) where there’s excellent evidence that basalt dyke intrusions in fissures from the NE (i.e. in the direction of Bárðarbunga) had forced their way into Torfajökull and triggered eruptions there of rhyolite (a more viscous and sticky magma type). The last one in c1477 was fairly benign, with minor explosions and two lovely rhyolite lava flows. One of which has a natural hot pool where once can sit and watch the Northern Lights. But I digress. The eruption prior to c.1477 took place in c.874 AD and this led to a powerful and explosive rhyolite eruption. The problem with explosive rhyolite eruptions is that it contains more gas than basalt (hence more bubbles) so it gets blasted apart more. And because rhyolite is less dense than basalt the rhyolite ash is less dense and can get transported further.

1477 AD rhyolite  lava flow at Torfajökull (Laugahraun). Grey matter to left is older subglacial rhyolite eruption of Bláhnúkur.

1477 AD rhyolite lava flow at Torfajökull (Laugahraun). Grey matter to left is older subglacial rhyolite eruption of Bláhnúkur.

Rhyolite lava flow erupted c.874 AD at Torfajökull. The author in 1983 inside a large bubble (vesicle for the pedants).

Rhyolite lava flow erupted c.874 AD at Torfajökull (Hrafntinnuhraun). The author in 1983 inside a large bubble (vesicle for the pedants).

The area between Bárðarbunga and Torfajökull is one where massive fissure eruptions have occurred and from where some of the largest flood basalts in Iceland have poured forth. The two recent eruptions (c.1477 and c.874) weren’t as massive, but magma-water interactions with the big braided river to the SW area did produce strings of maars, tuff cones, explosion craters and so on, and consequently lots of fragmented basalt that dammed waterways and created temporary but large lakes.

Large braided rivers in the area between Bárðarbunga and Torfajökull. with subglacial basalt 'Toblerone' ridges

Large braided rivers in the area between Bárðarbunga and Torfajökull. with subglacial basalt ‘Toblerone’ ridges

One of the flood basalts  in the area between Bárðarbunga and Torfajökull. For scale is Professor John Smellie.

One of the flood basalts in the area between Bárðarbunga and Torfajökull. For scale is Professor John Smellie.

 

I’ve also worked at the Askja volcano to the NE of Bárðarbunga, and so have some idea of how a large basalt-dominated volcano with a large caldera like Bárðarbunga may have been constructed.

From Askja looking over to Kverkfjöll (left) and with the Dyngjujökull glacier to the right, currently considered the most likely place where meltwater from a subglacial eruption  will pour forth from.

From Askja looking over to Kverkfjöll (left) and with the Dyngjujökull glacier to the right, currently considered the most likely place where meltwater from a subglacial eruption will pour forth from.

 

OK, time for a break while you buy me another ale.

Q. But I bought the first round!

A. Yes, but there’s no such thing as a free tutorial. And remember that my financial prudence has been enhanced considerably after some time living in Yorkshire….

Posted August 23, 2014 by davemcgarvie in Uncategorized

Press Release via Science Media Centre 22 August 2014 10:40 GMT   Leave a comment

The Science Media Centre (SMC) gathers information from scientists when relevant stories break. I provided them with their first update on Bárðarbunga and they asked me for an update this morning. Thought I’d share this with you.

Current situation at Bárðarbunga

Stable as at 10:30 GMT on 22 August 2014. No sign that an eruption is about to start.

Events on 21 and 22 August have raised anticipation (amongst some) that an eruption is imminent. Given the absence of escalation, these (i.e. summit earthquakes and slight subsidence) are best regarded as normal.

What’s happened so far?

“A magma filled fissure (dyke intrusion) some 25 km long has formed within the crust at 5-10 km depth. Sitting on top of this 25 km strip of crust is ice c.150-350 m thick. This dyke is on the NE flank of the main volcano, which is good news, as eruptions from beneath the main volcano itself have a higher probability of being powerful and explosive enough to generate sufficient fine ash to cause disruption to air traffic. There is no indication that the magma in the dyke is moving upwards, but if it did start moving upward this would heighten the possibility of an eruption.”

The NE flank zone – where magma is on the move

“Should an eruption occur from this flank dyke, the eruption style will be influenced by the presence or absence of ice above the eruption site, how much magma erupts, and the rate at which magma erupts. The likelihood of the magma currently in this dyke erupting to produce a substantial enough ash cloud to seriously affect international air travel is zero.

In summarising the flank dyke scenario, if this dyke grows at a similar (slow) rate to that of recent days, then it will either stall in the crust where it will cool and solidify, or it will gain access to the surface and erupt. A modest eruption is likely, with spectacular local explosions generated via interactions between magma and ice/water being observed unless the eruption is wholly covered by ice. Any subglacial eruption generates considerable amounts of meltwater as erupting magma can melt more than 10 times its own volume of ice (NB. variable – depends on conditions).”

Local flooding

“The authorities in Iceland have taken the precaution of evacuating everyone from an area where they would be cut off should a vital bridge be destroyed during a flood. The bridge crosses one of Iceland’s largest and most powerful rivers, and so authorities have alerted communities downstream of action they should take in the event of a flood. It should be noted that unlike the spectacular Amazon River sized flood following the subglacial Gjálp eruption of 1996, as there is no similar sub-ice topographic receptacle near the dyke intrusion in which to store meltwater till it escapes in one massive pulse, meltwater should escape rapidly and continuously from underneath the glacier which will help with managing and mitigating the effects of the flood.”

One worst-case scenario

“Although there are a number of ‘worst-case’ scenarios, one worth mentioning (because it is naturally on everyone’s radar because of the Eyjafjallajökull eruption) is a large and powerful explosive eruption from the main volcano itself that produces a sizeable ash cloud. I must stress that this is not even on the horizon at the moment – it’s somewhere well off the edge. Powerful and explosive eruptions from Iceland’s volcanoes are well documented, and there are many of them. Put simply, Icelandic magma contains enough gas to drive powerful explosive eruptions. The most recent  unequivocal evidence of this was the 20 km high eruption plume produced during the 2011 eruption of Grímsvötn. Evidence from ash layers in Iceland indicates that powerful explosive eruptions have occurred in the past from Bárðarbunga.

“The good news is that if a powerful and explosive eruption does happen, then the experience gained during the 2011 Grímsvötn eruption (which involved a relaxation of the rules for flying with volcanic ash in the atmosphere), would result in a carefully managed strategy to minimise the number of flight cancellations and diversions. Despite erupting twice as much  ash as Eyjafjallajökull 2010, flight cancellations during the Grímsvötn 2011 eruption were less than 1% of the number of flights cancelled during the Eyjafjallajökull 2010 eruption. An important factor in reducing the number of flight cancellations in 2011 was a wind direction that was favourable to UK and western Europe.

“In summarising the large and explosive eruption scenario, there are NO indications that this is about to happen. Even if it does happen we would not get a repeat of the disruption caused by the 2010 Eyjafjallajökull eruption, simply because if this same eruption happened tomorrow there would be far fewer flight cancellations (due to revised flight rules, better information on ash concentrations, and experience gained during 2010 and 2011).”

Concluding note

“Finally, volcanoes are complex natural systems, and when we know so little about a volcano such as Bárðarbunga because it hasn’t erupted in the modern era and thus we have no prior understanding of how it behaves when it stirs, it’s difficult to anticipate what might happen. If this current event does not last long then it will be a volcanic speed dating experience. If it lasts longer, then we may get to know Bárðarbunga’s volcanic personality a little better.”

Posted August 22, 2014 by davemcgarvie in Uncategorized

Bárðarbunga – everything you need to know (well, almost). 21 August 2014   5 comments

Yesterday a colleague decided to hold a ‘mock’ interview with me during our lunch break, which she recorded and I’ve just written up and tidied up. You may find it informative.

Q. So Dave, stop the 50:50 stuff when asked ‘will it erupt’. What do you really think?

A. It’s still 50:50! Whether it will erupt or not depends on a number of factors, some of which cannot be monitored. So that people can better understand why predictions are so difficult let me list some:

The magma is sitting at depth in a vertical fissure and slowly moving NE. It’s a dyke intrusion.

A key question is whether new magma is joining the magma in the dyke. If not (or it’s just a small amount), then there is unlikely to be an eruption. It will stall and cool.

However should a fracture suddenly appear above the dyke, then the magma is going to move upwards, and then it’s more likely to erupt.

Because as it moves up, it will reach a level where any dissolved gases (mostly water) will stop being dissolved, expand dramatically and accelerate upwards, and ‘push’ the magma to the surface. This is actually how eruptions are powered – bubbles.

Another scenario is if magma keeps being pumped into the dyke. The dyke has a number of choices: use the extra energy to keep moving NE; expand by moving to the SW, or grow up and/or down.

Get the picture?

Q. Thanks Dave, and stop calling me Bubbles. Right, we all love an apocalyptic story, so what’s the worst case scenario?

A. Ah, well, there’s more than one with this particular volcano – sorry. But these are nowhere on the horizon at the moment. Here are three.

  1. This presently benign little dyke intrusion is the forerunner to the uprise of large packets of melt from below (from the mantle) and it suddenly turns into a Laki-type flood basalt eruption. There’s still controversy over how these massive eruptions are fed in Iceland, but they always occur in fissures, and they have to involve the mantle because we have no definitive evidence that 10s of cubic kilometres of melt are stored under each central volcano just waiting to erupt. A little puzzle to solve is why these flood basalts (if they are fed directly from the mantle) have ‘shallow’ pressure signatures, but this might just mean they spend enough time at shallow dept in transit to ‘equilibrate’ to lower pressures.
  2. This event triggers activity within the heart of Bárðarbunga, beneath the summit, where there’s almost certainly some melt and or mush (melt+crystals) stored. This could be all basalt, or there could be some more ‘sticky’ magma around, such as rhyolite. Evidence from ash layers in Iceland indicates that explosive basalt eruptions from Bárðarbunga do happen, and that they are powerful. The good news is – and myself and John Stevenson have said this many times – is that we have less to worry about if this happens because we’ve already had one – Grímsvötn 2011. So we know that fewer flights will be cancelled simply because the old “ash in the sky you don’t fly” rules no longer exist. Everyone is much better prepared for a big and powerful explosive eruption. I’ve seen a few geologists say things like “Icelandic magmas do not contain enough gas to drive powerful explosive eruptions”. This is utter rubbish, incorrect, and misleading. These are invariably geologists who lack a true understanding of Icelandic volcanism because they have done little or no research there.
  3. Probably the worst-case scenario for Iceland is that this leads to a massive volcano-tectonic event in the fissure system to the SW of Bárðarbunga, as this is where a number of large flood basalt eruptions have occurred. The hydroelectric power plants on the rivers near to this fissure system would be in trouble, and we know that in the past large ash piles have dammed the rivers. The abundant water in this area results in spectacular (but fairly local) explosions and a high production of fragments as the abundant river water cools the erupting magma.

Q.Final question. You mentioned over coffee that you’d been very active on Twitter trying to get what you called the ‘right information’ out there. But isn’t there a danger that others will pinch your work and re-cast it as their own?

A. That comes with Twitter territory. I’d much rather try and provide an informed and scientifically-based set of views and ideas that can be pillaged and re-used (usually without credit) than leave it to those who don’t understand Icelandic volcano-tectonics to mislead (not always deliberately I hasten to add). I appreciate it when folks give me credit, but I don’t expect it. If you are being paid from public money to do your science, then put your knowledge to good use for the benefit of the public. Getting credit for it is a bonus, not a right.

Q. OK – late for the next meeting Dave. Maybe continue with a pint or two later?

A. Only if it’s a real ale acceptable to my palate.

Posted August 21, 2014 by davemcgarvie in Uncategorized, Volcanism

Askja Volcano – 26 July visit to the 22/23 July 2014 Landslide   1 comment

On the night of 23/24 July 2014 (around midnight) there was a large landslide in the SE corner of the steep inner wall of the 1875 AD caldera at the Askja volcano in central Iceland. This event is simply the latest (albeit large and spectacular) of many that have formed the current water-filled caldera of Öskjuvatn (Askja lake). It is part of the ongoing process of the formation of this youngest caldera at Askja, which is after all only 139 years old and which after its initiation in 1875 took several decades (until c.1932) to get close to the shape we see today.

This blog post contains images from before and after the landslide. I was fortunate to be doing fieldwork nearby (collecting samples from basalt subglacial mountains) when I heard that access to the ‘safe’ area above the lake had just been granted. So we went there on the first day that the area had been opened since the landslide. It was a bit special.  A. Takn in July 2011 - the site of the July 2014 landslide.

A. Taken in July 2011 – the site of the July 2014 landslide.

B. Taken in July 2011. Yellow line shows the major fracture system that was exploted during the July 2014 landslide.

B. Taken in July 2011. Yellow line shows the major fracture system that was exploited during the July 2014 landslide.

C. Taken in July 2011. Purple shaded area shows roughly the part of the inner wall that collapsed during the July 2014 landslide.

C. Taken in July 2011. Purple shaded area shows roughly the part of the inner wall that collapsed during the July 2014 landslide.

D

D. Taken on 26 July 2014, 3 days after the landslide.

Estimates of the volume of the landslide range from 24-60 million cubic metres, and no doubt this will become refined as Icelandic scientists either gain access to the area or use digital elevation models to obtain more precise measurements.

Hazards

The main hazard from the landslide was not the slide itself, as this occurred in a location well away from tourist trails. This location is visited only rarely by geologists utilising the superb exposures revealed by the caldera collapse to gain deeper insight into Askja’s past geological evolution. See Graettinger et al., 2013.

Nope, the main hazard from the landslide was the wave triggered by the sudden entry into the lake of a large mass of debris. Various people have called it a tsunami, a displacement wave, and a seiche. Tsunami will do, and estimates place it as 60-75 m high when it reached the opposite caldera wall there the vast majority of tourists gather to gaze over the lake and into the small crater of Víti (Hell) filled with turquoise coloured, warm, and sulphurous water. Fortunately nobody was in Víti at the time or they’d have had a shock (and an unwelcome cold shower) as the top of the tsunami wave spilled into Víti.

 

The small water-filled crater of Viti, which lies just north of the rim of the 1875 AD (youngest) caldera at Askja. It was one of the vents of the 1875 eruption - the rest are buried beneath the lake water.

Figure E. The small water-filled crater of Viti, which lies just north of the rim of the 1875 AD (youngest) caldera at Askja. It was one of the vents of the 1875 eruption – the rest are buried beneath the lake water. ‘Spillover’ marks the low point where water from the tsunami wave poured into Viti.

 

Aftermath

The image below shows the raft of rhyolitic pumice and ice that remains after the landslide, with the source of the pumice being loose and unconsolidated deposits from the 1875 eruption. It will be interesting to see how long this raft persists, as the strong winds of Autumn and Winter will deposit much of the material on the eastern shore.

Raft of rhyolitic pumice and ice occupying the northeast corner of Askja lake. Debris-covered areas clearly indicate inundation bythe tsunami wave. Access to these areas to check extent of inundation was not possibe as the area is closed.

Figure F. Raft of rhyolitic pumice and ice occupying the northeast corner of Askja lake. Debris-covered areas clearly indicate inundation by the tsunami wave. Access to these areas to check extent of inundation was not possible as the area is closed.

 

Images from the landslide source – 2010 and 2011

August 2010, at the eastern end of the headwall of the July 2014 landslide. Outcrops show downward movement relative to the ridge crest, and multiple parallel troughs indicating fault development.

Figure G. August 2010, at the eastern end of the headwall of the July 2014 landslide. looking to the west. Outcrops show downward movement relative to the ridge crest, and multiple parallel troughs indicating fault development. Some block rotation resulting in dip to the south (left) was apparent on closer inspection.

In 2010 and 2011 I was co-supervising a PhD student who was mapping the older rocks that lie on the east and south of the young 1875 AD caldera right down to the lower outer flanks of the volcano. I also visited the southeast corner with an Earthwatch group in 1985 and made the surprise discovery that there was an old rhyolite dome here, which I confirmed with a chemical analysis. It was apparent that the area around the top of the rhyolite dome and to the west was unstable and that a fault system had been active given that parts of the rhyolite dome had moved downslope and been rotated to dip 5-15 degrees to the south.

From the eastern end of the 2014 landslide headwall, looking across the lake to the Viti crater.

Figure H. From the eastern end of the 2014 landslide headwall, looking across the lake to the Viti crater.

To be honest, on the past occasions I was above the headwall of where the July 2014 landslide occurred (i.e. in 1985, 1987, 2010, and 2011) I was aware of the potential for a landslide in this area, but from the evidence I could see of other landslides (especially the older one immediately to the east) it looked like any future landslide may be a gentle slump rather than a headlong dash into Askja lake.

Older landslide immediately to east of July 2014 landslide. This older one contains large intact blocks of rotated rhyolite lava from the dome above. Look carefully and you can see these be seen on images A-D above.

Figure I. Older landslide immediately to east of July 2014 landslide. This older one contains large intact blocks of rotated rhyolite lava from the dome above. Look carefully and you can see these be seen on images A-D above. In the foreground is one of the basalt vents from the 1920s, when a number of basalt (and mixed-magma) eruptions occurred around the 1875 caldera margins. This vent erupted a number of silicic lithics (non-juvenile clasts), some of which have chemical affinities to the old rhyolite dome nearby, whilst some suggest that other rhyolite sources lie buried.

 

Consequences?

Well the eastern fringes of where the July 2014 landslide occurred formed a convenient way up to the top in this area, but this has now gone. And the landslide has covered over more (if not all) of what was a poorly exposed 1920s basalt lava. The debris dumped onto the lovely little 1920s basalt lava of Bátshraun (0robably 1921) will have covered some of the exposures I was working on – which provide evidence of lava-ice/water interactions at the time of its eruption.

Consequence. A straightforward route up to the rim at this point has now gone. It went up the eastern edge of what came down in July 2014.

Figure J. Consequence. A straightforward route up to the rim at this point has now gone. It went up the eastern edge of what came down in July 2014.

Photo taken August 2010 shows a basalt lava flow from the 1920s which is now largely/wholly covered by debris from the July 2014 landslide.

Figure K. Consequence. Photo taken August 2010 shows a basalt lava flow from the 1920s which is now largely/wholly covered by debris from the July 2014 landslide.

Flow front of the 1920s Bátshraun basalt lava, showing typical a'a upper surface (to left) with glassy and block-jointed lava at lake level indicating more rapid cooling of the loer part of the lava flow.

Figure L. Consequence. Flow front of the 1920s Bátshraun basalt lava, showing typical a’a upper surface (to left) with glassy and block-jointed lava at lake level indicating more rapid cooling of the lower part of the lava flow. See Figure F for location (debris-covered lava).

Figure M. Detail of blocky and glassy texture of Bátshraun basalt lava, showing pseudopillow fractures (long and curving with small joints perpendicular to main fracture). On right is actual pseudopillow fracture surface.

Consequence. Figure M. Detail of blocky and glassy texture of Bátshraun basalt lava, showing pseudopillow fractures (long and curving with small joints perpendicular to main fracture). On right is actual pseudopillow fracture surface.

Working in this area one is aware of the regular small rockfalls from the steep north-facing wall of the 1875 AD caldera, and of the larger slumps that have taken place. As mentioned above I was surprised at the rapid displacement of lake water that let to such a dramatic tsunami wave being formed, but then I’m a volcanologist and not a landslide expert.

No doubt landslide experts will evaluate the potential for additional landslides from the zones adjacent to the July 2014 headwall, as these may have been weakened and potentially be ready to go. However these zones appear fairly small in comparison to the estimated c.800-900 m length of caldera wall that collapsed on 23/24 July.

The Future?

There will be further landslides at Askja simply because the 1875 caldera is still ‘settling’ and will be for some time, with the southern and eastern caldera walls being likely sources because this is the area which underwent the largest amount of subsidence as a consequence of caldera formation (i.e. a sizeable chunk of pre-existing elevated terrain disappeared from the SE corner into the developing caldera). The southern walls of the caldera are particularly steep and consequently material shed from this area has a high probability of entering the lake and displacing water.

An interesting research project would be to look specifically for evidence of past tsunamis at Askja lake, to evaluate whether the July 2014 event was an extreme/low probability event, or just the latest in a number of larger events. My hunch (based purely on the pristine surface of the Bátshraun lava flow prior to this event that is now covered in debris – see Figure F) is that these larger events are infrequent.

The spectacular large landslide of 22/23 July won’t stop me working at interesting localities along the shoreline of Askja lake in future as the risk of a repeat seems very small (though this may change if the authorities carry out a more thorough examination of the source zone and say otherwise). At present the authorities are allowing access only to the relatively safe areas well above the lake level. It will be interesting to see whether this changes over the next few weeks.

In any case, Askja is a truly spectacular place to visit even if you don’t get to go down to the lake edge. And its dynamic nature has been superbly illustrated by this recent landslide, along with its effects and aftermath.

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Posted August 2, 2014 by davemcgarvie in Volcanism

Tagged with , , , , , ,

The Spider Stone of Dalmellington, Scotland   1 comment

Introduction

A short walk from the Ayrshire town of Dalmellington in southern Scotland is an old driveway into what was once the Camlarg Estate. Much of this driveway is still flanked by enough boulders to suggest that the original driveway was flanked by two continuous files of silent rocky sentinels.

However there is one stone that is markedly different from the rest – this is The Spider Stone, and why it’s called this will become apparent soon.

Nothing reliable is currently known about why The Spider Stone came to be where it is now, or how it got its web-like pattern. Encouraged by my old friend John Paterson who’s lived in the area for 57 years, I went to have a wee look after being told “you know about rocks Davie, come and have a look”.

What follows are some preliminary and relaxed observations made over the course of twenty minutes on a pleasant early summer evening. I’ll keep terminology out of this account as much as possible, but when you see [PN with some text] – this stands for ‘pedant note’ and will keep those with greater geological expertise slightly happier.

Caveat. I spend all my time looking at volcanic rocks, so my sedimentary work is rusty to say the least. The observations below should be OK, but don’t take the interpretations too seriously as more work (especially lab work) is needed to validate these.

I’ll put the more interesting stuff at the start (Overview, what I think, and some hypotheses), with more information at the end on rock types and further notes etc.

 

John Paterson of Dalmellington beside The Spider Stone.

John Paterson of Dalmellington beside The Spider Stone.

Overview

The Spider Stone lies with its web-like face upwards, but John reminded me that when we were boys it was more upright (about 30 degrees off vertical). The stone is roughly 1.6-2 metres in diameter. The Spider Stone is formed of two units of sedimentary rock – a thick lower unit and a thinner upper unit (more details are at the back). The web-like pattern is formed in the upper unit. The grooves forming the web-like pattern extend 1-6 cm into the upper unit, with most being in the 1-2 cm range. The grooves were not observed penetrating more than 1 cm into the lower unit.

 

Grooves on The Spider Stone, showing they are only 1-6 cm deep.

Grooves on The Spider Stone, showing they are only 1-6 cm deep. Pen is 15 cm long.

The Spider’s Web

Actually it’s not a very good spider’s web, as those constructed by spiders have long and continuous radiating spokes whereas those on The Spider Stone are discontinuous. But hey – it’s a great name and I can’t think of a better one.

IMG_20140524_181124815

The spokes and elliptical grooves of The Spider Stone. Pen is 15 cm long.

 

Observations (the web)

  1. The web pattern is not circular – it is elliptical.
  2. The ellipse is not symmetrical, it is skewed (bulges) to the top left looking up the stone from the base.
  3. The ‘spokes’ are discontinuous, although the segments of the ‘spokes’ are generally within a cm or two of each other.
  4. The elliptical grooves dominate the structure.
  5. The elliptical grooves are clearly more continuous than the spokes, but still appear discontinuous because no single groove can be traced through 360o without making a slight step to enable continuation. (This could easily be investigated further by a small and detailed study.)
  6. There appears to be no difference in depth between the spokes and the elliptical grooves. (Another prime candidate for a small-scale detailed study.)
  7. Some of the grooves forming the web pattern extend beyond the web into the surrounding rock.
  8. Many of the elliptical grooves terminate by becoming shallower (pinching-out).
  9. The grooves form ‘blocks’ and the shapes of these vary in a regular fashion.
  10. In general, the blocks at the centre of the web have side lengths that are broadly similar (within a few cm) whereas those at the margins have long sides that are markedly larger than the short sides. [PN shapes are more equidimensional towards the centre.]
  11. The grooves are more ‘U’ shaped than ‘V’ shaped. And in the bottom of many grooves is a white crystalline mineral that forms a flattish base to the groove.
  12. We didn’t have a decent bit of steel to test the hardness of this mineral. (Quartz is harder than steel, so a metallic smear will be left on the quartz if you try scratching it with steel; calcite is softer than steel so the steel will score a scratch in the calcite. But it could be one of the many white minerals that exists, such as barite.)
Whitish mineral occupying base of grooves. Pen nib for scale.

Whitish mineral occupying base of grooves. Pen nib for scale.

What do I think about The Spider Stone?

  • The stone is not a lava, it is sedimentary in origin.
  • The web pattern is developed in a specific ‘layer’ of rock that is (subtly) different to the rock type forming the bulk of the stone.
  • The web pattern is a surprisingly mix of regular and irregular grooves. None of the grooves appears to be continuous.
  • How did it get the web pattern? At the moment I favour a semi-natural origin modified by human action. I think the grooves were largely/partly formed by natural processes, and were enhanced/extended by people. The reasons for this are within the hypotheses below.

 

What am I going to do next?

Make a visit to carefully sample some of the whitish mineral to determine its precise chemical composition using some expensive equipment. Take small samples of the two rock units (away from the web itself) to characterise these properly.

 

Concluding note

No need to read further. But if you want to see the preliminary hypotheses and the descriptions of the two rock units, feel free to read on.


 

Hypotheses

Geologists construct hypotheses based on preliminary observations to focus their thinking, and then undertake further research to prove/disprove them, or to modify them, or to formulate completely new ones.

Popular thinking about The Spider Stone considers two possibilities: it’s either natural or people carved it. From this I’ll construct four hypotheses, with some comments on how likely they are, and with notes on how to prove/disprove them. Although there are many more hypotheses than these, I’ll stick to those that are at least partly supported by my preliminary observations.

  1. One or more people decided to carve a web-like pattern into the surface of a large sedimentary boulder. To enhance the web pattern they poured a mineral solution into the grooves to form the whitish mineral. Problem – why not make the spokes and the elliptical grooves continuous and spaced more regularly?
  2. One or more people decided to carve a web-like pattern into the surface of a large sedimentary boulder, and simply deepened some existing grooves (formed by weathering, erosion, dissolution etc), with/without creating new grooves to complete their pattern. To enhance the web pattern they poured a mineral solution into the grooves to form the whitish mineral. Problem – the know-how to create a suitable mineral solution from which the whitish mineral would precipitate and crystallise.
  3. Entirely natural. The web pattern is simply a product of weathering, erosion, dissolution etc. The whitish mineral occupying the grooves was introduced during a separate event in which the stone was buried and fluids were circulating in the crust and then precipitated in the grooves. Problem – requires two separate geological processes, which is a big ask, bus in not impossible in an area with dynamic earth movements (i.e. there are many known faults in the nearby coalfield, and we are not far away from the big Southern Uplands fault zone).
  4. Entirely natural. The web pattern is simply a product of weathering, erosion, dissolution etc. The origin of the whitish mineral occupying the grooves is from the dissolution of the lime-rich upper surface that produced a purer calcium carbonate liquid which then crystallised in the grooves. Problem – I don’t know if this can actually happen, and in any case it assumes that the upper later is more lime rich and that the whitish mineral is calcite (as yet unproven).

At the moment I slightly favour hypothesis 2. One important key to unlocking the origin of the web pattern is the nature of the whitish mineral occupying the grooves. If this is something that could easily be produced by people then this argues for the grooves being of natural origin but modified by people, and then being filled with a solution that precipitated the whitish crystals. If this mineral could only form via geological processes (i.e. requiring specific pressure and/or temperature conditions outwith the human ability of the time) then the entirely natural hypotheses 3 and 4 come into play.


 

Rock types [PN lithologies]

Lower unit. This is a fragmental rock that displays distinct layering. Hand lens examination suggests fine-medium sand. The distinct layering is on the sub-mm scale and individual layers [PN laminae] cannot be traced across the entire exposed surface [PN so it’s not planar lamination]. The layers have numerous shallow-angle cuspate/lenticular structures. Preliminary interpretation: This is a fine-medium sandstone with a planar structure containing numerous small-scale lenticular structures. [PN. Likely to be lenticular and/or flaser bedding.]

 

Upper unit. This is a fragmental rock with fine-scale layering at the base that becomes less well defined towards the uppermost surface. The grain size is slightly larger than that of the lower unit, and medium sand is suggested. In places the boundary between the lower and upper units is indistinct, but elsewhere it is marked by a distinct groove presumably reflecting an impersistent joint/fracture plane at the boundary. The uppermost part of this unit has a finer-grained appearance, but lack of a fresh surface prevented further examination. (Next time I’ll bring a wire brush to gently remove the surface crud.) Preliminary interpretation: A medium sandstone representing a separate packet of sediment to the lower unit. The larger clasts at the base grading upwards to smaller clast would suggest [PN assuming present orientation is same as original] that larger clasts settled first followed by successively smaller clasts. [PN graded bedding.] A tentative suggestion based on the finer-grained nature of the top 1-2 cm along with the weathering pattern on the exposed edges  is that the uppermost portion of the upper unit is more lime rich. [PN it’s more calcareous, and the matrix especially may be more calcareous.]

 

Edge of The Spider Stone, showing lower unit with distinct laminations and upper unit showing less distinct laminations with coarser clasts.

Edge of The Spider Stone, showing lower unit with distinct laminations and upper unit showing less distinct laminations with coarser clasts.


 

Further Notes

What is known about The Spider Stone? Very little. Even my friends who have lived in the area for over 50 years know nothing definitive, just vague rumours. The Camlarg Estate had ceased to be long before this, and so reliable local knowledge on the origins of Spider Stone probably disappeared long ago. There may be older folks in Dalmellington who can recall knowledge passed down to them, but they would need tracking down and interviewing.

A web search gives no images, and only one decent link to a Geocache site. This gives no references to the description provided, and no observations to support the interpretation.

http://www.geocaching.com/geocache/GC53RF7_the-spider-stone

The cache is located a short distance beyond The Spider Stone. It is thought that the Web shaped fissures in the face of the rock were created by gas bubbling up to make a mound and then cooling. It is found in volcanic conditions, probably in the lava flow. There are a number of rocks along the former driveway to the old house which are thought to have been imported in Victorian times. We are doing extra research on this stone in collaboration with the local community. Locals have been known to make wishes at the stone. A rare geological gem.congratulations to stewart57 on FTF

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