Archive for the ‘Chile’ Category

30-January-2016. Nevados de Chillan volcano, Chile – is it about to erupt?   8 comments

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This is the highest part of Nevados de Chillan – the aptly named Cerro Blanco, with a young (Holocene) basaltic andesite lava flow in the foreground. Cerro Blanco lies in the NE part of the volcanic complex, whereas the part of the volcanic complex that is showing signs of unrest is at the SE end, in the Las Termas area. In this blog post I’ll discuss the types of eruptions that might happen in the near future. Then I’ll provide some background on the Nevados de Chillan volcanic complex itself. I’ll finish off with some examples of lava-ice interactions at Nevados de Chillan.

Nevados de Chillan volcano in southern Chile has been showing signs of unrest since September 2015, and this is likely to lead to an eruption within the next few weeks-months. On 8 January a new vent was observed, and there are now 3 webcams focused on the volcano as well as a host of instruments measuring the developing unrest. See this link. So if an eruption does occur and the visibility is good, you’ll get a ring-side seat courtesy of the Chilean geologists and authorities.

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Webcam capture on 30 January 2016 at 20:00 GMT from Portezuelo webcam. See hyperlink above.

What makes me think that this unrest is likely to lead to an eruption? Well there are two main reasons.

Firstly, there’s clearly been a new heat source introduced into the plumbing system beneath the volcano, and this has drilled a new pathway to the surface leading to bursts of heat escaping through a new vent. This heat source is almost certainly due to magma rising up in the plumbing system. And at the moment there’s a ‘vent-clearing’ phase in place, with bursts of heat interacting with water contained within the cone (hydrothermal). There are probably magmatic gases involved as well. These energetic outbursts are clearing out material in the developing conduit, and possibly also pulverising (fragmenting) material being blown out.

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Webcam capture at 09:53 GMT on 30 January 2016 showing small and ground-hugging low plume of particulates.

Secondly, this new vent has developed on the youngest cone at this volcanic complex, which has developed through a long series of eruptions, punctuated by time gaps of a few years to decades.

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The twin cones of Nuevo and Arrau, that form part of the Las Termas, a particularly active part of the volcanic complex and one that erupts evolved and viscous dacite/rhyolite magmas.

 

In essence, this is quite a simple situation. An eruption from the new vent would simply be just the latest stage in the development of Nevados de Chillan’s youngest cone.

So what sort of eruption might we expect?

To answer this we can look at recent eruptions, as well as the series of eruptions that constructed this part of Nevados de Chillan. And this is a key point – Nevados de Chillan is not a simple cone-shaped volcano like the current Mount Fuji – instead it comprises a number of volcanic centres aligned in a roughly NW-SE direction, along with older rocks and remnants of caldera walls that are considered to have formed when large ignimbrite eruptions took place. I’ll return later to some of the older rocks – especially those that show evidence of lava-ice interactions.

But back to recent eruptions. The most recent eruption was a small eruption that took place in 2003 after a gap of 17 years (1986), lasted about a week, and produced episodic and small pulses of ash ejection that rose up to 500 m above the cone See LINK

The new vent formed on the saddle between the two newest and overlapping cones in this part of the volcanic complex – Nuevo and Arrau. (See diagram below.) A small ~64 m double crater was formed, with measurements of the two craters being 25 x 14 m and 39 x 28 m.

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Nice diagram from Naranjo et al., (2004) – see link above. Shows the 2003 vent in relation to the young cones of Nuevo and Arrau. Also shows how the young cones have been build upon the older stratovolcano of Democratico.

A thin and restricted carpet of ash was produced, much of it on snowpack. An interesting point is that if this eruption had occurred a few centuries ago, it might not have been recorded as it was so small, plus the thin ejecta blanket (being on snow) would not have been preserved for study by present-day volcanologists. The point being made is that such small eruptions in the past on ice/snow-clad volcanoes relatively remote from local populations are unlikely to have been observed, plus as the deposits have very poor preservation potential (being emplaced on snowpack), there will be no record of such eruptions. One of the little challenges for volcanologists working on snow/ice-clad volcanoes.

Such a small eruption could indeed be what Nevados de Chillan is building up to produce. But speaking personally, I’d rather see an eruption of the kind that produced the two newest and overlapping cones in this part of the volcano. These are called Nuevo and Arrau, with Nuevo being formed in the period 1945-1945, and Arrau in the period 1973-1986. The small 2003 eruption was the first since Arrau stopped erupting in 1986.

 

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The Las Termas area in the SW of the Nevados de Chillan complex, showing the two young cones Nuevo and Arrau. The small 2003 vent (mentioned above) has been given the name Chudcún. The new vent that opened on 8 January is indicated. Source: LINK

These two newest cones have been built on top of (and largely obscure) an older stratovolcano (see figure above). These new cones are constructed of interesting rock types (dacite and rhyolite) that are extremely viscous and gas rich, and so eruptions tend to be explosive as gas escaping from the magma blasts apart the magma into ash and pumice. (It’s one of those counter-intuitive quirks of science that escaping gas is stronger that molten rock and can actually fragment it. This leads to the production of enormous eruption clouds composed of fragmented magma, such as the 30 km high eruption plume formed when Chaitén volcano unexpectedly erupted in 2008. But I digress….)

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Chaiten volcano erupting in 2008. A classic example of the eruption of a massive amount of viscous and gas-rich rhyolite magma is a short time (i.e. high mass eruption rate). Source LINK

However, when these viscous magmas can rise to the surface without being completely blasted apart, then they will form slow-moving lava flows that combine to form steep-sided cones just like the Nuevo and Arrau cones at Nevados de Chillan. So this is what I’d like to see – a number of lava flows effusing from the new vent and building a new cone that, given the position of the new vent, might grow to be the highest point on this part of the volcano and exceed the heights of the Nuevo and Arrau cones.

Is such an eruption likely to be dangerous? What hazards are likely? The Chileans are aware of the potential hazards from their volcanoes, and of course those that are snow/ice-capped present an additional hazard if there is a great deal of melting, as the meltwater will move downhill into drainage systems, where it can entrain particulate matter and eventually develop into a lahar. However at the moment it is late summer in Chile and consequently snowpack is close to its minimum annual mass. Good news. Sernageomin has prepared hazard maps and will work with the local authorities to ensure that populations in danger are informed and if necessary evacuated http://www.sernageomin.cl/volcanes-mapas.php. After all, this is considered the 7th most active volcano in Chile, so it’s an obvious target for hazard planning and mitigation. See LINK

So, to sum up the current activity. This latest unrest could be a damp squib and we may see nothing more than the current puffs of particulate matter being ejected a few hundred metres above the new vent, rather like the tiny 2003 eruption. Or we may (more excitingly) see magma rise to the surface and effuse out of the new vent to produce steep blocky lava flows that slowly pour down the sides of the Nuevo-Arrau cones. The above two scenarios are the most likely ones based on the past activity that built the Nuevo and Arrau cones. I’d be surprised if there was a massive explosive eruption that produced a Plinian eruption column, but volcanologists always have in the back of their minds a number of ‘worst-case’ and ‘unlikely’ scenarios – just so they have done a bit of thinking about them on a just-in-case basis.

The volcanic complex

I’ve described Nevados de Chillan as volcano complex, and not a volcano. The reason for this is that unlike, for example, Villarrica which has a single prominent cone that is the focus for most eruptive activity, Nevados de Chillan has a number of recent vents/cones (estimated to number c.13) that are aligned NW-SE for c.10 km.

 

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The Nevados de Chillan volcanic complex. Source: LINK

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Geological map of Nevados de Chillan, from Dixon et al., (1999). LINK

 

There is evidence of an ancient volcanic complex that predates the newer c.10 km long volcanic complex, in which there are remnants of caldera walls. Some work has been done on these and can be found at LINK

 

This is a pattern that occurs at many Chilean volcanoes – an early volcano complex that produces caldera-forming eruptions (with associated ignimbrite sheets), and then a younger volcanic complex develops within the caldera(s). Another quirk that occurs fairly often at Chilean volcanic complexes is that two discrete centres of volcanic activity can develop, which erupt quite differing magmas. This is the case at Nevados de Chillan, where the NW volcanic centre (Cerro Blanco) erupts mostly magma around basaltic andesite in composition, whereas the SW centre (Las Termas, which encompasses the youthful Nuevo and Arrau cones) erupts the more evolved rock types of dacite and rhyolite. I’ll be honest with you, I’ve not yet read a thoroughly convincing model that fully explains why this happens.

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The two main centres of the Nevados de Chillan volcanic complex that have been active in recent centuries. On the left (NW) is Cerro Blanco, and on the right (SE) is Las Termas where the Nuevo and Arrau cones are. This is where the January 2016 vent has appeared.

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The prominent dark lava in the aerial image above is a relatively young dacite lava flow. This image shows what this lava flow looks like on the ground, with glassy dacite (foreground) plus piles of blocky rubble that characterise the surfaces and sides of these lava lobes. This is the sort of lava that might be expected to appear if there was an effusive eruption from the new vent that opened on 8 January 2016.

 

The most recent eruption from Cerro Blanco took place from 1861-1865 and produced a lovely basaltic andesite lava flow that melted a fair bit of snow and sent meltwater down into the Santa Gertrudis valley. Not always as a steady trickle, as a portion of the meltwater was dammed and escaped as a flood.

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In the foreground is Cerro Blanco, with the 1861-1865 flank eruption of the basaltic andesite Santa Gertrudis lava with its source cone (both characteristically dark). The lava lobe on the lower left is plunging into the upper parts of the Santa Gertrudis valley.

 

As part of a wider study of lava-ice interactions at Nevados de Chillan I visited this lava flow in 2001 whilst I was supporting a PhD student based at Lancaster University (UK), and discovered a number of fractures indicative of enhanced cooling of lava, which implied water/snow/ice was involved. This was followed up the following year by the PhD student (Katy Mee) and a colleague (Hugh Tuffen) who did a more detailed study and wrote this up as a paper. See LINK

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The 1861-1865 Santa Gertrudis lavas. Taken in 2001.

The wider study of lava-ice interactions at Nevados de Chillan along with a geochemical and geochronological study was published as a separate paper, which can be accessed at   LINK or at LINK

Lava-ice interactions

One the aspects of research I am particularly interested in is what happens when lavas interact with ice and snow (or the cryosphere if you prefer a more grandiose name). What happens with sustained eruptions (point or fissure) under thick ice is arguable fairly straightforward with different volcanic rock types (lithofacies) produced in response to varying ice/water conditions, and enhanced complexity occurring as edifices collapse, vent positions shift, and intrusions permeate the edifice.

Anyhow, the point I am making is that unlike eruptions into thick ice sheets, lava-ice interactions at stratovolcanoes is a subject that has not been much studied, and yet these interactions produce a surprising diversity of landforms because of the varying thicknesses and properties of ice/snow, the type of lava erupted, how fast it is erupting/flowing, and topographic aspects such as slope angle and so on.

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A small tongue of lava displaying characteristics that indicate it has been more rapidly cooled than just by interactions with air. It has a glassy texture (indicating rapid cooling) and a set of closely-spaced fractures – the latter are produced when heat is rapidly extracted from a lava, and water/ice is very effective at doing this. Air is not.

At Nevados de Chillan as well as good examples of lava-snow/ice interactions during the 1861-1865 eruption of Santa Gertrudis lavas, there are older examples and I’ll deal with a few of these. I’ll say now that the images (taken in 2001) are a combination of early digital camera (175 kb max) and slide scans from a rusty old scanner, so don’t expect crisp and sharp images.

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A tongue of lava that has a vague similarity to an Elephant’s trunk. Structures within the lava (e.g. flow bands and folds) indicate that it started at the high point and then propagated downwards. Pale yellow material consists of quench fragmented lava. The interpretation is that a lava was ‘perched’ on the flanks of an ice stream, and then decided to exploit a meltwater tunnel in the ice, which it enlarged as it melted its way downwards. With difficulty.

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Lava lobe that encountered ice. This has a confined shape, consists of glassy and fractured lava, and in this case has a reddish (oxidised) base. The ‘Elephant Trunk’ lobe above came from another exposure of this lava, just out of shot to the right.

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Reddish base of the lava lobe shown in the previous image. This shows intense fracturing (hackly fractures for the pedants), above which a zone of platey fracturing develops. Above this occurs microcrystalline to glassy lava. The degree of fracturing is much greater than is seen in a typical subserial flow (i.e. erupted in the absence of water/ice/snow), and so it is interpreted that ice/snow was present at the time of eruption. The ‘confined’ morphology of the lava lobe along with its glassy and fractured carapace helps to corroborate this interpretation.

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The prominent scarp in the foreground has been interpreted as a caldera fault, and it curves round (to the right) to pass in front of the highest peak (above the pool).

 

Endnote

This has been a bit of a long and rambling blog entry, but as it’s such a contrast to the limitations of writing a scientific paper I have perhaps gone a bit over-the-top. Well done if you get this far! All good wishes, Dave.

 

 

 

 

 

Posted January 30, 2016 by davemcgarvie in Chile, Eruptions, Lava, Nevados de Chillan, Volcanism

Recent volcanism at Volcán Quetrupillán, Chile   Leave a comment

This short blog entry is rich on images and short on text. Its purpose is two-fold. First – to provide a brief introduction to the recent (Holocene) volcanism of this poorly-understood volcano, and second – to provide a bit more information for candidates interested in applying for the currently-advertised PhD project at the University of Edinburgh on this very topic, on which I am a supervisor.

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The ‘beheaded’ Quetrupillan stratocone from the SW, with Laguna Azul in the foreground. The sheets and lobes on the ridge in front of the main stratocone are formed from a subglacial (Pleistocene) dacite eruption of unknown age.

So why is so little known about Quetrupillán? One key reason is that with one of Chile’s most active volcanoes (Villarrica) being such a close neighbour and with Villarrica’s past reputation for causing death and disruption, a nearby volcano that hasn’t erupted within living memory and has no obvious signs of current/recent unrest, won’t be given much if any attention. And that’s fair enough, because when you have finite resources to monitor potentially dangerous volcanoes, you need to focus those resources wisely. Even knowing what I now know about Quetrupillán, with limited resources I’d still put my monitoring equipment onto volcanoes such as Villarrica, Llaima, Calbuco, Puyehue-Cordon Caulle, and so on.

Quick big-picture context. Running through this part of Chile is a c.1200 km long approximately N-S fault zone called the Liquiñe-Ofqui fault zone LOFZ, along which many of the volcanic centres of Chile are associated. Quetrupillán lies in the middle of a NW-SE chain of three volcanoes that cuts obliquely across this fault zone, with Villarrica at the NW end and Lanín at the SE end.

I’m only mentioning this because one of the key features that distinguishes Quetrupillán from its two neighbours in the chain is that it sits astride part of the LOFZ, and on closer examination it is clear that many volcanic vents to the south and around the east and west flanks are aligned along roughly N-S fissures. This has given Quetrupillán a rather mixed morphology, with focused vent activity producing a ‘beheaded’ stratocone developed to the north, and more dispersed fissure-controlled activity producing a fascinating volcanic field to the south with abundant evidence of (Pleistocene) volcano-ice interactions as well as recent (i.e. Holocene) explosive and effusive eruptions.

I’m currently writing a paper on the volcano-ice interactions that have taken place during the Pleistocene, and I’ll write another blog entry when this is close to publication with more images than a journal will allow. Previous blog enties contain some information: Blog 1 and Blog 2

There’s also evidence of lateral transport of pyroclastic material in surrounding valleys (i.e. pyroclastic flows – or pyroclastic density currents for the pedants), as well as pyroclastic deposits formed via sedimentation from volcanic plumes. What we call ‘fall’ deposits which are important as they represent the remnants of sizeable eruption plumes in the past.

So enough of the text and onto the images.

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Western side of Quetrupillan (north to top) showing the beheaded stratocone to the north with ice-filled summit crater, along with two key geographic features – Laguna Azul and Laguna Blanca.

 

 

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Eruption 1 is a dog-leg fissure eruption (probably dacite), which in the south has been highly productive in producing lavas. In the north it has mainly resulted in initial vent clearing with the formation of craters that have cut into pre-existing Pleistocene deposits. Eruption 2 comprises part-eroded lavas that are more likely to be early than late Holocene. Eruption 3 is a thin lava flow (basaltic andesite?) that is partly covered by aeolian deposits and the rising waters of Laguna Blanca. Eruption 4 is a small lava flow than has not travelled far from its vent (hidden) to the SE.

Read the rest of this entry »

Posted November 23, 2015 by davemcgarvie in Chile, Eruptions, Lava, Volcanism