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.
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.
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.
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.
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.
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….)
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.
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.
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.
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
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.
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.
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.