Tag Archives: Southern Volcanic Zone

The fate of volcanic ash in the environment

4 Dec

Over the past few years, we have been working to piece together the record of major post-glacial volcanic eruptions in southern Chile that have occurred over the past 18,000 years. This work started off with a search for volcanic ash layers that were preserved in road cuttings, or cliff faces other accessible geological locations in the region. Since then it has expanded to include the search for pumice and ash layers (or, more generically, ‘tephra’) in peat bogs and lake core sediments.

Sampling a peat bog in southern Chile.

Seb Watt sampling a peat bog in southern Chile.

By using the chemical compositions of the volcanic glass from each eruption to ‘fingerprint’ the deposits, we can now start to develop a framework in time and space of when volcanoes erupted, where they left deposits, and how large those eruptions were.

Depositional environments for volcanic ash in southern Chile, from Fontijn et al. (2014).

Depositional environments for volcanic ash in southern Chile, from Fontijn et al. (2014).

As well as looking at the deposits of past eruptions, which we can find in these cores and cuttings, recent eruptions in the region have also given us some new information on how the ash ends up where it does after an eruption.

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Pale coloured band of volcanic ash at 44-46 cm depth in a peat core sample, Cuesta Moraga, Chile.

One of the fascinating stories that is starting to emerge from this work is just how patchy the preservation record can be – even for moderate to large explosive eruptions. In a really nice piece of work which has just been published, Sebastien Bertrand and collaborators looked at how volcanic ash and pumice ended up in the nearby lake Puyhue, after the 2011 eruptions of the volcano Puyehue – Cordon Caulle.

In this case, the dispersal of the ash clouds during the explosive phases of the eruption were very well constrained. As with most eruptions in this region, winds blew most of the ash clouds to the East, across Argentina, and there was no major phase of the eruption that deposited pumice and ash into lake Puyehue, to the West of the volcano. Instead, the thick deposits of ash and pumice that ended up in this lake – up to ten cm thick in places – must have been transported there by fluvial processes. Rainfall during and after the eruption would have helped to remobilise freshly fallen pumice and ash from the upper reaches of the watershed. This tephra would then have been washed downstream, and into the lake, where lake currents at different water depths then helped to redistribute the tephra across the different parts of the lake system.

Cartoon from  Bertrand et al. (2014) showing the fate of pumice and ash from the 2011 eruptions of Puyehue - Cordon Caulle, Chile

Cartoon from Bertrand et al. (2014) showing the fate of pumice and ash from the 2011 eruptions of Puyehue – Cordon Caulle, Chile

This study provides a really nice example of the complexities of trying to piece together the deposits from ancient eruptions from the sparse environmental records that are eventually preserved. In the lake Puyehue example, the sediments accumulating at teh bottom of the lake will be an excellent archive for the deposits – since they will eventually be buried and preserved. But since the deposits are entirely reworked, their characteristics in terms of both grainsize and thickness could be quite misleading, unless they are recognised as ‘secondary deposits’. Since volcanologists usually rely on pieceing together the areas affected by tephra deposition from the few locations where the deposits are both preserved, and then accessible to later discovery, and then use these data to work out how big the eruption was and which way the winds were blowing at the time, this new work will make us all have to think a little bit harder about our interpretations in the future.

References.

S Bertrand, R Daga, R Bedert, K Fontijn, 2014, Deposition of the 2011-2012 Cordon Caulle tephra (Chile, 40 S) in lake sediments: implications for tephrochronology and volcanology, Journal of Geophysical Research (Earth Surface), in press.

K Fontijn et al., 2014, Late Quaternary tephrostratigraphy of southern Chile and Argentina, Quaternary Science Reviews, 89, 70-84.   doi:10.1016/j.quascirev.2014.02.007  [Open Access]

 

 

Friday Field Photos: the Southern Volcanic Zone of Chile

30 Aug

If you are ever in Chile and have the chance to take a mid-morning flight south from Santiago towards Puerto Montt or Concepcion, make sure you try and book a window seat on the left hand side of the plane.  Once the early morning cloud has cleared, you could be in for a treat as you fly along the ‘volcanic front’, with spectacular views of Chile’s brooding volcanoes popping up from the landscape. Be sure to take a map, too, so that you can work out which one is which. The pictures below are roughly in order, flying from north to south – and several major volcanoes of the chain aren’t included.

There are several things to notice about these volcanoes – they are often in pairs, either as distinct but closely spaced mountains (Tolhauca and Lonquimay), or as ‘twin peaks’ forming the summit of an elongated massif (e.g. Llaima, Mocho Choshuenco). Many of the volcanoes are also clearly very young structures – forming wonderfully characteristic conical shapes (e.g. Antuco, Villarrica, Osorno). These cones must be younger than 15 – 20,000 years (and perhaps much younger than this), based on what we know about when the last major glaciation in the region ended. These cones sit on top of the lower-relief and older parts of the volcanoes, many of which have been reshaped by caldera-collapse, perhaps shortly after the ice retreated during deglaciation. The accessibility of the volcanoes of the Southern Volcanic Zone of the Andes makes this a wonderful place to study volcanic processes and volcano behaviour, both at the scale of individual eruptions, as well on the regional scale.

The river Cachapoal runs out of the Andes mountains, past the city of Rancagua

The river Cachapoal runs out of the Andes mountains, past the city of Rancagua

The saddle-shaped volcanic complex of Planchon-Peteroa (35.2 S), which last erupted in 2011.

The saddle-shaped volcanic complex of Planchon-Peteroa (35.2 S), which last erupted in 2011.

Cerro Azul volcano, Chile.

The spectacular ice-filled summit crater of Descabezado Grande volcano, Chile, at 35.6 S. The last eruption from this complex was in 1932, shortly after an eruption of the  nearby volcano Cerro Azul (or Quizapu).

View across the volcanoes of Tolhuaca (or Tolguaca, near ground) and Lonquimay (38.3 S). Both volcanoes are young, but it is not known when Tolhuaca last erupted. Lonquimay last erupted from 1988-1990.

View across the volcanoes of Tolhuaca (or Tolguaca, near ground) and Lonquimay (38.3 S). Both volcanoes are young, but it is not known when Tolhuaca last erupted. Lonquimay last erupted from 1988-1990.

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The young cone of Volcan Antuco, 37.4 S. Its last known eruption was in 1869.

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Twin-peaked Llaima (38.7 S) is one of the most active volcanoes of southern Chile, and last erupted in 2009.

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Volcan Sollipulli (39 S) has a spectacular ice-filled summit caldera, but is not thought to have erupted since the 18th Century

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Panorama across three young volcanoes, looking east: Villarrica (39.4 S) in front; the snow-covered sprawl of Quetrupillan in the middle ground; and the peak of volcan Lanin, on the Chile – Argentina border, in the distance.

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Villarrica, with a characteristic thin gas and aerosol plume rising from the open crater at the summit.

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The twin-peaked volcanoes Mocho Choshuenco (39.7 S). Choshuenco, thought to be the older vent, is the angular crag nearer the camera; Mocho is the small cone in the middle of the summit plateau. Mocho last erupted in 1937.

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Looking across a bank of cloud towards volcan Osorno (front, 41.1 S), and volcan Tromen, in the background. Osorno last erupted in 1869; Tromen is thought to have last erupted in 1822.

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Volcan Calbuco (41.3 S), which last erupted in 1972.

Data source: information on the recent eruptions of these volcanoes is all from the Smithsonian Institution Global Volcanism Project.

Further reading:

CR Stern, 2004, Active Andean volcanism: its geologic and tectonic setting. Revista geologica de Chile 31, 161-206 [Open Access].

SFL Watt et al., 2009, The influence of great earthquakes on volcanic eruption rate along the Chilean subduction zone. Earth and Planetary Science Letters, 277 (3-4), 399-407.

SFL Watt et al., 2013,The volcanic response to deglaciation: evidence from glaciated arcs and a reassessment of global eruption records, Earth-Science Reviews 122, 77-102.

Acknowledgements: my fieldwork in Chile over the past 10 years has been funded by NERC, IAVCEI and the British Council. Many thanks to my parents for introducing me to Chile and its volcanoes at the age of 7; and to Jose Antonio Naranjo and many others at SERNAGEOMIN for facilitating our continuing work in the region.