Tag Archives: volcanic ash

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.

Cuesta

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]

 

 

Chaiten: anniversary of an eruption

1 May

May 1st marks the anniversary of the start of the first historical eruption of Chaiten, a small volcano in southern Chile, in 2008. A lot has been written on the eruption elsewhere, starting with Erik Klemetti’s eruptions blog which first reported on the event at the time. This is an opportunity to share some field photos, which I took during field visits to Chaiten in 2009. At the time of the eruption, Chaiten was not well known,  but it was recognised to be an old dome of obsidian lava, last thought to have erupted about ten thousand years previously. In fact, we now know that Chaiten has a long history of explosive eruptions of  rhyolite magma, and is probably one of the most prolific producers of rhyolite in southern Chile.

The snapshots illustrate some of the transient consequences of explosive, ash-rich eruptions for both people, and the environment; and some of the excitement of  trying to read the deposits before they have been washed away. Enjoy!

Further reading: a special issue of the Open Access journal ‘Andean Geology‘ on the Chaiten eruption was published in May 2013. This issue contains a number of papers that describe the 2008 eruption and its consequences, and others that reconstruct the past history of this volcano.

IMG_4506

Ash and leaf litter

IMG_4552

Prints in the ash

Impressions

Impressions in ash

IMG_4677

Ash in the undergrowth

IMG_4690

“Chaiten will not die”

IMG_4710

“We want to return to Chaiten, our little town”

IMG_4382

Wood shavings

IMG_4724

Chaiten bay, choked with pumice

Approaching Chaiten

Approaching Chaiten

IMG_4728

Survey spot

IMG_4965

Field volcanology

IMG_5052

Evening glow

Acknowledgements: funding for fieldwork on Chaiten and elsewhere in southern Chile was provided by grants from NERC and the British Council. Field collaborators included Fabrizio Alfano, Constanza Bonadonna, Chuck Connor, Laura Connor and Seb Watt.

Further reading:

JJ Major and LA Lara, 2013, Overview of Chaiten volcano, Chile, and its 2008-2009 eruption, Andean Geology 40 (2), 196-215. [Open Access]

SFL Watt et al., 2009, Fallout and distribution of volcanic ash over Argentina following the May 2008 explosive eruption of Chaiten, Chile, Journal of Geophysical Research 114 (B04207).

SFL Watt et al., 2013, Evidence of mid- to late-Holocene explosive rhyolitic eruptions from Chaitén Volcano, Chile,  Andean Geology 40 (2), 216-226. [Open Access]