VolcanicDegassing

Mocho Choshuenco

Taking the pulse of a large volcano: Mocho-Choshuenco, Chile

Taking the pulse of a large volcano: Mocho-Choshuenco, Chile

As the recent eruptions of Calbuco and Villarrica in southern Chile have shown, the long arcs of volcanoes that stretch around the world’s subduction zones have the potential to cause widespread disruption to lives and livelihoods, with little or no warning. Fortunately, neither of these eruptions has, so far, led to any reported loss of life – but the consequences  of these eruptions for the communities living within reach of the ash plumes and beyond will continue to play out for months or years into the future.

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The young cone of Mocho volcano, southern Chile, which may have erupted as recently as 1937. Mocho-Choshuenco volcano is one focus of our ongoing work in the region.

We have been working in southern Chile for a few years now, helping to extend what is known about past explosive eruptions at some of the region’s most active volcanoes. In this part of Chile, the written records of past eruptions only extend back a few hundred years – at the most – so most of our work has involved digging into the geological records of the region, to try and piece together the fragmentary stories of past eruptions. This can be slow and painstaking work, both in the field and in the laboratory, but is always exciting when things start to come together.

Field sampling on Mocho-Choshuenco volcano: deposits of the ‘Enco’ eruption.

This week, Harriet Rawson has published her first major scientific paper on the volcanic eruption history of Mocho-Choshuenco over the past 18,000 years. The 18,000 year timescale spans the volcanic activity that has taken place since the end of the last ice age; and we can be fairly confident that by visiting hundreds of sites around the volcano, we have found most of the ‘major’  eruptions, and many of the ‘moderate’ explosive eruptions from this volcano over this time period. The results of Harriet’s work are summarised in the picture below – which shows the timing, composition and sizes of eruptions through time. Just for context, the March 3 eruption of Villarrica was small (10 million cubic metres of ash, or magnitude 3 on the y axis), with a composition represented by an orange colour (so a bit like the Mocho eruptions around 4,000 years ago); while the April 22 eruption of Calbuco was moderate (210 million cubic metres of ash, or magnitude 4.5 on the y axis), and a composition in the green to pale blue range (like the eruption around 2000 years ago).

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The record of explosive volcanic eruptions at Mocho-Choshuenco volcano over the past 18,000 years (from Rawson et al., 2015). The x axis shows time, as ‘thousands of years before present’, based on radiocarbon dating of flecks of charcoal preseved within the deposits. The y-axis shows the ‘size’ of the eruption, in terms of the eruption magnitude, which is a logarithmic scale of erupted mass or volume of ash and pumice. The coloured curves represent the age and erupted composition of the volcanic events that have been recognised – with the ‘peak’ of the curve showing the best estimate of the eruption age, and size. The width of the curve gives an indication of the uncertainty in the timing of the eruption. The cartoon parallel to the x-axis shows how regional climate and ice cover at the volcano are thought to have changed over the same time period.

In many ways, this work is just the start of the forensic process of understanding how this particular volcano works and of the threats it might pose for the future;  but it is also a critical piece of the jigsaw in terms of understanding the pulse of the volcanic arc, and crossing the gap between the geological past, and the volcanic present.

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The wonderful ‘Salto Huilo Huilo‘ in the Huilo Huilo ecological reserve, at the foot of Mocho-Choshuenco.

Acknowledgements

This work has been funded primarily by the UK Natural Environment Research Council, and represents the outcome of many years of collaborations with colleagues from the Chilean Geological Survey, SERNAGEOMIN, with field work in the region supported by numerous colleagues and assistants, and with the support of CONAF and Reserva Huilo-Huilo.

References

K Fontijn et al., 2014, Late Quaternary tephrostratigraphy of southern Chile and Argentina, Quaternary Science Reviews 89, 70 – 84. [Open Access]

H Rawson et al., 2015, The frequency and magnitude of post-glacial explosive eruptions at Volcan Mocho-Choshuenco, southern Chile. Journal of Volcanology and Geothermal Research, doi:10.1016/j.jvolgeores.2015.04.003 [Open Access] Datasets available on figshare.

DM Pyle, 2015, Sizes of volcanic eruptions, Chapter 13 in Sigurdsson et al., eds, Encyclopedia of Volcanoes, 2nd edition, pp 257-264. doi:10.1016/B978-0-12-385938-9.00013-4

Friday Field Photos: the Southern Volcanic Zone of Chile

Friday Field Photos: the Southern Volcanic Zone of Chile

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.

Field report: Pumice

Field report: Pumice

One of the most rewarding parts of fieldwork on volcanoes is when the parts start to fit together, and hunches turn into firmer ideas. When piecing together ancient volcanic eruptions, the process often starts with the discovery of the trace of a new deposit in a road cut section. This might be something as simple as the appearance of a scruffy yellow or orange band that catches our eye as we pass by (as, for example, in the photo below from an earlier blog post).

Karen Fontijn and Harriet Rawson measure up an ancient pumice deposit - the orange layer - in a road cut.

Karen Fontijn and Harriet Rawson measure up an ancient pumice deposit – the orange layer – in a road cut.

A quick clean up of the surface with a trowel , and running the sample through your fingers, is usually enough to tell whether this really is a pumice deposit – and then the hunt is on. Where did it come from? How big was the eruption? How old is the deposit? Where does it fit into the record of eruptions we already know about?

We can usually get fairly good answers to most of these questions from the field, but to understand why we need to know a little bit about pumice. Everybody knows what pumice is: it’s that frothy rock that floats on water, and many people might even have a small lump of it in a corner of their bathroom. But to the volcanologist, pumice is the grail: a gobbet of magma, frozen in mid-flight that captures in its essence the story of the eruption: where the molten rock came from, how it matured before the eruption began, and perhaps even how the eruption started.

A block of pumice. This pumice sample had a chequered history even before it was erupted: it was broken apart while rising up through the plumbing system, and shortly afterwards healed up again, leaving the broken pieces of a pinkish pumice welded to the main white pumice.

A block of pumice. This pumice sample had a chequered history even before it was erupted. It broke apart, and then welded back together, as it rose up through the volcanic conduit towards the surface. The break separates the pink from the white areas. Coin is about 3 cm across.

Most explosive eruptions produce pumice, and it is this that gets carried high up into the atmosphere in the eruption plume and blown by the atmospheric winds, before raining down onto the surface below to form a pumice fall deposit.

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Pumice fall deposit, near volcan Mocho Choshuenco

These sorts of deposit are remarkably similar from volcano to volcano the world over. Typically, they are made up of angular fragments of white, cream, yellow or grey pumice that just pile on top of each other where they fell. Pumice blocks can range in size up to about 20 – 30 cm across, but are more usually centimetre to millimetre sized. And because these deposits are formed from strong eruption plumes – similar to that described by Pliny the Younger at Vesuvius in AD 79 – the pumice deposit covers the landscape like a carpet, gradually getting thinner and finer away from the volcano. We can use these changes in thickness and fragment size to work out the size and strength of the eruption; and we can also use the map of the deposit to work out not only where the eruption began, but also the wind direction at the time.

But we can also go further. Pumice is mainly volcanic glass: the molten rock, flash frozen as it erupted. This freezing process often catches the processes that actually cause the eruption in the act. Whether it is the expanding bubbles of volcanic gases that have caused the magma to froth in the first place,

Large bubble in a pumice block, formed as several smaller bubbles coalesced. Bubble is about 3 cm across.

Large bubble in a pumice block, formed as several smaller bubbles coalesced. Bubble is about 3 cm across.

or whether it is the droplet of once-hot basalt magma that might have stirred up the eruption in the first place.

Frozen droplet of basalt, on the edge of a large gas bubble, trapped within a larger block of rhyolite pumice. Did the arrival of some hot basalt into the rhyolite magma chamber cause the eruption?

Frozen droplet of dark-grey basalt, on the edge of a large gas bubble, trapped within a larger block of cream coloured rhyolite pumice. Did the arrival of some hot basalt into the rhyolite magma chamber cause the eruption? Bubble is 2 cm across.

The clues to both why an eruption happened, and its consequences at the time, can all be extracted from the buried remains of that ancient carpet of pumice.

After a day of successful pumice-hunting in the field here in Chile, we have returned to base camp (well, it’s actually a very well appointed wooden cabin) laden with samples, and many more questions to think about.

Field report: beware of the sealions

Beware of the sealions (‘cuidado lobos marinos’) declares the sign at Valdivia fish market, which stretches along the docks in this southern Chilean city.  And it’s no joke either, as a large clan of these creatures has set up stall in the estuary beneath the fish market, ready to feast on the daily pickings cast over the sea rail.

Valdivia fish market, with attentive sea gulls and sealions (not shown)

Valdivia fish market, with attentive cormorants and sealions (out of sight)

We are not here, however, to admire the sealife; or to see how the city has recovered from the devastation of past earthquakes. Instead, Valdivia is our gateway to the volcanoes of southern Chile; the chain of snow-capped mountains that adorn the landscape for over a thousand kilometers south from Santiago, like a trail of giant milk chocolate Hershey’s kisses.

Villarica, through the window glass

Villarica, through the window glass

For the past few years, we have been piecing together the trace of past eruptions from some of these volcanoes by scouring the landscape for layers of ash and pumice deposited long ago, and now buried deep in the soil. This time, the target is Mocho Choshuenco, a twin-peaked volcano that looms majestically over lakes Panguipulli and Rinihue.

Volcan Mocho Choshuenco, Chile. Choshuenco, to the left, is probably no longer active. Mocho, to the right, last erupted in 1864.

Volcan Mocho Choshuenco, Chile. Choshuenco, to the left, is probably no longer active. Mocho, to the right, last erupted in 1864.

Mocho, the younger part of the volcano, is last known to have erupted in 1864 and has shown no signs of activity since. This is not at all unusual for these volcanoes –which have lifetimes of hundreds of thousands of years, and where the intervals between eruptions can often stretch from decades to centuries. But it is the past activity of Mocho that we are looking for. Earlier work by colleagues from the Chilean geological survey suggests that there may have been two or three ‘large’ eruptions in the past 10,000 years, as well as numerous, undocumented smaller events. We plan to track down the deposits of these eruptions wherever we can find them – and this usually means buried deep within the rich soils of the temperate rain forests of the region. Rather than excavating pits ourselves,  we rely either on nature (erosion) or human activity to expose the pumice in road-side cuttings. A little bit like the sealions, perhaps, reliant on the pickings of the day.

Karen Fontijn and Harriet Rawson measure up an ancient pumice deposit - the orange layer - in a road cut.

Karen Fontijn and Harriet Rawson measure up an ancient pumice deposit – the orange layer – in a road cut.

This is only the first step in the detective work, because to identify which eruption the pumice has come from we need to collect samples for chemical analysis. In southern Chile, this is easier said than done – as the several metres of rain annually, and the warm summer temperatures, leave ten thousand year old pumices with the consistency of warm butter. Over the next few days of this field season, we will be completing the first stages of this work, and starting to put together a time-line of the past eruptions for which we can find a trace.