VolcanicDegassing

volcanoes

Into the Inferno: an anth(rop)ology of volcanoes

Into the Inferno: an anth(rop)ology of volcanoes

What do volcanoes mean to you? This is perhaps not a question to ask a volcanologist (cue: a paean to their current flame); but what do they mean to the publics? Fire and brimstone? Death and destruction? Of humans pitted against mountains? Or is it something else? Perhaps the answer is obvious, but it is certainly something we need to think about when preparing for an audience: what will they expect to find? and how can I surprise them?

It is a question that the volcanologist Clive Oppenheimer has pondered for some time. Five years ago his book ‘Eruptions that Shook the World‘ caught the attention of film director, Werner Herzog. They had already met earlier (on the Antarctic volcano, Erebus, of course); and have now released the fruits of their collaboration on Netflix.

Running at 1 hr 44 minutes, Into the Inferno is part anthology, part anthropology, and part conversation between volcanologist and film-maker. Taking in breathtaking footage of some of the volcanoes ‘du jour’, Herzog and Oppenheimer explore their own, rather different, attractions to volcanoes – as objects; as cultural symbols; as places incidental to the life and living that goes on around them.

Clive Oppenheimer introducing 'Into the Inferno' at the Cambridge Film Festival

Clive Oppenheimer introducing ‘Into the Inferno’ at the end of the 36th Cambridge Film Festival

For Herzog, volcanoes are not the main attraction. Sitting on the rim of Erebus, the world’s most southerly volcano, it is almost as if he couldn’t care less about the fuming crater below. It’s the people, and their stories that he wants to capture.  So it is that the film takes us from the roiling chasm of the crater of Mt Yasur on Tanna, to the verdant slopes of Ambrym: devastated in a hurricane just the year before, but with the scars hidden by the new tropical growth. Here, Chief Mael Moses explains how the volcano is a part of the neighbourhood, but not a part of their lives. Later, he relates a story of how, glimpsing into the crater, he explains how the swirling fires looked like the tumbling waters of the sea; and of what this implies for the future.

Exploring the origins of the film takes Werner Herzog and crew back to Erebus; while the origins of Clive Oppenheimer’s relationships with volcanoes takes him to Toba, Sinabung and Merapi: into an observatory bunker room, stocked with a month’s supplies of food, and a Church in the shape of a chicken; or is it a dove? The search for human origins and volcanoes takes us to the Danakil desert of Afar, Ethiopia; where Tim White has just found the skeleton of another early human. Entombed in the ashes a hundred thousand years ago, and exhumed as the visitors arrive. The origin tale takes us to the Codex Regius and Iceland, where we hear the stories of eruptions of Heimay, Eyjafjallajokull and the great Laki fires of 1783. En route, newsreel of past eruptions are spliced with some footage from the peerless volcano filmmakers, Maurice and Katia Krafft. Katia sampling, as surging rolls of lava surf in a channel, and a moment of Chaplin-esqe fun as a foil-coated Maurice toddles towards a wall of fire. Footage of the hot rock avalanche and surging pyroclastic cloud that brought their lives, and the lives of 41 others, to an abrupt end in Japan in 1991 creates a pause for reflection.

To conclude his curious tales of people around volcanoes, Herzog takes us to the mysterious and closed world of DPRK (North Korea). This is a country under international sanction, with rhetoric that is throwback to the last century. Its northern border is anchored by the ‘long white mountain’, the volcano known as Paektu-san in Korea, and Changbaishan in China. This volcano looms large in the national psyche, with links to the nation’s origins, and those of the present regime; and where the Paektu-san song is almost a national anthem.

Volcanoes as icons: Mt Paektu, on the DPRK (North Korea) - Chinese border

Volcanoes as icons: Mt Paektu, on the DPRK (North Korea) – Chinese border

This is a delightful film that shows off volcanoes and their context in a quirky and entertaining way. In a cinema setting, the mesmerising footage and soundscape pull you right in to the crater. But ‘Into the Inferno’ is about much more than fire and brimstone; it gives voices to the people who live, work on or are drawn to volcanoes, revealing in their own words what it is, or is not, that volcanoes symbolise for them, and their kin.

 

The smallest volcanic island in the world?

The smallest volcanic island in the world?
Oshima Ko Jima on May 4, 1805

Oshima Ko Jima on May 4, 1805. Sketch by Wilhelm Tilesius.

One of the delights of talking to children of primary school age is their disarming ability to ask really simple questions that demand straightforward answers, but leave you struggling to throw your academic caution to the wind. Even with the questions of the biggest, the smallest, the oldest and the youngest there are still different ways of (over)interpreting the question, that can leave you floundering. So for those of you looking for a clear answer to ‘what is the smallest volcanic island in the world?’, here is a suggestion of an answer from the early 19th Century. In 1805, Wilhelm Gottlieb Tilesius was engaged as the physician, naturalist and draftsman on the Russian ship Nadezhda, on the first Russian circumnavigation of the world. In May 1805, they were sailing across the sea of Japan and past the Matsumae peninsula in northern Hokkaido, heading for Kamchatka, when they came across two small volcanic islands ‘Oosima and Coosima’. Tilesius described Coosima (Oshima Ko Jima) as ‘perhaps the most diminutive volcanic island in the world’, noting that it was essentially a small pointed rock, which was incessantly smoking. The island was just ‘150 fathoms’, or 270 m, tall; lacking in vegetation and made of dark blue lavas with a weatherbeaten dark-red skirt around the base. Tilesius made all of his observations from on-board ship, as it took a quick tour around the island, and describes ‘light coloured smoke’ and ‘blue sulphur flame’ from the summit crater. Tilesius’ paper suggesting that this was the world’s smallest volcanic island soon found it’s way into Charles Daubeny‘s ‘Active and Extinct Volcanoes‘, and shortly afterwards in a number of popular periodicals of the time. Whether – with an area of 1.5 square kilometers – it really is the world’s smallest volcanic island; and whether it is actually ‘active’ are both moot points; in fact, so little seems to be known about the island that the Smithsonian catalogue still lacks a photograph of it.

Dr W Tilesius, 1820, On the Volcano called by the Japanese Coosima and situated in the neighbourhood of Cape Sangar, in the archipelago of Japan, Edinburgh Philosophical Journal, Vol III, no 6, October 1820, pp. 349-358.

William Dampier and the Burning Islands of Melanesia

William Dampier and the Burning Islands of Melanesia

A tweet from Jenni Barclay about a Pirate Scientist gave me an excuse to visit the newly opened reading rooms in the Bodleian’s Weston Library..

William Dampier was a seventeenth century pirate, and later maritime adventurer, whose several books of ‘Voyages and Discoveries’ make for fascinating reading. In 1699, he set sail in HMS Roebuck to try and find Terra Australis, a mythical ‘southern continent’. His journey took him past the Cape of Good Hope to the north-western coast of New Holland (now Australia), and then on to Timor, New Guinea, and New Britain. On the way back, he only got as far as Ascension island, in the Atlantic, before his ship sprung a leak and had to be abandoned. His crew were rescued some weeks later by a party of British naval ships on their way to Barbados.

In the preface to his 1703 book ‘A Voyage to New Holland..’ he writes ‘The world is apt to judge of everything by the success, and whoever had ill fortune will hardly be allowed a good name. This, my Lord, was my unhappiness in my late expedition in the Roebuck, which foundered thro’ perfact age near the island of Ascension. I suffered extreamly in my reputation by that misfortune..’

Dampier 1703 preface

Part of the preface to Dampier’s ‘Voyage to New Holland’.  Bodleian ms. 8 S 43 Jur

Despite his pessimism over the loss of the ship, and his failure to reach Terra Australis, the expedition was a great success as a ‘voyage of discovery‘. Dampier’s book has some lovely descriptions of active volcanoes that he encountered to the east of Timor island – part of what we now call the Banda Arc – and of others along the north coast of (Papua) New Guinea, part of what is now known as the Bismarck Arc. Several of his reports of volcanoes that he encountered are the earliest written records of activity.

Banda

Banda island volcanoes – likely Wurlali and Banda Api. ‘No 6. A burning island to the East of Timor shows thus. Distance 4 leagues.’ ‘No 7 Thus shows 2 of the Bandy islands. Distance 12 leagues.’          1 league is 3 nautical miles.  Bodleian ms. 8 S 43 Jur

 

On 13 Dec 1699, the Roebuck sailed from Babao, east past Timor towards New Guinea.  On December 27th ‘ we saw the burning island.. It lies in lat 6 deg 36 minutes. It is high but small. [The top] is divided in the middle into two peaks,  between which issued out much smoak’. This is most likely Wurlali volcano,  on Damar island, Indonesia. Later they sailed past another burning island, most likely Banda Api. Then around the northern coast of present day Papua New Guinea, they encountered several volcanoes of the New Guinea arc. Wally Johnson has interpreted this portion of Dampier’s trip in his recent book  Fire Mountains of the Islands, and identified several of Dampier’s unnamed ‘burning islands’.


PNG volcanoes

Two Burning Islands offshore from the northern coast of Papua New Guinea – most likely Kadovar (to the north), and Karkar (to the south). Bodleian ms. 8 S 43 Jur

 

On the 24 and 25th March 1700, Dampier spent some time watching an eruption, now thought to be of Ritter Volcano which lies in the straits between Papua New Guinea and New Britain

Dampier passage

Dampier’s passage between New Guinea and New Britain. The erupting volcano (far right) is Ritter island. Bodleian ms 8 S 43 Jur

At ten o clock I saw a great fire, blazing up in a pillar, sometimes very high for three or four minutes..  In the morning I found out that the fire we had seen was a burning island, and steered for it.’ ‘March 25th the Island all night vomited fire and smoak very amazingly and at every belch we heard a dreadful noise like thunder and saw a flame after it. The intervals between its belches were about half a minute. Some more, some less, neither were these pulses of eruptions alike, for some were but faint convulsions in comparison of the more vigorous yet even the weakest vented a great deal of fire, but the largest made a roaring noise and sent up a large flame 20 or 30 yards high, and then might be seen a great stream of fire running down to the shore‘.

Little more is known of Ritter volcano until 1888, when it collapsed catastrophically in a non-eruptive submarine avalanche.

Further reading.

There is a wonderful interactive map of Dampiers voyage in Google Maps. Wally Johnson’s Fire Mountains of the Islands (ANU Press, 2013 – available online) is a splendid resource on the volcanoes of Papua New Guinea and the Solomon islands, and their eruptive histories. There is a also a short piece on the eruption history of Ritter island in the Cooke-Ravian Volume of Volcanological Papers (editor, R. W. Johnson), Geological Survey Of Papua New Guinea Memoir 10, 115-123 (1981).

Original source: William Dampier, A voyage to New Holland &c. in the year 1699. 3 volumes. 1703. [Several e-versions are available].

Volcanoes Under the Ice

The ice-filled summit crater of volcan Sollipulli, Chile.

The ice-filled summit crater of volcan Sollipulli, Chile.

A fascinating story has emerged this week from a paper in Nature Geoscience by Amanda Lough and co-workers (Lough et al., 2013), on the discovery of a new volcano deep beneath the ice of the Western Antarctic Ice Sheet (WAIS).  The discovery is partly a story of scientists looking in a place where no-one had looked before; this case, using a network of seismometers, as a part of POLENET/ANET – a polar earth observing network. The instruments were laid out across a section of Western Antarctica, which is known to contain some large and relatively young volcanoes. The surprise from the data was the discovery of a deep source of seismic activity (earthquakes), in a location that didn’t match any of the existing volcanoes. The authors were able to show that the earthquake signals look like those from volcanoes, and not like those made from breaking rock or creaking ice; and they were also clustered into small swarms, both in time and space. All of this evidence points to the idea that these earthquakes relate to the movement of magma deep in the crust beneath a small volcano that was not previously known, and which hasn’t yet emerged above the ice. This is fascinating ‘discovery’ science, and poses all sorts of interesting questions about how and why the volcanoes in this part of Antarctica exist in the first place – a topic where little is known, but where there is a very timely recent paper by Wesley LeMasurier.  Of course, much of the press coverage has focussed on speculation about whether, or not, there might be calamitious effects for the West Antarctic Ice sheet in the case of an eruption (The short answer is no!).

One broader reason for my interest in the paper is that it links to the long-standing question of what happens when large volumes of ice are removed from volcanoes? On Earth, the main example of this relates to the end of the last glaciation, which began between about 18,000 and 20,000 years ago. The current hypothesis, which has been around for some time, goes like this. During the peak of glaciation, vast areas of the land surface can be buried under great thicknesses of ice – perhaps as much as hundreds of metres to kilometres. Then, during deglaciation, the ice retreats rapidly reducing the pressure on the rocks beneath. So far so good, but how could taking away some ice have any effect on volcanoes? One clue to the answer may come from Iceland. This is a part of the world where the mantle is hot enough at shallow enough depths, that there is a column of partially-molten rock that probably extends from about 40 km depth down to about 100 km depth. Here, where the rock is above its melting point, Charlie Langmuir and Dan McKenzie have shown that that amount of melt increases by about 1% for a pressure drop of 100 MPa. So removal of a 1000 m thick column of ice (at an Icelandic volcano) will translate into a pressure drop of 10 MPa, and an increase of melt fraction of 0.1% along the length of the 60 km melt column. This is to a change in the amount of melt produced of 60 m. This might not seem like much – but when you consider the full size of the melting region in the mantle, this could correspond to a considerable volume of melt produced simply as a result of ice removal. In Iceland, there is geological evidence for a pulse of eruptive volcanism that occurred very shortly after deglaciation – around 11,000 years ago. 

Moving back to Antarctica, the current idea is that the volcanoes beneath West Antarctica might also be fed from a plume, similar to that beneath Iceland and the Canary islands. So perhaps it is plausible that a similar explanation might also apply in Antarctica, and there might be a feedback on geological timescales between large-scale mass changes in the ice sheet, and the magma supply from depth? What is not clear, yet, is whether this same mechanism applies to the volcanoes that form above subduction zones. Here the melting process is a little different, driven primarily by the arrival of water into the mantle. So it is not clear how pressure changes due to changing ice load might lead to changes in melt production deeper down. It is also not yet clear, from the geological evidence, whether or not there were changes in rates of eruptions at the subduction zone volcanoes that were once covered with ice. This remains a work in progress!

References

Lough, AC et al., 2013, Seismic detection of an active subglacial magmatic complex in Marie Byrd Land, Antarctica,  Nature Geoscience (Advanced Online Publication, 2013/11/17/) http://dx.doi.org/10.1038/ngeo1992

Watt, SFL et al., 2013, The volcanic response to deglaciation: Evidence from glaciated arcs and a reassessment of global eruption records, Earth-Science Reviews, 122, July 2013, Pages 77-102, http://dx.doi.org/10.1016/j.earscirev.2013.03.007

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.

IMG_6868

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

IMG_6877

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

IMG_6887

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

IMG_6900

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.

IMG_6893

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

IMG_6917

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.

IMG_6939

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.

IMG_6972

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: 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.

Polygons, columns and joints

Over on her Georney‘s blog, Evelyn Mervine has recently posted a nice piece with some spectacular images of columnar jointing. This seemed like a good opportunity to dust off some field photos, with some more examples of polygonal joint sets in lavas from a variety of settings, to illustrate the diversity of forms that cooling-contraction joints may take in volcanic rocks.

The first example is a late Pleistocene lava flow from the Afar, Ethiopia.

Image

Berihu Abadi, with an example of columnar jointing in a late Pleistocene basalt, Afar, Ethiopia, exposed in the face of recently-opened fracture. Fieldwork carried out as part of the NERC-funded Afar consortium.

Young lavas in the Afar are predominantly fissure-fed basalts, erupted across the topography and air-cooled. Columnar jointing is pervasive in the upper surfaces of these young lava flows, and seems to develop immediately beneath the surficial glassy and vesicular crust that forms on the top surfaces of the modern lava flows.

Image

Uppermost surface of a fresh basaltic lava flow, Afar, Ethiopia. Field of view is about 30 cm. Beneath the surface layer formed by the scales of glassy, vesicular lava, the lava is weakly polygonally-jointed (not visible in this picture). Fieldwork carried out as part of the NERC-funded Afar consortium.

Moving across to Europe, and southern Spain. Here, in the Cabo de Gata, there are some fabulous examples of polygonally-jointed andesitic lava domes, emplaced in a shallow submarine environment. These are Miocene in age, and formed in a transient volcanic arc, which has now been faulted against the Spanish mainland.

At Playa Monsul, polygonally-jointed dykes can be found criss-crossing a fabulous series of hyaloclastite bodies. Monsul is also well known as the location in Indiana Jones and the Last Crusade, where Sean Connery fends off an aerial attack with the help of an umbrella and a flock of seagulls.

Image

Polygonal jointing in the vertical face of a dyke, intruding a submarine sequence of wet sediment and hyaloclastite lavas. Polygons are typically 10-20 cm across. Photograph taken on a University of Oxford undergraduate field trip.

The southern-most cape of the Cabo de Gata has a wealth of collonaded andesites; presumably submarine domes.

Image

Columnar joints in an andesite ‘dome’ of Miocene age, southernmost Cabo de Gata, Spain. This locality offers some spectacular views both of polygonal joints in planform, and also large-scale structures showing the fanning of columns that reveal the orientations of the original cooling surfaces. Location visited on an undergraduate field trip.

The final examples of polygonal jointing come from higher (southerly) latitudes, and higher elevations, where jointing has developed as a result of ice- or snow-contact volcanism. The first example is from the flanks of volcano Osorno, Chile, where characteristic hackly jointing has developed on the outer margins of an andesitic lava flow. This differs from the regular pattern typical of columnar jointing, and is thought to be one characteristic of rapid quenching of lava.

Image

Jose Naranjo, SERNAGEOMIN, and the hackly-jointed lavas from volcan Osorno, Chile. Fieldwork carried out as part of a NERC-funded project by Sebastian Watt.

A little further South, again in Chile, and here is another classic example of a small sub-glacial ridge of andesite, this time near the summit of volcan Apagado, on the Hualaihue peninsula.

Image

Complex polygonal jointing in a ridge of andesite, from the margins of volcan Apagado, Chile. Fieldwork carried out as a part of a NERC-funded PhD project by Sebastian Watt.

A final example is from volcan Sollipulli, a little further North in the Chilean Lake District. This example is of a dyke, thought to have intruded along the contact between a glacier and the volcanic edifice, near the present-day crater edge. Here, the portion of the dyke that was originally in contact with the ice has developed a very strong platy fabric, and is falling apart to form a scree of what looks like slate.

Image

Platy jointing developed in the margins of an originally ice-bounded dyke, volcan Sollipulli, Chile. View from above, looking down the dyke margin. Platy scree now partially fills the channel which was once filled with ice. View – about 1 m across. Fieldwork was carried out as a part of a IAVCEI-funded project on the hazards of snow-capped volcanoes.

While there is general agreement that these patterns of jointing form because of the contraction of magma as it cools, there is not yet a concensus model for what it is that controls the size of the polygonal joints, or the typical number of sides. Theoretically, it is argued that a hexagonal jointing pattern would be the lowest-energy, and favoured, solution. But in reality, cooling rates might be too fast for full energy minimisation, and this might explain why many polygonal joints have fewer than six sides. This is the conclusion of the most complete study to date, in which Gyoergy Hetenyi and colleagues argue that field evidence points to two major controls being the size of the cooling body, which influences how fast it cools, and composition, which influences the physical properties of the cooling magma.

Chilean volcanoes: shaken, but not always stirred?

November 7th marked the 175th anniversary of one of the largest earthquakes to have struck northern Patagonia. The earthquake, which is estimated to have had a magnitude of 8, had an epicentre close to Valdivia, and was accompanied by significant ground shaking and subsidence as far south as Chiloe island, and a major tsunami that reached Hawaii.  The eyewitness reports of the time have been well documented. From a geological perspective, the key feature of the 1837 earthquake is that it occurred along a section of the plate boundary that has ruptured repeatedly, with great earthquakes in 1575, 1737 and, most significantly, in 1960  – which, with a magnitude of 9.5, is still the largest recorded earthquake globally. The 1837 earthquake struck just two years after the great Concepcion earthquake, of February 1835, which was exquisitely documented by Charles Darwin, among others.  Because of the location, adjacent to the long southern Chilean volcanic arc, and the frequency of large earthquakes in this region, both the 1835 and 1837 earthquakes have become critical pieces of evidence for the ongoing question of whether, and how, large earthquakes might lead to small triggered volcanic eruptions. Historical records from the region that include maps, expedition reports and navigational charts mean that the record of past eruptions in the southern parts of the central valley of Chile extend back into the 16th century and the earliest Spanish colonists.

Osorno map view from 1747

Map of southern Chile, and the northern part of Chiloe island extracted from ‘A new and accurate map of Chili, Terra Magellanica, Terra del Fuego etc.’ compiled by Emanuel Bowen in 1747. Active volcanoes of Osorno (vul. of Osorno) and, probably, Hornopiren (vul. of Quechucabi) are shown. From the Bodleian libraries, University of Oxford.

The 1837 Valdivia earthquake was followed by reported eruptions, on the same date, both at volcan Osorno, and Villarrica. Both volcanoes had already been in a state of activity on and off in the months or years prior to the earthquake, and both have long historical records of activity, so the observations are not necessarily surprising. One of the challenges of testing for cause and effect when it comes to possible earthquake-triggered eruptions is the likelihood of false reporting that arises from the natural tendency of people to conflate all sorts of observations and speculations in the aftermath of major events, like earthquakes. For example, both Villarrica and Osorno have also been recorded as having erupted shortly after the great earthquake of 1575, but neither observation is necessarily secure.

Volcan Osorno, overlooking Lago Llanquihue, Chile

Volcan Osorno, overlooking Lago Llanquihue, Chile

In contrast to the reports from 1837, the 1960 earthquake did not appear to have any major consequences for either system. Osorno has now been dormant since the late 19th century, while activity at Villarrica has rumbled on into the 21 st century.

Volcan Villarica, map view from 1759

Map of volcan Villarrica, from 1759. Reproduced in ‘Cartografia hispano colonial de Chile’, published in 1952 to mark the centenary of the birth of Don Jose T Medina. From the Bodleian libraries, University of Oxford.

Volcan Villarrica

Volcan Villarrica, with steam plume, viewed from Pucon.

Work is still in progress to investigate the consequences of the most recent great earthquake in the region: the Maule earthquake of February 2010. It remains possible that reported changes in activity at Villarrica in March 2010, seen in thermal infra-red satellite imagery, and subsequent eruptions of Planchon-Peteroa and Puyehue – Cordon Caulle may ultimately be linked to the rejuvenating effects of the earthquake, but this remains to be properly tested.

Of course, this is a question that is mainly of academic interest (in terms of understanding how eruptions are triggered), since most of the eruptions documented to have occurred in the immediate aftermath of great earthquakes are very small, and are most likely to occur at systems which have already been in eruption. The consequences of these eruptions are usually negligible, compared to the effects of the large earthquakes themselves. In recognition of the frequency of these potentially devastating earthquakes, the Chilean authorities (ONEMI, Oficina Nacional de Emergencia) are today holding an earthquake simulation across the schools of Santiago, as part of the programme of national preparedness for future emergencies.