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Geochemistry, Mineralogy, Petrology & Volcanology

Great walls of fire – Vitrification and thermal engineering in the British Iron Age

It’s long been recognised the peoples of European prehistory occasionally, and quite deliberately, melted the rocks from which their hilltop enclosures were made. But why did they do it? In today’s blog post Fabian Wadsworth and Rebecca Hearne explore this question.

Burning questions

Throughout the European Bronze and Iron Ages (spanning 2600 years from 3200 BC to 600 BC), people constructed stone-built, hilltop enclosures. In some cases, these stone walls were burned at high temperatures sufficient to partially melt them. These once-molten forts are called vitrified forts because today they preserve large amounts of glassy rock. First described in full in 1777, the origins and functions of these enigmatic features have been the subjects of centuries of debate.

Today, researchers generally agree that the glassy wall rocks are the result of in situ exposure to high temperatures in prehistory, similar in magnitude to those temperatures found in volcanoes on Earth. This was sufficient to partially or wholly melt the stonework, and the resulting melts are preserved as glass upon cooling.

There are still many outstanding questions concerning vitrified enclosures and forts, but the most immediate and arresting are: how and why were they burned?

Vitrified fort walls are mostly found in Scotland and are built from a diverse range of rock types

Vitrified enclosures occur throughout Europe but the best known examples are found in Scotland. Using the compilation created by Sanderson and co-workers (link provided below) we can map the distribution of forts, categorized by the rock-type from which they were built, and compare this with a simplified geological map of Scotland (from the British Geological Survey; Image 1). This shows that the building stone used in fort walls is not always the same and was more likely to be found locally.

Map of Scotland with simplified basement geology and cover-sediments marked. Vitrified fort positions are numbered such that 1- Finavon, 2- Craig Marloch Wood, 3- Tap O’North, 4- Dun Deardail, 5- Dunagoil, 6- Craig Phaidrig, 7- Laws of Monifieth, 8- Knockfarrell, 9- Dunskeig, 10-Dumbarton Rock, 11- Carradale, 12-Dun MacUisnichan, 13- Art Dun, 14- Mullach, 15- Trudernish Point, 16-Cumbrae, 17- Dun Lagaidh, 18- Sheep Hill, 19-Urquhart Castle, 20- Eilan-nan-Gobhar, 21- Eilan nan Ghoil, 22- Duntroon, 23- Torr Duin, 24- Trusty’s Hill, 25- Doon of May, 26- Castle Finlay, 27- Mote of Mark. (From the British Geological Survey).

Map of Scotland with simplified basement geology and cover-sediments marked. Vitrified fort positions are numbered such that 1- Finavon, 2- Craig Marloch Wood, 3- Tap O’North, 4- Dun Deardail, 5- Dunagoil, 6- Craig Phaidrig, 7- Laws of Monifieth, 8- Knockfarrell, 9- Dunskeig, 10-Dumbarton Rock, 11- Carradale, 12-Dun MacUisnichan, 13- Art Dun, 14- Mullach, 15- Trudernish Point, 16-Cumbrae, 17- Dun Lagaidh, 18- Sheep Hill, 19-Urquhart Castle, 20- Eilan-nan-Gobhar, 21- Eilan nan Ghoil, 22- Duntroon, 23- Torr Duin, 24- Trusty’s Hill, 25- Doon of May, 26- Castle Finlay, 27- Mote of Mark. (From the British Geological Survey).

How hot? How long?

A key question surrounding the melting and glass-formation processes that occur to form vitrified fort walls is what temperature was required and how long must the fires have burned? As we know, the required minimum temperature for melting rocks is the solidus, which varies by hundreds of degrees from rock-type to rock-type. Above the solidus, the partial melt fraction will increase until the material liquidus, above which all components of the rock are molten. In mineralogically diverse rocks, the partial melt fraction between the solidus and liquidus not only increases, but changes composition. Pioneering investigations of vitrified fort wall materials (for example, by Youngblood and co-workers) and others have explored this by looking at the composition of the glass that is formed when the partially molten fort walls are cooled. The composition of these materials and an understanding of the thermodynamics of the melting process yields temperatures of the prehistoric fires in question. Youngblood and colleagues found that temperatures were likely to be, on average, 900-1150 ºC.

In a recently published study, we used a different technique in an attempt to answer the same question. Samples of fort walls from Wincobank vitrified hillfort, Sheffield, UK, were used in high-temperature tests to measure the melting process in situ. We measured the amount of each mineral phase as it decreased above the solidus. This technique allowed us to extract additional information that previous investigators have not been able to probe. As well as suggesting a temperature window within which melting occurs, we were able to find the timescale of burning required to achieve the degrees of partial melting seen within Wincobank’s vitrified enclosure wall. Wincobank was constructed from a local sandstone; we found that the quartz in this sandstone was steadily removed upon experimental heating, matching the final quartz content of the enclosure wall rocks at a temperature window of 1050-1250 ºC for burning events of more than 10 hours.

When taken together, such investigations of the conditions required to form glass in prehistoric enclosure walls can more reliably inform the debate about why the fires were set in the first place. If glass is consistently found around a fort’s circumference (as is often the case, particularly in Scotland), and its particular building stone type dictates a heating event requiring a duration of 10 hours or more at peak temperature, it seems unlikely to us that such walls were burned accidentally or during periods of conflict or events of warfare (see below). In that case, if the enclosure walls were burned deliberately by the occupants, the outstanding question remains: why?

Why set fire to a stone wall?

In any archaeological investigation, a key goal is to attempt to explore and extrapolate the beliefs, motives, and desires of people in antiquity from their material culture. For practitioners of any discipline this is no easy task, with subjective interpretation of evidence and difference of opinion often resulting in vibrant discussion!

Consequently, numerous possible prehistoric motives for burning a fort wall to the point of melting have been posited. First is the possibility that the fires were lit during enemy attack or some other act of violence (mentioned above). Second, there’s the possibility that the fires were a product of deconstruction of the fort walls at the end of occupancy. Third, the conflagration was part of a ritual or display of prestige. Finally, there is the possibility that the fires were set with the intention of strengthening the stonework during construction. Each of these explanations has received attention; however, the last option had, until recently, been dismissed by previous researchers as largely unlikely, once it was recognised that heating rocks in general weakens them by the proliferation of microcracks from thermal stresses.

We revisited the idea that fort walls could have been burned during the construction in order to strengthen them. In our latest work we explored this in a simple way by showing that while the blocks in a fort wall will get weaker during high temperature burning, the more fine-grained rubble interstitial to these blocks will get much stronger. This strengthening occurs simply because the fine grained materials between larger blocks can fuse together by sintering when they are partially molten. And indeed, it is so often reported that large blocks are surrounded by a glassy mass that is fused to them (we show this in Image 2 from Wincobank fort, Sheffield, U.K.). We pointed out that this is contrary to the conventional view that fort walls must be weakened by the fires.

A block from the Wincobank enclosure wall in Sheffield, UK. This piece shows the typical feature where fine grained glassy material is welded to the larger, less altered blocks. In detail, this demonstrates that thermal gradients resulting from heating blocks of different sizes play an important role in determining which blocks melt and weld, and which blocks do not. (Credit: Fabian Wadsworth)

A block from the Wincobank enclosure wall in Sheffield, UK. This piece shows the typical feature where fine grained glassy material is welded to the larger, less altered blocks. In detail, this demonstrates that thermal gradients resulting from heating blocks of different sizes play an important role in determining which blocks melt and weld, and which blocks do not. (Credit: Fabian Wadsworth)

The debate rages on

We acknowledge that the strengthening effect does not rule out other motives. Indeed, the strengthening may be incidental to the true motive for the wall burning. It is also important to take into account the fact that, in many of the known examples of vitrified enclosures, where dated, the burning event takes place, in some cases, many hundreds of years after the fort’s initial construction.

A little-discussed possibility which is gaining momentum is that some of these forts may not have been forts at all. The very term “fort” is loaded and implies inherent military purpose, which remains a hypothesis with little solid evidence to its claim. Rather, they may have been monuments that were built and burned as displays of power and prestige or in some ritual event. In periods of our history where only the stones upon which we can base our suppositions remain, it is difficult to differentiate between these possibilities.

These acknowledgements highlight not only that there are outstanding questions requiring future investigation, but that each fort is different and there need not be a common explanation for them all.

The debate continues and, although the evidence sheds new light on the possible truths, we still do not know why Iron Age peoples throughout Europe set fire to stone enclosures and stoked those fires to volcanic temperatures. Rocks melt and crystallize and re-melt in volcanoes frequently and as a matter of natural process. To combine our understanding of these rock-forming materials and Earth processes as they are melted in anthropogenic conflagrations is essential to understand these curiosities of our Iron Age.

By Fabian Wadsworth, PhD student Ludwig Maximilian University of Munich, and Rebecca Hearne, Department of Archaeology, University of Sheffield

The Wincobank hillfort is in Sheffield in South Yorkshire, U.K. If the stone walls of its  enclosure were deliberately burned   this feature potentially extends Sheffield’s heritage of high temperature expertise – exemplified by the once-prolific steel works of the city – much farther into the region’s past than hitherto imagined.

Further reading

Imaggeo on Mondays: Moving images – Photo Contest 2016

Since 2010, the European Geosciences Union (EGU) has been holding an annual photo competition and exhibit in association with its General Assembly and with Imaggeo – the EGU’s open access image repository.

In addition to the still photographs, imaggeo also accepts moving images – short videos – which are also a part of the annual photo contest. However, 20 or more images have to be submitted to the moving image competition for an award to be granted by the judges.

This year saw seven interesting, beautiful and informative moving images entered into the competition. Despite the entries not meeting the required number of submissions for the best moving image prize to be awarded, three were highly ranked by the photo contest judges. We showcase them in today’s imaggeo on Mondays post and hope they serves as inspiration to encourage you to take short clips for submission to the imaggeo database in the future!


Aerial footage of an explosion at Santiaguito volcano, Guatemala. Credit: Felix von Aulock (distributed via imaggeo.egu.eu)

During a flight over the Caliente dome of Santiaguito volcano to collect images for photogrammetry, this explosion happened. At this distance, you can clearly see the faults along which the explosion initiates, although the little unmanned aerial vehicle is shaken quite a bit by the blast.


Undulatus asperitus clouds over Disko Bay, West Greenland. Credit: Laurence Dyke(distributed via imaggeo.egu.eu)

Timelapse video of Undulatus asperitus clouds over Disko Bay, West Greenland. This rare formation appeared in mid-August at the tail end of a large storm system that brought strong winds and exceptional rainfall. The texture of the cloud base is caused by turbulence as the storm passed over the Greenland Ice Sheet. The status of Undulatus asperitus is currently being reviewed by the World Meteorological Organisation. If accepted, it will be the first new cloud type since 1951. Camera and settings: Sony PMW-EX1, interval recording mode, 1 fps, 1080p. Music: Tycho – A Walk.

Lahar front at Semeru volcano, Indonesia. Credit: Franck Lavigne (distributed via imaggeo.egu.eu)

Progression of the 19 January 2002 lahar front in the Curah Lengkong river, Semeru volcano, Indonesia. Channel is 25 m across. For further information, please contact me (franck.lavigne@univ-paris1.fr)

 

Imaggeo on Mondays: Half dome at sunset

Imaggeo on Mondays: Half dome at sunset

Yosemite’s Half Dome stands, majestic, over a granite dominated terrain in the Yosemite Valley area;  one of the most beautiful landscapes in northern America, and arguably, the world – it is also an Earth scientist’ playground.

Stamped into the west slope of the Sierra Nevada range, the Yosemite Valley is a collection of lush forests, deep valleys, meandering rivers and streams, all punctuated by huge domes and cliffs of ancient volcanic origin.

Come and explore this part of the world and you’ll not miss Half Dome. Standing at the head of the valley, the quartz monzonite (a coarse grained orthoclase and plagioclase feldspar dominated rock) structure rises a little short of 2700 m above sea level.

Despite standing proud in the present landscape, it was once a magma chamber, buried deep below a volcano. Over a long period of time, the molten magma cooled and crystalised to form the coarse granite rock we see today. Erosion and exposure did the rest, eventually exhuming the dome and cutting deep valleys into the surrounding landscapes.

For more information on the geology of the Yosemite Valley and Half Dome, please refer to these United States Geological Survey (USGS) resources:

The Geological Story of Yosemite Valley
How did Half Dome, acquire its unique shape?
Bedrock Geology of the Yosemite Valley Area

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

 

 

 

 

GeoSciences Column: Hazagora – will you survive the next disaster?

GeoSciences Column: Hazagora – will you survive the next disaster?

There is no better thing, on a cold and stormy winter’s evening, than to gather your friends for a night of games / board games. Fire blazing (if you have one), tasty snacks laid out and drinks poured, you are all set to indulge in a night of scheming (if you are playing battle ship), deceit (Cluedo), or even all out comedy (think Pictionary or Charades).

The main purpose of the games you are likely to enjoy, in the relaxed setting described above and in the company of your nearest and dearest, is to entertain. You might not be aware that in playing board games you are also boosting your cognitive, decision-making and social skills. Serious games exploit this notion in order to support learning and raise awareness of important issues, as Dr. Mirjam S. Glessmer previously wrote about in our GeoEd column. With this in mind, could a board game be used to raise awareness about the complexities of geohazards and disaster risk reduction management?

A team of Belgian researchers set out to test the idea by developing Hazagora: will you survive the next disaster? Its effectiveness as an educational tool, both for those living in disaster prone areas, as well as stakeholder and scientists involved in risk management activities, is discussed in a paper recently published in the EGU’s open access journal Natural Hazards and Earth System Sciences.

Playing the game

The game is set on an island, with a central volcano surrounded by forests, agricultural lands and coastal areas. Immerse yourself in the game and you’ll have the option to embody one of five characters: the mayor, the fisherman, the lumberjack, the farmer and the tour guide.  Potential locations where players can settle, with their families, road networks and wells to provide water supply, are drawn on the board game. The board is divided into different sectors which can be affected by a geohazard. The game is led by a game master, bound to follow the Hazagora guidelines.

 Setup of the game: (a) board game; (b) character cards with from left to right: the mayor, the fisherman, the lumberjack, the farmer and the tour guide; (c) resource cards: bread, water and bricks; (d) resource dice; (e) water well and food market; (f) hut (one chip with one family), house (two chips with two families), and road; (g) cost information card for building new streets, huts, and houses and buying protection cards. Taken from Mossoux, S., et al. (2016).

Setup of the game: (a) board game; (b) character cards with from left to right: the mayor, the fisherman, the lumberjack, the farmer and the tour guide; (c) resource cards: bread, water and bricks; (d) resource dice; (e) water well and food market; (f) hut (one chip with one family), house (two chips with two families), and road; (g) cost information card for building new streets, huts, and houses and buying protection cards. Taken from Mossoux, S., et al. (2016).

The outcome of a natural disaster, contrary to common reporting in the media and popular belief, is not exclusively controlled by the force of the natural hazard. The livelihood profile of each of the characters in the game is specifically chosen to highlight the important role economic, social, physical and environmental circumstances play in shaping how individuals and nations are affected by geohazards. A fisherman will inevitably be limited in his choice of settlement location, as he/she is bound to live close to the coast, while at the same time his/her occupation controls its income. On a larger scale, political and socioeconomic factors mean that victims of natural hazards in developing countries, especially Asia and Africa, are more vulnerable to geohazards when compared to residents of developed nations.

Life on the island unfolds in years, with players establishing his/her family on the land by providing shelter, bread (food) and water. Income is received each round table and can be used to a) provide for the family or b) invest in further developing their settlement by adding more housing for extra families. At any given time, and without warning, the game master can introduce a natural hazard (earthquake, tsunami, lava flow, ash fall). All players watch a video clip which illustrates the hazard and outlines the impacts based on recent disasters. The players then discuss the potential damage caused by the hazard to infrastructure, resources and people involved in the game based on factors such as their geographical location relative to the disaster, economic potential and available natural resources. The outcome is displayed on an impact table and the damaged infrastructure removed from the board. Affected families also receive no income during the following roundtable and neighbouring natural resources become contaminated. In this way, the players visually experience complex situations and are able to test new resilience strategies without having to deal with real consequences.

(a) Game session organized with citizens in Moroni (Comoros Islands). (b) Interaction among Belgian students to develop a resilient community. Taken from Mossoux, S., et al. (2016)

(a) Game session organized with citizens in Moroni (Comoros Islands). (b) Interaction among Belgian students to develop a resilient community. Taken from Mossoux, S., et al. (2016)

Players also have the opportunity to acquire protective action cards which can be used to mitigate, prepare or adapt to hazards. The cards can be used by individuals, but also be part of community actions. During a natural hazard, players can decide to use their cards, individually or as a team, to avoid (some) of the impacts caused by the geohazard. This approach stimulates learning about the risks and mitigation strategies associated with natural hazards, by allowing players to test, experience and discuss new management ideas.

The game lasts for a minimum of five years, or equivalent to three hours game time, after which the resilience of the community (which takes into account factors such as number of living families with permanent shelter and access to natural resources) is evaluated using a resilience index. Players are ranked according to their resilience index, thus generating discussion and analysis of strategies which lead to some players fearing better than others.

Following the game, do players better understand natural hazards?

To test the success of the game at raising awareness of natural hazards, the researcher’s carried out a number of game sessions. A total of 21 secondary school and university students from Belgium, as well as a further 54 students, citizens, earth scientist and risk managers from Africa took part in the sessions. Players completed questionnaires before and after the games to evaluate how their understanding of natural hazards and risk management strategies changed after having played Hazagora.

Appreciation of the game by the players (n=75). (∗) Results are significantly different between European and African players (p <0.05). Taken from Mossoux, S., et al. (2016). Click to enlarge.

Appreciation of the game by the players (n=75). (∗) Results are significantly different between European and African players (p <0.05). Taken from Mossoux, S., et al. (2016). Click to enlarge.

The questionnaires revealed that participants found the game fun to play and greatly appreciated the flexibility offered to players to come up with their own adaptation and mitigation strategies. The scientific information regarding the physical processes driving natural hazards was the main thing European players learnt from the game. In contrast, West African players highlighted the usefulness of the game to develop personal and professional mitigation plans; the learning outcomes reflecting the differing life experiences and geological situations of the participants.

Hazagora succeeds in making players more aware of the mechanisms which drive natural hazards and how communities’ vulnerabilities differed based on social-economic factors, rather than depending solely on the potency of the geohazard. By driving discussion and collaboration among players it also stimulates engagement with the importance of disaster risk reduction strategies, while at the same time developing player’s social and negotiation skills. And so, following an enjoyable afternoon of gaming, Hazagora achieves its goal and becomes a great addition to the tools already available when it comes to raising awareness of geohazards.

By Laura Roberts Artal, EGU Communications Officer.

 

References

Mossoux, S., Delcamp, A., Poppe, S., Michellier, C., Canters, F., and Kervyn, M.: Hazagora: will you survive the next disaster? – A serious game to raise awareness about geohazards and disaster risk reduction, Nat. Hazards Earth Syst. Sci., 16, 135-147, doi:10.5194/nhess-16-135-2016, 2016.

Hazagora is a non-commerical game that is available upon request – please contact the study authors for more details.

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