Our Changing World

Thursday 6 August 2015, with Alison Ballance & Veronika Meduna

On This Programme

Testing Volcanic Ballistics with a Cannon and Catapult

By Alison Ballance

In a world-first, geologists are using a medieval siege catapult, or trebuchet, to hurl projectiles made from silly putty wrapped in condoms. The comic sounding experiment is part of a serious project gathering information on the behaviour of volcanic ballistics that will be used to improve New Zealand’s volcanic hazard modelling.

“We want to simulate the passage of volcanic bombs as they fly out of volcanoes, to see how they change shape as they fly through the air. Does it fly through the air like a cowpat? Or does it splat on the ground like a cowpat? Our hypothesis is that the volcanic bombs may be becoming cowpat shaped, like a discus, as they fly through the air. And that affects how far they fly, it affects their drag, which are all very important parameters in eruption models.”
Ben Kennedy, geologist, University of Canterbury.

The video shows a rubber ball filled with sand as it is fired from the trebuchet, arcs up through the air and lands about 80 metres away

George Williams holds an analogue lava bomb, made from silly putty enclosed in a condom (left). After being fired from the trebuchet the silly putty ballistic spun in mid-air and then landed like a cowpat (right).

George Williams holds an analogue lava bomb, made from silly putty enclosed in a condom (left). After being fired from the trebuchet the silly putty ballistic spun in mid-air and then landed like a cowpat (right).

Photo: RNZ / Alison Ballance


On 6 August 2012, a phreatic (gas and steam driven) eruption took place at the Upper Te Maari Crater on Mount Tongarairo, in the central North Island. The Te Maari crater had erupted a few times between 1855 and 1897, and was then inactive until August and November 2012.  During the August 2012 eruption volcanic ash and thousands of blocks and lava bombs were fired out of the volcano, and the damage from the ballistics covered an area of 6 square kilometres. Several blocks went through the roof of the Ketetahi Hut, which was fortunately unoccupied. As the eruption occurred during bad weather in the middle of the night there were luckily no people walking the popular Tongariro crossing.

In November 2012, Upper Te Maari Crater on Mount Tongariro had a short eruption which spread ash and ballistics around the crater. A more damaging eruption in August 2012 covered a much larger area

Geologist Ben Kennedy holds a sand-filled rubber ball that has just been fired over 80 metres by the trebuchet - it started spherical, and as it flew it flattened and became more tyre shaped

Geologist Ben Kennedy holds a sand-filled rubber ball that has just been fired over 80 metres by the trebuchet.

Photo: RNZ / Alison Ballance

For her Master’s degree PhD student Rebecca Fitzgerald carried out an assessment of ballistic hazard and risk from the Upper Te Maari crater eruption. She used aerial photographs to identify 3,587 impact craters with a mean diameter of 2.4 metres, but when she combined this information with ground surveys, which allowed her to find much smaller impact craters, she estimated that approximately 13,200 ballistic projectiles had been thrown out during the eruption at an average speed of 200 metres per second. She also modelled the risk of serious injury or death to walkers along the Tongariro alpine crossing during this size eruption, and found that in some places on the track the probability of death was 16%. Her modelling also showed that a larger eruption had a much higher – up to 100% - risk of death.

Rebecca is building on her Master’s research for her PhD, and will be travelling to Japan to map ballistics from the September 2014 Mount Ontake eruption in which 57 people were killed. She says “I’ll also be doing more numerical modelling as this will allow us to look at future eruption scenarios.” This work will include identifying potential ballistic hazard zones in the Auckland volcanic zone.

Testing the Trebuchet

The trebuchet, a 4-metre high medieval siege catapult, is in a cocked position ready to fire (left) and (right) the arm is extended vertically after it has fired and flung the ballistic.

The trebuchet, a 4-metre high medieval siege catapult, is in a cocked position ready to fire (left) and (right) the arm is extended vertically after it has fired and flung the ballistic.

Photo: RNZ / Alison Ballance

During the test run, the team had two missiles to test with the trebuchet. One was a rubber gym ball filled with sand, and the other was “a more realistic analogue material,” according to Ben, “which is silly putty wrapped in a condom. Silly putty has similar properties to magma in that its visco-elastic. Everyone know that silly putty bounces if you throw it on the floor but if you leave it there it’ll just spread out slowly. Hopefully it’ll represent the behaviour in flight of volcanic bombs, which are also visco-elastic.”

Ben says depending on the type of volcano and the type of eruption, a range of projectiles ranging in viscosity or stickiness from “snot dribbles’ to solid blocks several metres in size can be thrown out. Future plans for the trebuchet include doing a systematic study of viscosity in different sizes and under different conditions.

Firing the Cannon

The pneumatic cannon (left) fires a rock towards the ground using compressed air. The rock made a small crater as it impacted (right) - the water-filled bladders prevent the cannon lifting off the ground as it fires

Cannon-builder Pete Jones and Rebecca Fitzgerald with the pneumatic cannon (left) that fires a rock towards the ground using compressed air. The rock made a small crater as it impacted (right) - the water-filled bladders prevent the cannon lifting off the ground as it fires

Photo: RNZ / Alison Ballance

Masters student George Williams will test various roof and wall constructions by firing ballistics from the vertical cannon, while PhD student Rebecca Fitzgerald is interested in measuring the impact and craters of different kinds of ballistics.

Masters student George Williams will test various roof and wall constructions by firing ballistics from the vertical cannon, while PhD student Rebecca Fitzgerald is interested in measuring the impact and craters of different kinds of ballistics.

Photo: RNZ / Alison Ballance

As well as the trebuchet, the team are running a second set of experiments using a vertical cannon that fires rocks down towards the ground. The cannon has been made out of a piece of old drilling pipe. It works using pressure created by compressed air, and is capable of firing rocks weighing up to 8-10 kg at 100 metres per second. In a real eruption rocks can be blasted out of the vent faster than the speed of sound, at 400 metres per second. They then slow down as they travel, but can speed up again as they begin to fall, which makes ballistic modelling very complex.

Rebecca plans to use the cannon to build a relationship between the size of the ballistic, the speed it is fired at, the size of crater it creates and whether it fires out any shrapnel. She will be able to relate this information to craters she has mapped in the field.

Masters student George Williams is using the cannon to fire rocks against common roofing materials such as corrugated iron. He has collected various roof and wall materials from demolished buildings in Christchurch, and explains that “what we’re doing is calculating the impact energy required to cause different amounts of damage to different building materials. We’ll be testing, for example, whether a small rock moving fast is able to puncture through more than one that’s moving with the same energy but is big and slow. We’ll also be using silly putty.” The team are particularly interested in reinforced concrete, which is the material that volcanic shelters are built from, and say that this is the first time work like this has been carried out.

The pneumatic cannon is powered by compressed air, and the three tonnes of water in the large plastic tanks are to prevent it from taking off as it fires. The first rock is fired out at about 30 kilometres per hour while the second is at about 70 km/hr - these speeds are much slower than occurs during a real volcanic eruption

What To Do During An Eruption

What’s the best advice if you’re caught in an eruption that is firing out ballistics? Rebecca says that all the evidence to date from eruptions like Mount Ontake in Japan is that most people are killed from injuries sustained to their backs and necks as they run away.

“You want to make sure you watch where the ballistics are coming from and move away from that area,” says Rebecca.

Ben stresses that you should seek shelter if you can. “I had a conversation with the military recently, and their advice was that if there are lots of objects falling around you make yourself as small as possible, and try and shelter behind a large block or something solid.”

If there are fewer objects falling then Ben advices keeping an eye on the sky, and watching individual objects that seem to be falling towards you so you can step out of the way as it lands.

“We call it an auk step. There’s a bit in Lord of the Rings where there’s a big bit of concrete coming at an auk and he just moves to the side,” says Rebecca.

In this video, shot by Alistair Davies, the trebuchet is in the far distance, in the corner of the field. The trebuchet fires and then about 18 seconds into the video you can see a small dot appear which is the sand ball rising against the blue sky and then the white cloud before beginning to fall, then bounce and roll across the grass after it lands.

Our Changing World has previously featured stories on the Taupo Volcanic Zone and volcanic hazard planning:

Harry Keys from DoC looks after the lahar eruption detection system on Mount Ruapehu

Massey University have a volcanic eruption simulator

Brad Scott from GNS explains the new, simplified volcanic hazard alert system

GNS volcanologist Gill Jolly explains how data from a volcano monitoring network help forecast volcanic activity

And Brad Scott takes Alison Ballance around the Waimangu Valley near Rotorua, which is one of a number of volcanic monitoring sites in the Taupo Volcanic Zone


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Making Urban Bushland Better

Parks and forest remnants are becoming increasingly important aspects of urban planning, and Kiri Cutting, a PhD student at Waikato University, is interested in working out how cities can improve and better maintain their green areas.

Kiri Cutting, taking environmental data in Glenpark Avenue Reserve in New Plymouth. This research site has a mostly non-native tree population, and the data is compared to nearby sites which have been restored with native trees.

Kiri Cutting, taking environmental data in Glenpark Avenue Reserve in New Plymouth. This research site has a mostly non-native tree population, and the data is compared to nearby sites which have been restored with native trees.

Photo: RNZ / Ruth Beran

Kiri chose three different sites in New Plymouth, New Zealand’s greenest city, which still has fragments of original native forest within its urban landscape. The study sites include such forest remnants as well as parklands that have been restored and replanted with native plants, both of which she finds off the Huatoki walkway. At the Glenpark Avenue Reserve, she studies an area that has been left to regenerate by itself and is dominated by exotic plane trees.

She says she is interested in studying the ecological complexities of urban forest fragments that have never been degraded and to figure out how to make urban restoration projects more successful. To get an idea of the ecology of each site, Kiri has been measuring soil moisture, air temperature and humidity, and how much leaf litter accumulates on the ground.

“Leaf litter is immensely important in the life of a forest. It forms an insulating blanket on the ground, allows native plants to regenerate, and if seeds drop into it, they germinate and are protected.

Kiri says she’s been finding that “in sites where restoration plantings are very young, and the sites are very open and the plants aren’t very big yet, there’s hardly any leaf litter and regeneration of native plants takes a little longer because there’s no nice nursery for the seeds to germinate in”.

All the data is compiled into a modelling equation which relates the different parameters to each other. “Eventually this becomes applied when we can say things and make predictions about these sites that allow us to manage them better.”

The data gathered at the New Plymouth sites provide a benchmark in comparison with other sites from Hamilton, which has fewer green urban spaces. “The two cities are far apart on the spectrum of forest cover. Hamilton has 2.5 per cent of its original forest left, New Plymouth has 8.5 per cent, so that is an interesting comparison for this study because connectivity [between park areas] is really important. Connectivity can change the dynamics and affect the restoration success.”

The first results from the study show a clear connection between the age of a site and the density of the canopy.

As trees grow, the crowns close in and produce the canopy roof – and that prevents sunlight from getting to the forest floor and keeps weeds out. We’re finding that it takes 15 to 20 years for that to happen, but then weed control can be reduced.

Kiri says urban tress have many benefits. “They clean our water and air, they reduce noise pollution and abate flooding, and they help reduce stress.”


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How Nature is Good for our Well-being

By Alison Ballance

What makes us happy? Environmental scientist Lin Roberts and colleagues argue in a new report that nature is a key factor in our happiness, and that ecosystem services delivered by indigenous biodiversity and natural ecosystems contribute in a wide variety of ways to the wellbeing of New Zealand and New Zealanders.

View across beech forest to mountain tops - the Arthur Range in Kahurangi National Park

The Arthur Range in Kahurangi National Park

Photo: RNZ / Alison Ballance

“The ecosystem services idea was developed as a way to communicate to people what we get from nature, and how dependant we are on it. Because many of us living in cities forget how much we use and rely on nature to provide us things like air to breathe, water to drink, food to eat. [They] also provide all sorts of other benefits like flood protection, water and air purification, pollination.”
Lin Roberts, Lincoln University

The explosion of research into wellbeing has come about because, Lin says there is a “recognition that our economic wellbeing has been increasing steadily, GDP has been growing, incomes have been growing – but people are not getting any happier in developed countries.”

A lichen on the forest floor

Nature underpins happiness and wellbeing, according to a new report.

Photo: RNZ / Alison Ballance

The report was commissioned by the Department of Conservation and is called ‘The nature of wellbeing: how nature’s ecosystem services contribute to the wellbeing of New Zealand and New Zealanders’. It uses a framework developed by Chilean Max-Neef who recognised that all people have the same basic nine needs, which he called satisfiers: subsistence, protection, affection, understanding, participation, creativity, identity, freedom and leisure. Lin says that different people satisfy these needs in different ways and it is not necessarily through consumer goods.

“Our need for affection will usually be satisfied through close relationships with other people. Or our need for leisure could be just spending time reading a book, lying in the sand.”

Lin says the report aims to encourage people to think about how they’re living, and how it impacts both on themselves and on the planet.

“Thinking more about how we satisfy our needs and what are the best types of satisfiers gives you an opening to explore some of the ways we might satisfy our needs without actually getting much wellbeing return and at the same time damaging the systems we rely on.”

To quote the report, ‘if we can become better at identifying and choosing high-happiness-return/low-impact consumption over high-impact/low-happiness-return consumption, we will not only improve our own wellbeing and that of supporting ecosystems, but will also enhance the opportunity for our grandchildren and others on the planet to meet their basic needs and enjoy “the good life”.’

The Valuing Nature conference, held in Wellington in 2013, discussed ecosystem services and the links between economy and the environment. Lin Roberts was one of the panellists, and Pavan Sukdevh presented a talk on TEEB, The Economics of Ecosystems and Biodiversity.


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