Tephra sampling has the widest spatial range of any Erebus project this year. Nels Iverson is a postgraduate student at New Mexico Tech. studying towards his Masters in Geology, and his fieldwork in Antarctica has taken him from the top of Erebus volcano to the sea ice, and out to the Polar Plateau over 200 kilometres away.
Nels did his undergraduate degree at the University of Hawai’i at Hilo, and has also worked in Washington and Oregon, mapping basaltic rock. Although he has spent a lot of time in the field, this is his first project in tephra and geochemistry. He is in his second Erebus field season.
Volcanofiles: What is tephra, and why are you sampling it?
Nels: Tephra is anything that erupts from a volcano from the size of ash to blocks the size of cars. We are sampling it to look at Erebus’ past geochemistry and to date it, to better understand the volcano’s past eruptive activity.
Volcanofiles: How do you collect tephra samples? Do you find any blocks the size of cars?
Nels: Fly around in helicopters ‘til we see black stripes on glaciers! We use satellite imagery first to scope out spots, do reconnaissance by helicopter, then sample via chainsaw. The tephra grainsizes here range from less than 30 microns (a micron is a thousandth of a millimetre), to one millimetre.
Volcanofiles: How exactly does the chainsawing and collection work?
Nels: Each one of the black layers is hopefully an ash layer, so we cut a block of ice out of the glacier, then trim it up to just get the black layer. We throw them in coolers, take them back and melt the ice blocks to extract the tephra.
Volcanofiles: Is a black layer always ash?
Nels: We also get windblown sediment. Some places it’s easy to tell because there are melt pits of gravel on the glacier, and it’s not glassy like tephra would be. We have also found places where one ash layer has contaminated another layer, with three chemical signatures in one place.
Volcanofiles: Are these layers only found in glaciers?
Nels: No, there are also layers in the Dry Valleys on the Antarctic continent but it’s much easier to spot tephra in glaciers. Some of these areas will have forty ash layers in one section of blue (glacial) ice, compared to maybe two layers in an equivalent area of the Dry Valleys. The tephra layers in the glaciers are found where ablation (removal of ice from the surface) has brought the ash up from depth.
Volcanofiles: Is all the tephra you find from Erebus, and how can you tell?
Nels: All but two ashes we have found are from Erebus. We use an electron microprobe to do chemical analysis of major elements, from which we can discern whether the ash is from Erebus or not. We have two ashes that are not of the normal Erebus lineage. Possible sources are the Pleiades (northwest of us), or West Antarctica (to the east of us).
Volcanofiles: How do you do the dating?
Nels: We use the 40Ar – 39Ar method. It is a proxy for the K-Ar system. In order to use it, we have to irradiate the samples in a nuclear reactor.
Volcanofiles: The quick version: K-Ar dating method relies on the slow natural decay of a radioactive form of potassium, Potassium-40 (40K), into Argon-40 (40Ar) over time. It takes about 1.3 billion years for half of the total amount of 40K to decay (this is the ‘half-life’ of 40K). So, assuming that no new argon can get into the mineral sample, the amount of 40Ar can indicate how much time has passed since the potassium-bearing mineral was formed.
40Ar – 39Ar dating involves irradiating the samples in order to convert the 39K (which is stable and does not decay under normal conditions) to 39Ar, which does not occur naturally. This argon isotope indicates the amount of 39K originally in the sample. Since the usual ratios of different potassium isotopes are known, the amount of 39Ar can also be used to indicate the amount of 40K originally present – so just by measuring the ratios of different argon isotopes, a relative age can be found for the sample.
Volcanofiles: What other sorts of geochemical analysis do you do?
Nels: We do Laser Ablation ICP-MS to look at trace elements, which is another way to understand the evolution of the volcano. Erebus is very stable geochemically – at least over the past 30 000 years it has been fairly chemically stable.
Volcanofiles: What kind of evolution are you expecting to find?
Nels: There is a time where the composition changed from a tephriphonolite to a phonolite. The main part of the project is to date the ash layers – we already know about the different layers but we can constrain them in time.
Volcanofiles: Why is it important to study tephrochronology and tephrostratigraphy?
Nels: It’s important because these ash layers can be traced over large areas and can have large effects on snowmelt. Tephra layers can be used to correlate ice cores and their ages. Erebus doesn’t have any known tephra layers in ice cores, but other volcanoes in Antarctica can be traced across several difference ice cores.
Volcanofiles: So how far away are you finding these Erebus layers?
Nels: We have one layer 200 kilometres away in Manhaul Bay in the Allan Hills, and one in Mount Dewitt which is about 39 000 years old.
Volcanofiles: This is your second field season here – what were your fieldwork goals for last year and this year?
Nels: Last year we sampled many different locations to get a good spatial coverage. This year is for collecting more samples to get more dates. We did sample a few new spots that were either previously covered by snow and are now uncovered and locations we didn’t get to last year. There were some this year which we wanted to sample but were covered by snow.
Volcanofiles: How have you found the transition to working in Antarctica and geochemistry, after your work in Hawi’i with lava flows?
Nels: I love it. It’s such a treat to come down here and work. It’s been the experience of a lifetime. It’s a whole different ballgame flying around looking for black spots in the ice and chainsawing them out. You’re only working with a few grams of ash – you have to be a lot more careful. It’s meticulous work. It’s very interesting – it’s a different side of a volcano that I was never exposed to before this.
For a published study on Erebus tephrostratigraphy: Harpel, C.J., Kyle, P.R., Dunbar, N.W., 2008. Englacial tephrostratigraphy of Erebus volcano, Antarctica. Journal of Volcanology and Geothermal Research, 177(3): 549-568.
Among the unique features of Erebus are the systems of ice caves and towers formed by fumaroles on the volcano. Aaron Curtis is a PhD student at New Mexico Tech., currently in his third Erebus field season studying the ice caves. The research he is undertaking here is the first of this kind to be published since 1976. He tells us about the work he has been doing in the caves over the past few weeks.
Volcanofiles: What is an Erebus ice cave?
Aaron: I’m studying fumarolic ice caves and towers – these caves are more like glacier caves than like limestone caves containing ice. There are similar things at a few other volcanoes – Mt Rainier, Mt Baker in the Cascades of the northwest US, Mt Melbourne in Antarctica and several in Iceland – whenever you get a volcano under a pretty large amount of snow. There’s nothing like Erebus in terms of the number and diversity though.
Volcanofiles: So how are these fumarolic ice caves and towers formed?
Aaron: Well, nobody knows exactly, but that’s what I’m studying. The volcano releases gases and heat from the entire summit caldera area, and the permafrost and perennial snowpack is reshaped by that heat. One of the common results is that you get voids developing at the bottom of the snowpack and some of those are spectacular networks of passages. Some of the cave entrances have towers on them, and are linked up to towers.
Volcanofiles: Are they are all formed in the same way?
Aaron: It seems like there could be two major mechanisms of heat transfer. The first is conduction through the floor. Werner Giggenbach, in the 1980s, wrote about areas of warm rock causing caves to appear above. But what I’ve been finding, which wasn’t reflected in earlier research, is that there are discrete fumaroles that have a huge impact on the cave geometry. I think what the networks we can see are most directly a result of airflow from discrete vents, because there are fractures and shallow structure in the rock that controls where the heat goes and where the cave forms.
Nial: The scalloping on the walls shows that there is hot gas flowing through the cave, rather than just hot ground. (Although Nial is currently doing his own PhD work here, his first Erebus season was as Aaron’s field assistant, so he has spent a lot of time in the caves!)
Aaron: Exactly, and the microclimates of the caves are completely different from the surface. Our average surface temperature is -32.6oC over the year, whereas the cave temperatures are between 0-6oC all year. There is really strong airflow in some caves which indicates that hot air is coming through the bottom and spewing out the top.
Volcanofiles: What about the humidity?
Aaron: It’s pretty close to 100% relative humidity. We see liquid water in some of the caves. The presence of water in environments such as these is a huge deal for biologists – it’s kind of like an oasis in a polar desert. We at the Mount Erebus Volcano Observatory are interested in how closely the steam coming out of the vents is linked to the magmatic system – whether it’s degassing directly out of magma, whether it’s recycled melted snow, or whether it reflects a hydrothermal system.
Volcanofiles: So how do you carry out these studies?
Aaron: The main focus in my research for the last couple of years has been fibre optic distributed temperature sensing (DTS) investigation of the cave microclimates. That allows us to see all of the discrete fumarolic vents and compare them to the environment in the rest of the cave.
Volcanofiles: What is DTS and how does it work?
Aaron: The cables are strung out in the cave, and we fire a laser down the cable. This gives us reflections from down the length of the cable. The temperature at every point on the cable affects the spectra that we get back. We send out a specific frequency signal and we get light back at several different frequencies. Some of those frequencies are affected by temperature so we can compare them to get a precise temperature. The electronics allow us to measure very finely the time when the response gets back, which relates to the distance along the cable, so we can measure different temperatures for every metre of cable.
Volcanofiles: How much distance are we talking about?
Aaron: Probably up to 1 km for one of the caves I’m looking at. The large ones are on the order of hundreds of metres, but some of them are barely big enough to get your head into – those ones are a bad idea to put your data loggers into because you don’t tend to get them back!
Volcanofiles: So the caves change a lot?
Aaron: I’ve been astonished by the differences in caves just from one field season to the next. The passages seal off and whole new sections will open up – it’s kind of a weird feeling because the caves are vaguely familiar from last year so it can be quite disorienting. We just finished the first repeat LIDAR 3D scan (which Volcanofiles will be posting more on soon!) of Warren Cave – we mapped it in my first year and did the LIDAR the two following years. Drea Killingsworth will be able to tell you more about that.
Volcanofiles: Are there any other instruments you’ve got in the caves?
Aaron: I’m monitoring about 15 different caves using various data loggers for temperature, windspeed, barometric pressure, and CO2 concentration. Other work I’m doing includes taking a bunch of gas samples and biological samples, and thermal imagery. I also have a multigas meter which I can take in with me, that checks for CO, SO2, H2S and CO2 – primarily for safety, but it gives us a bit of science as well.
Volcanofiles: Can you tell us more about the unusual crystal formations in the caves?
Aaron: They’re spectacular and hugely variable. The cold areas in particular have crystals that can be up to a metre long, and clearly reflect the microclimate in that particular area. They can even tell you the wind direction – some of the hoarfrost grows into the wind. It’s almost like having a vector field mapping out the wind direction – the kind of thing it would take a really advanced instrument to be able to do, but nature does it for us.
Volcanofiles: How accessible are the caves?
Aaron: Access varies from being able to walk in to pretty technical rope setups. Every cave is different. Some caves we have not been able to get into at all.
I feel fortunate because I get to stay down in the nice warm protected environment while other people are up at the rim – but it’s humid down there, and when you come back to the surface, your clothes turn into armour when everything freezes. Snowmobiles don’t have seatbelts so it can be convenient when your butt freezes to the seat…
Volcanofiles: Thanks, Aaron! Good luck with your work down Warren Cave today.
For more about the DTS work Aaron is doing here, here’s the reference for his paper recently published in Geophysical Research Letters:
Curtis, A & Kyle, P., 2011. Geothermal point sources identified in a fumarolic ice cave on Erebus volcano, Antarctica using fiber optic distributed temperature sensing. Geophysical Research Letters 38:L16802.
In order to spend time at the Erebus camp, we first need to have a couple of days of acclimatisation at a lower altitude. The usual site for this is at the Fang glacier on Erebus (about 2900 m), where we are based in tents for a couple of days as our bodies adjust to the altitude.
Those of us in the second group had fairly good weather. We spent most of our time there in Scott tents that had already been set up by the field support people at McMurdo.
When acclimatising, it’s better to stick to light exercise, so we went for a short walk near camp and visited the glacial moraine, consisting of eroded volcanic rocks that were redeposited by a glacier.
To the east, we could see Mount Terror, another of Ross Island’s volcanoes.
Terror and Erebus were named after two ships in Sir James Ross’ expedition in the 1840s.
Our Antarctic Volcanofiles, Nial and Tehnuka, have arrived safely at McMurdo base, Antarctica, the largest Antarctic base, via a C-17 flight from Christchurch, New Zealand.
The plane ride takes about 5 hours, but the adrenaline makes the whole thing go by all too fast. Boarding a huge military aircraft like the C-17 is a thrill, and seeing the continent out the window for the first time — unforgettable.
Now that they’re there, new members of the team will go through basic survival and Antarctic skills training while the experienced members will take some refresher courses. Safety is a big deal in Antarctica, especially when you’re deployed to a remote field area like Erebus. Although Erebus is only about 37 km (23 mi) away from McMurdo, the only way to get there is via helicopter. And helicopters will only fly when the weather is right. So team members need to be trained to make sure that nothing goes wrong — and know what to do if something does.
The weather in McMurdo is warm but snowing, and visibility is low. On a good day, you could see Erebus clear as day from the ice runway where the C-17 lands. But, today, it was too cloudy to see her. We’ll post some good pics when visibility clears up and we can get a good look at Erebus in all her glory. Here’s the view from today:
I’ve received word from our Antarctic team members that everyone made it safely to Christchurch, New Zealand yesterday. After only a few days to catch up on the jet lag, they’ll be issued their gear and put onto a C-17 aircraft that will take them to the great white south.