We’re sitting on an airforce Hercules on the way back to Christchurch. An 8-hour flight is an excellent opportunity to catch up on sleep, data processing, and blog posts.
Erebus itself is quite an unusual volcano – it is home to one of the world’s few persistent lava lakes; large crystals, also called megacrysts, of anorthoclase feldspar are found in its lava; and it is one of a handful of ice-covered volcanoes where flank degassing results in the formation of ice caves (other examples include Mt Rainier and Mt St Helens in the US). As we mentioned in a post a few years back, these ice caves are interesting to scientists for a number of reasons.
While we know of active glaciated volcanoes in the present day, there have been times in the past when most of the earth is thought to have been ice-covered. One theory on how the world ‘defrosted’ is that volcanic carbon dioxide (CO2) emissions caused warming of the atmosphere. One way to better understand what happens to CO2 coming out of glaciated volcanoes is to measure it at a place like Erebus. Since much of the gas that is escaping seems to be associated with heat and steam, we find steaming warm ground, and ice caves that have been shaped by escaping gases – sometimes together with ice towers from steam freezing around gas vents.
A second reason for our interest in Erebus is more local – how does the flank degassing relate to the activity we see at the summit (which includes the lava lake(s) and a number of fumaroles)? Erebus is in an area where the lithosphere is thinning as it is pulled apart along the West Antarctic Rift system – specifically, it is associated with the Terror Rift – so we might expect to see more gas escaping along fractures related to the rifting. But where is the gas really coming from? Is it escaping at shallow depths from the magma that supplies the lava lake? Or is it sourced at much greater depths, escaping from the mantle and finding pathways directly to the surface? Then, as it approaches the surface, with what else can it interact – is there substantial water underground that adds to the gas emitted at the surface?
Our work in these two field seasons is to start answering some of these questions by collecting and analysing gas samples from different sites around the volcano. We sampled gas from fumaroles near the main crater rim, down through warm ground, to ice caves in the lower part of the summit caldera. These sites span about four or five hundred metres vertically, but will hopefully also give us an idea of how emissions vary with distance from the lava lake.
We used a number of sampling methods, partly so we could measure different things, and partly to keep our bases covered in case something went wrong! One set-up for gas sampling is a soil probe connected to a series of copper tubes, which in turn connect to a pump. We usually connected the output to a glass vial that can collect another 12 mL of gas. The pump was left running to flush out ambient air, so that the gas we wanted to measure could fill up the tubes. When we returned, it was time to crimp the ends of the copper tubes. This cold welds them shut and stop the gas escaping.
Another type of measurement was carbon dioxide flux. Unlike the copper tube and glass vial samples, which we must analyse back in the lab, this CO2 analyser immediately tells us the concentration and flux of carbon dioxide. The cylindrical chamber is put on the ground and it measures the changing concentration of CO2, using this to calculate flux (i.e. the amount of CO2 emitted per unit of time). By taking flux measurements at points along a grid, we can extrapolate to get an idea of the rate at which CO2 is emitted from the volcano’s flanks more generally. The main technical challenge here was keeping the instrument warm enough to operate. After one incident of emergency rewarming inside Lyra’s jacket, I ended up making it a sort of tea cosy out of two rock bags separated by a layer of bubble wrap.
The CO2 flux meter and soil probe can both be used to collect gas in a vial or in a special plastic bag. The samples in the bags don’t last long, but can be used with an instrument that we were given a last minute opportunity by the DCO to take along with us – an infrared isotope ratio spectrometer. We set this up in the garage hut and it could, in theory, be used to measure carbon isotope ratios. While a first look suggests some good data, we also had enough technical issues to keep us busy in the garage on bad weather days.
Isotopes are variation on an atom distiguished by the number of neutrons they contain. Having more neutrons does not affect the charge of the atom, but does affect its mass. Carbon has two stable isotopes that occur naturally (as well as carbon 14, which is radioactive). Of the two stable isotopes, carbon 13 has greater mass than carbon 12, because its atoms each contain one more neutron. Carbon from different sources has a different balance of isotopes depending on how and whether it fractionates – that is, how it separates according to mass. Carbon dioxide from deep down in the mantle usually has a relatively heavy isotope ratio, whereas if it undergoes phase changes, such as becoming dissolved and then exolving into a gas again, the heavier isotopes may be separated out. So by looking at the isotope composition of the carbon dioxide, we can start to understand where it has come from and how it has been modified, helping us to address some of those questions I mentioned at the start, about the depths from which the gas is sourced and how it interacts with water.
Also headed back to Christchurch today were a case of copper tubes, glass vials, and sampling bottles. These, we will analyse back in the lab for gas and isotope composition.
We are now back at McMurdo after two and a half weeks in the field. This post was started partway through our season. Unfortunately, I didn’t get a chance to finish and post it in the few hours that the internet connection was up – apologies to our readers for the delay (it’s a harsh continent!)
Our field camp was at Lower Erebus Hut (LEH), on the north side of Erebus. Most of our work, however, was a half hour snowmobile drive around the caldera, at Ice Tower Ridge. This is a line of ice towers and caves that extends southwest from an old Erebus caldera rim up to the summit area. We are looking at gas compositions from this area to see how they vary with distance from the main crater – but this first post will focus more on the practicalities of our work.
Having spent most of my previous two seasons working at LEH and the summit area, Ice Tower Ridge is still fairly new to me. None of the three members of my team are familiar with the area either, so a large part of a day in the field can be spent driving to the site, trying to find cave entrances, and choosing a good place to position instruments.
It usually falls to our mountaineer, Lyra Pierotti, to make sure we can safely access a cave. She often goes ahead to check whether the ice beneath us is sturdy, and to decide how best to enter – whether we can simply crawl in, or whether crampons or ropes might be required. Having someone to get us to our sites makes it a lot easier for us to focus on getting our science equipment set up!
Once inside, we must find suitable places to set up our gas sampling equipment and measure carbon dioxide flux. This can involve some exploration – looking for soft ground or a vent to place soil probes for gas sampling, checking soil temperatures, and ensuring the site is safe to access.
One spot I found was a deep hole with warm air coming out at up to 6 m/s, warmer than the cave air by over ten degrees Celsius.
It is quite warm in the caves – often above freezing, compared to -20°C or lower outside. This can be nice to work in – except that it is also much more humid in the caves, and all the liquid water dripping on us freezes rapidly when we head back out. Not all of our work is done in the ice caves, either. We are also working outside to take gas measurements in areas of warm ground, where heat and gas move more diffusely through the soil.
The Erebus ice caves and geothermal areas are home to unique micro-organisms. This means we do not enter any caves that are pristine (have not been entered previously) or certain areas of warm ground. We also need to take precautions to reduce contamination that in some sites involve sterilising the instruments that come into contact with the ground, and in others require protective clothing.
Some of our work doesn’t require so much preparation, however! One of the sites nearby, Hut Cave, is a good place to work during bad weather since it’s just a few minutes’ walk from camp. I made a couple of trips out to place and retrieve a soil probe, with some copper tubes and a pump attached to collect gas.
Carbon dioxide levels can be high inside some of the caves, so it’s good to have a handheld monitor and someone for backup when investigating the smaller passageways and crevices in search of gas vents.
We made it into about five cave systems this season, and set up multiple sampling sites in some of them. We also spent time above ground collecting gas in areas of warm ground and around the crater rim. Collecting the samples is just the first step, though – I’ll be writing more about what we’re actually looking for in the next post!
As we are just a couple of days out from our planned departure for Erebus, we are finishing off training and testing out some of our equipment to make sure it will work in the field.
On Saturday, we went out for crevasse rescue training, along with another team. They will be working on a glacier, so the training is a useful precaution – in our case, although the upper slopes of Erebus are crevasse-free, our work comes with a small risk of breaking through the top of an ice cave. We may also need to use ropes to access some of the caves.
In the classroom, we went through some knots, principles of crevasse rescue, and self-rescue using prusiks, which are friction hitches tied around a rope. These can attach you securely to the rope when your weight is hanging on them, but slide freely when they are not loaded (usually when your weight is being held by a second prusik). We then headed out to the simulator – an artificial crevasse – a short way by hagglund from McMurdo. Unfortunately, photos are a little scarce as I was busy trying to learn things! We practised self-arrest (to stop ourselves from sliding, or being pulled, into a crevasse) using ice axes, creating anchors to which a rope can be attached and used to rescue a crevasse-fall victim, then put all the elements from our training together to pull either ‘Mr Orange’ or a heavy bag out of the crevasse.
The first part, which was hard enough, was self-arresting with a bag about half my own weight falling down the crevasse attached to the rope behind me. It was then up to my supervisor to secure a second rope into the snow, set up a pulley system, and rescue the bag (which had, by then, hit the bottom). I am not yet confident that I could rescue anyone from a crevasse or ice cave (unless it were myself), so here’s hoping for a safe field season!
It has been snowing a lot this weekend, so I put off going for a walk (until after I finish this post!) and spent another day in the office today.
Our preparation for fieldwork included putting together the system that we would use for gas sampling. This starts with a soil probe, which will go into a vent. Flexible tubing connects it to a series of copper tubes that will be used to collect gas so that we can measure its composition, and analyse helium and other noble gases. A tiny pump draws air through this system so that the air already inside will slowly be flushed out, and the gas from the vent will fill the tubes. On the other side of the pump, some glass vials will collect the outflowing gas for carbon isotope analysis. All of these samples will need to come back to UNM for analyses.
We tried testing the system with the soil probe in a beaker of water. We wanted to find out how long it took for the air in the system to be flushed out, which in this case would be when it filled up with water. It’s much harder for the pump to draw up water than air, though, so while we found a few leaks to deal with, we didn’t manage to time the flushing. Instead, we found the volume of the sampling train by filling it with water. It’s about 120 mL so, at a pumping rate of 10 mL air/min, it would take (in theory) 12 minutes to flush the system. In practise, we think that flushing for a couple of hours should be enough to ensure that we are measuring gas from the vent and not the ambient air.
After dinner, I went back to pack things up…
…and finally, I can head out into the sunshine for a walk, before a busy day tomorrow – getting our cargo ready to fly, more snowmobile training, and packing up our personal gear.
The Volcanofiles have had a long break between fieldwork posts, and there are a few more days to come before I get into the field, so this season’s blog posts will start with today’s non-scientific update on our trip thus far, and some photos from Ross Island, where we are based.
It’s exciting to be back on Ross Island after four years. A few things are different this time around. Instead of working on gas emissions from Erebus lava lake, we are collecting gas samples in the ice caves. Last time, with G-081, (who will also be heading up in January this season) I was in a group of about twelve. Now, I’m part of a much smaller group (event number G-411) – just me, my supervisor, and a mountaineer to help us in the field. This means I’m much more involved in planning our fieldwork, and realising just how much effort goes into supporting Antarctic science.
Since arriving, we’ve had meetings and training to cover several topics, including communications, field safety, working at altitude, and our environmental responsibilities. I’ll go into more detail on the Antarctic Specially Protected Area, or ASPA, and environmental concerns around the ice caves, in a later post.
We completed our food pull today with the help of field centre staff here, so that we have supplies both for the acclimatisation camp at Fang glacier, and for Lower Erebus Hut, or LEH. We also went through the camping and caving equipment that we are borrowing for the trip. For those who didn’t follow our previous trips to Erebus, we spend a couple of nights at an intermediate altitude, in order to help us acclimatise to the lower oxygen availability. Fang is a tent camp, with no buildings, so people from here at McMurdo have to go and set it up every season before science events like ours start coming through. LEH has two huts – one where we cook and work, and a ‘garage hut’ with tools and storage. We can set up our own tents when we arrive.
Now we’re hoping for good weather on Monday, so that Fang camp can be put in. Apparently this was due to happen last week, but weather conditions have intervened. Once Fang is in, we can head up – in the meantime, the carpenters will be opening up Lower Erebus Hut. As you’ve probably gathered, there are a lot of people working hard to make the research down here happen.
Tomorrow, we are planning some crevasse rescue training to prepare for our work in the ice caves. This is weather dependent, of course – as I write this I can see the islands of the Ross Archipelago to the south appearing and disappearing due to what I think is snow! An update on our training will follow, but in the meantime, here are a few photos from our trip so far.
We travelled from Albuquerque, New Mexico, in the USA, where I started a postdoctoral research position in August, to Christchurch, New Zealand, where we were issued clothing and waited for our flight south.
After an early morning start the next day, we got on a C-17 at Christchurch airport, and spent a few hours getting to Ross Island.
One difference from my previous seasons is that we landed out at Pegasus air field, an hour’s drive from McMurdo on ‘Ivan the Terrabus’, as opposed to the sea ice runway that we used in the past, which was much closer to McMurdo.
Unfortunately, I was so disoriented on getting off the plane that I’m not actually sure which direction Erebus is in relative to anything else in these photos!
McMurdo is as built up as ever. We sleep in dorm buildings, work in the Crary lab where we have lab and office space, go to the ‘galley’ for meals, with visits to places like the Science Support Centre for training, or the Berg Field Centre for our field gear. We spend most of our time indoors – but if you remember to look up (provided the visibility is not too bad) it still looks like Antarctica.
When the weather is good, it’s easy enough to take a walk out of town. Last night I visited Scott Base, which was a chance to meet some fellow Kiwis.
The main reason for the walk, though, was to get outside and find some nice views…
…including a first look at Erebus.
If you squint, you may be able to see the summit cone and a tiny plume coming out of it…but in any case, please keep an eye on the blog. We hope to be reporting from up there in a week or so.
Infrared cameras are a great way to take thermal measurements of a volcano from a distance. A thermal camera has been used at Erebus for several years. There, it provides the opportunity to look not just at changes to heat output, but also at the activity of the lava lake. Nial Peters, one of the Volcanofiles and a PhD student at Cambridge, has been operating the camera for the past three field seasons and looking at the data from it. Nial first went to Erebus as a field assistant for Aaron Curtis, who we interviewed last season, working in the ice caves – so he knows the volcano well. Here’s his email interview from the 2012-13 field season, telling us about his work.
Volcanofiles: What is a thermal camera?
Nial: Pretty much the same as an ordinary camera, except that the sensor records IR radiation rather than visible. Objects that are radiating a lot of heat show up as bright. Note that is not necessarily the same as saying hot objects show up bright – a high temperature silver object may show up as less bright than a cooler black object!
Volcanofiles: Perhaps it’s fairly obvious why you’d want to use a thermal camera on a volcano, then? What sort of things can you look for in the footage from the Erebus lava lake?
Nial: Well, perhaps not so obvious. Of course you can use a thermal camera to do the obvious things like measure heat output from the lava lake and many people have done such studies in the past. The reason I use a thermal camera is because it is capable of imaging the lake through a far thicker volcanic plume than a normal camera. Even on days when the lake is invisible to the naked eye, you can still record clear IR images. I am not so interested in the actual temperature readings from the camera, I am using the data to look at the surface velocity of the lake as it convects. You can also record the Strombolian eruptions of of the lake and measure things like refill time.
Volcanofiles: You’ve spent a lot of time building things so that you can collect data with the thermal camera. How do you set up the camera in the field? And what have you been working on these past few months?
Nial: The thermal camera we have does not store images on-board like most cameras do. Instead it is designed to stream images over an Ethernet link, using the GenICam interface. This means that it requires a computer to operate. The first year we used the camera, we set up a microwave Ethernet link to the crater rim and ran the camera from a PC in the hut. However, my goal was to have the camera run year-round and this setup was too power hungry and unreliable for this.
The system this year is totally different. The camera is being controlled by a ARM based single board computer (SBC) (it’s a Blue Chip Technologies RE2 board if anyone is interested) running Ubuntu Linux. I have written some custom control software for the camera based on the open-source GenICam project Aravis.
The software captures images, does a very limited amount of preprocessing, and then compresses the images into PNG files. The images are stored locally on a solid-state hard-disc. The SBC also runs a server program which can send the images and some environmental data (power consumption, temperature, etc.) in realtime over the microwave ethernet link (when it is operational – in other words, during the field season) so that we can keep an eye on things while we are here. The whole system is designed to run reliably by itself for an extended period, with lots of error checking and correction built into the software, GPS time synchronisation and a watchdog program to restart the whole system should something go badly wrong.
The weak link in the system is the power supply. In total the camera system uses about 11W, which is generated by a solar array and some wind generators situated 0.5km away (conditions on the rim are too harsh for solar panels and wind generators). An inverter is used to boost the voltage to 230V AC and power is then transmitted up a cable to the rim where it is stepped down and rectified to 12V DC. The whole power system has been replaced this year using low-temperature rated components and tougher cable. Hopefully this will mean that we can sustain power to the rim year round, but this is a challenging environment so we will see!
Most of my work for the past months has been developing and testing the new camera system (both software and hardware). It is probably the most complex thing I have ever made and I am really pleased that so far it has worked flawlessly (over 600,000 images captured so far!).
Volcanofiles: This is your third season running a thermal camera on Erebus, and there is data from older field seasons too. What have you measured with it in the past? And what else are you hoping to do with it this year?
Nial: The motion tracking is probably the most important thing that I have done with the data so far. This picks up the periodic behaviour of the lake very well, and shows that the lake has been doing the same thing for as long as we have been measuring it! It also shows the recent decrease in the size of the lake (it is now a quarter of the size it was two years ago). This year will be more of the same, but with a year-round dataset hopefully we can see in more detail how the lake is changing.
Volcanofiles: You’ve had four field seasons on Erebus. Does the novelty start wearing off? How have things changed for you since the first time you were up there?
Nial: Certainly it is not as exciting as it was the first time, but then nothing is, once you have some idea of what to expect. It is still an awesome place to come to though, and I am still thrilled that I get the opportunity to work here.
I guess the biggest change for me since my first season has been the transition from working in the caves to working at the crater rim. I’m still really interested in the work that is being done in the caves, but particularly this year I have not had the time to get involved. Season to season there is always a bit of change as different people come and go, but I suppose that it is more similar than it is different.
Volcanofiles: How’s the season going so far?
Nial: Pretty well I suppose. Everything was up and running in record time this year. Of course no field season would be complete without everything breaking and that has started to happen now with one broken gas sensor, a broken spectrometer and no liquid nitrogen left for the FTIR. But these things are to be expected, most of the equipment is being pushed to its limits here and so some downtime is inevitable. As I already said, the new power system is almost complete and the thermal camera system is working well – I am confident that we will get many more months of data after we leave, even if it doesn’t quite make it through the winter.
Volcanofiles: Thanks, Nial – we look forward to seeing some winter data from the thermal camera!
When everything’s running during the field season, you can see the most recent thermal camera images on the Mount Erebus Volcano Observatory site here.
Our next interview is about a long running project at Erebus. Three dimensional imaging through LiDAR (Light Detection and Ranging) terrestrial scanning technology is a useful way to look at changes in the Erebus landscape – within the crater, and around the ice caves. Drea Killingsworth is the latest student to undertake the scanning work, which began with a scan of the Erebus lava lake in 2008.
Drea did her undergraduate degree at Washington State University in Pullman, Washington, and is now doing her Masters’ degree at New Mexico Tech. For the past two field seasons at Erebus, she has been working with Jed Frechette, from the LiDAR Guys in Albuquerque, New Mexico, and with Marianne Okal and Brendan Hodge, from UNAVCO. And not only has she been doing her own fieldwork but, as expedition chef, she has also been keeping the team fed!
Drea’s project on Erebus has included scanning the lava lake, the main crater, and two ice cave systems: Warren, and Mammoth/Cathedral, both from the inside and on the surface. At Warren Cave, by combining her work this year with previous years’ LiDAR scans and a survey carried out by Aaron Curtis and Nial Peters, Drea can get a detailed image of how things have changed over the last four years.
With several years of lava lake and crater data, it is also possible to find out how the lake surface level has fluctuated, to quantify its surface area, and follow changes in the shape of both the lake and the crater. LiDAR allows us to see parts of the crater that aren’t normally visible when walking around the crater rim.
Volcanofiles: How does LiDAR work?
Drea: The instrument fires a laser (either visible green or near-infrared light), which is reflected off surfaces and returns to the scanner. Using the amount of time it takes to receive the reflection (or backscatter) when it returns, and the known speed of the laser, it is possible to map the point on the surface where the reflection occurs. Thousands of such points (about 50,000 per second) form a point cloud in three dimensions over a ~7-minute medium-resolution scan at each scanner position. In the ice caves, scans from over 40 scan positions throughout the cave are registered together, to form a detailed 3D model of the entire accessible portion of the cave.
Volcanofiles: Can you tell us about the different LIDAR instruments you use? What are some of the features you have to consider when selecting the right instrument for a particular scan?
Drea: We used three different scanners on Erebus this field season: the Leica Scanstation C10, which has a class 3R green laser (with 532 nm wavelength), the Riegl VZ400 and the Optech ILRIS-3D, which both use near infrared (with 1535 nm wavelength).
All of the scanners have similar accuracy (to < 1 cm) and cold ratings (functioning in 0° C – 40 °C) but, because of the different wavelengths, power capabilities and designs, they have different ranges and fields of view (FOV). The Leica is very useful for cave mapping because it has a wide FOV of 360° x 270° while its smaller range of < 1 m – 300 m allows for manoeuvrability in tight squeezes.
The Optech has a maximum range of 1200 m but a FOV of only 40° x 40°. In the caves we are scanning entire rooms so a large field of view allows us to use fewer scan positions to scan a given area. For the lava lake, we do a series of scans from a single FOV, so aren’t limited by this smaller area. The longer range of the near infrared scanner is needed to reach the lava lake, which is 300 m down from our scan position at the crater rim.
Volcanofiles: In addition to doing scans of the caves, you have been working on combining GPS data with LIDAR scans, and matching surface features to those underground. Has this been done before? Can you describe your instrument setup for us?
Drea: The GPS does not work beneath the ice of the caves because it cannot receive satellite messages. To accurately locate the caves on the slopes of Erebus, we had to set up known survey positions outside the caves on the surface. Four known surface positions were scanned using LiDAR and a traverse was made down through the cave entrance to four more survey positions set up inside of the cave. By combining the exterior and interior scans, the location of the interior points can be extrapolated from the GPS locations of the exterior points.
This is not as easy as it sounds! Access to the caves is by rappel and all equipment must be lowered by rope and pulley. To tie the scans together, we had to set up the scanner on a level tripod at the edge of the cave entrance and scan down into the cave to hit an interior target, also on a level tripod. The scanner and target then had to be reversed, carefully lowering the scanner into the cave without moving either of the level tripods.
The now-known exterior and interior survey points are semi-permanent and can be used for later scans to locate the caves in relation to a coordinate system.
Volcanofiles: You’ve spent quite a bit of time in very different Erebus environments. What are some of the challenges specific to your work on the crater, and in and above the caves?
Drea: In the caves, the biggest issue we face is the change in temperature and humidity between the outside entrance and the inside of the cave. On the surface, the air is very dry and the average temperature is -20°C before wind chill. Inside the caves, temperatures are just above freezing and humidity is extremely high, causing problems with fogging of lenses in the scanners as they are moved from cold to warm areas.
These differences can also cause problems in the opposite direction as water condenses and freezes onto the equipment (and the equipment operators!). The ice caves are not formed completely of ice – the floors of the caves are volcanic rock, covered by a layer of decomposing volcanic glass particles from lava bombs. A day of ice cave scanning involves lugging the scanner, tripods, targets and other equipment through tight squeezes, and navigating and levelling equipment by the light of a headlamp.
The lava lake presents a different set of challenges. The scanners are rated to operate down to 0° C -but the average temperature at the rim, before wind chill, can be around -30° C.
We have had to come up with some very interesting make-shift ‘cosies,’ involving everything from hand warmers to garbage bags, just get our equipment to turn on. Wind and cold makes setting up and levelling tripods difficult.
Despite any difficulties, it is such a privilege to be working on such exciting science in such an amazingly beautiful place.
Volcanofiles: Thanks to Drea for talking about her work, sharing her photos, and for the delicious cooking! To finish, here’s another image from this year to show the kind of results she’s been coming up with.
The sampling that is being carried out this year at Erebus also includes work on lava flows around the volcano. Dave Parmelee is a Masters student at New Mexico Tech., and over the past month he has been travelling all around the Erebus caldera by snowmobile and foot to collect samples from lava flows.
Dave completed his undergraduate study at the University of Connecticut in 2006. He worked for a large environmental consulting company, doing fieldwork in and around Connecticut. Dave and his girlfriend, Danielle, have done a lot of climbing and hiking around the USA, as well as spending some time in Guatemala. In 2010 they went to Peru, where they worked as volunteer scientists at a national park.
Earlier this week, Dave let me accompany him on one of his sampling trips. It was one of the highlights of this field season for me, to get out to one of the more remote areas of the volcano and see what he does. We walked around the Side Crater and down to the southern flanks of Erebus, where Dave collected some lava samples for Helium dating.
Previous studies of Erebus lavas by Chris Harpel have involved Argon dating (see the reference below and the previous post for more information), which gives a possible age range for the flows; Dave is collecting samples for Helium (3He) and Chlorine (36Cl) dating on the same flows. Argon dating at Erebus has high uncertainties, resulting in overlap of the possible ages of different lava flows, so it’s hard to tell which order the flows were emplaced in. In addition to this, excess argon found in Erebus rocks can cause ages to appear higher than they actually are. Excess argon can be cleaned out in the lab, but the results from Argon dating at Erebus are considered maximum possible ages.
The isotopes used in 3He and 36Cl dating are formed when an outcrop is exposed to cosmic rays – so by measuring these isotopes, it is possible to calculate the length of time for which the lava flows have been exposed to the surface. Snow cover around Erebus this year is greater than usual, but this is helpful for Dave’s sampling, as those locations currently exposed are likely to be the same ones that are usually exposed.
There are many challenges in sampling from these lava flows. Snow and ice cover change from year to year. It takes two metres of snow cover to completely shield a rock from cosmic rays, so Dave has to make a judgement on whether a flow is likely to have been exposed constantly. Not all of the flows marked in the published map are well exposed. Dave uses a combination of this map and aerial photographs to identify sample locations.
Near the summit cone, there are large volcanic bombs in addition to the lava flows, so Dave has to distinguish between the two before he can select a place to sample. The location must be relatively flat, to ensure that there is little shielding from cosmic rays and that the production of cosmogenic isotopes will have been at the highest rate possible. He has to avoid locations with steep slopes or topography that may have blocked the outcrop from exposure to cosmic rays – the more open the area is, the better. Samples have to come from the top few centimetres, also to ensure exposure.
Another requirement for 3He dating is the abundance of suitable crystals: He isotopes for dating are, at Erebus, mainly found in a mineral called pyroxene. This tends to occur as small crystals, often incorporated into larger crystals of Erebus anorthoclase. Helium 3 occurs naturally, before exposure to cosmic rays, so the helium inherited from eruption needs to be isolated from that which is produced cosmogenically. By contrast, 36Cl is only produced by cosmic rays.
Once he has found a suitably flat area on the flow (with visible pyroxenes if the samples are to be used for 3He dating) Dave can take notes and photos of the sample location. Things he has to consider include whether the rocks are still in their original location; usual extent of snow cover; the shielding from the horizon, which he measures by taking the inclination of the topography above the flat horizon at different points; and the extent to which the rocks have been eroded.
At this stage, Dave gets to work with a hammer and chisel. Although the rock can be difficult to break up, it also requires some precision to avoid losing any pyroxenes for 3He dating. A minimum of 300 milligrams of pyroxene are required, which may mean carrying back a couple of kilograms of rock. Chlorine sampling, on the other hand, requires a bulk sample of rock, so can be slightly quicker. It can be several hours work to drive and then walk out to a sample location, find a good site, and get enough sample.
Over the next few months, Dave will be isolating the isotopes that are of interest – the 36Cl at New Mexico Tech., and the 3He at Woods Hole Oceanographic Institute in Massachusetts. The amounts of Cl and He isotopes can then be measured. The production rate for the cosmogenic isotopes is calculated, using published equations, from measurements of the isotopes in the lab combined with the field notes, but the result is an ideal production rate – in reality there may have been shielding from topography and snow which is no longer present. As a consequence the age calculated from the ideal production rate may well be younger than the actual age. Combined with the argon dates, this gives a better constraint on the ages of the flows and we can start to understand how periodic the eruptive behaviour of the volcano has been.
Despite all his experience in the outdoors, spending a month on a volcano has been a new and exciting experience for Dave. He has learned to drive a snowmobile on technical terrain and travelled out to parts of the caldera that have not been visited in a long time.
Thanks to Dave for the information and a great afternoon out in the field!
For the paper on Argon dating of Erebus lava flows: Harpel, C.J., Kyle, P.R., Esser, R.P., McIntosh, W.C., Caldwell, D.A., 2004. 40Ar/ 39Ar dating of the eruptive history of Mount Erebus, Antarctica: summit flows, tephra, and caldera collapse. Bulletin of Volcanology, 66(8): 687-702.
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.