A sedimentologist’s guide to volcanic particle grain size (and foetal development)

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Volcanology has a lot of jargon.  This can obscure simple information, even from trained geologists.  Students are often put off by words like lapilli or tephra that sound terribly technical, even though they only mean little stones (in Latin) and ashes (in Greek).  For this reason, I find it helpful to ignore the volcanology part and just treat the deposits as you would any other sedimentary rock.  After all, studying explosive eruptions is just sedimentology in the atmosphere, and a volcanologist’s lapilli tuff is simply a breccia to any other geologist.

Pregnancy books are obsessed with fruit and vegetables, which are compared to the size of the growing foetus in week by week guides to development.  Such comparisons are helpful to people who have never seen a ruler.  They are no substitute for real numbers, though.  Is a kumquat bigger than a fig?  I have no idea.  Nevertheless, they are fun to visualise.

For these reasons, I have compiled the following table that compares the size of volcanic particles, sediment grains, fruit and growing foetuses.  I hope that it will go some way towards demystifying volcanology to sedimentologists, and gynaecology to gardeners.

Comparison of volcanic and sedimentary grains, and fruit and foetuses


Click to enlarge.

Using this table, you can imagine volcanic processes in a whole new way. For example, a block and ash flow caused by a collapsing lava dome can be imagined as a hot fruit salad, containing everything from poppy seeds to (whole) pumpkins, thundering down the side of the volcano.  There is also the question of the size of grains in a volcanic ash cloud;  aircraft that sampled Icelandic examples them found grains about the size of sperm-heads, while the grains that are found on the ground can be two sperm long.

Further reading

  • Metric babies: an old rant of mine against reporting birth weights in imperial units.
  • The table includes a column for the phi scale of grainsize, which is used when data are collected by sieving sediments.  Read more about it on the Wikipedia grain size page.


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Bárðarbunga: satellites and computer models quantify sulphur dioxide release

This is a guest post by Anja Schmidt, an Academic Research Fellow at Leeds University, and Claire Witham of the Met Office’s Atmospheric Dispersion Group. It describes Anja’s paper quantifying the sulphur dioxide gas release from the 2014 eruption of Iceland’s Bárðarbunga volcanic system at Holuhraun that she wrote with Claire and a large, international, team of scientists, including myself.

The biggest Icelandic eruption in more than 200 years

It is very likely that you remember the 2010 eruption of Eyjafjallajökull in Iceland and the ensuing travel chaos that resulted from volcanic ash drifting in and out of European and North Atlantic airspace for more than a month. In total 10 million travellers were stranded and the airline industry incurred financial losses in excess of 1.7 billion US dollars. Then, in 2011, another Icelandic volcano called Grímsvötn had an eruption about 100 times as powerful as Eyjafjallajökull, producing twice the amount of volcanic ash. Yet far fewer people will remember it due to its limited impact on aviation.

Ash-producing Icelandic eruptions affect Europe frequently – about once every three to five years. However, there are other types of volcanic eruptions in Iceland that typically produce very little volcanic ash but lots of lava and toxic volcanic gases. We generally refer to these as effusive eruptions and specifically as fissure eruptions. The biggest of these fissure eruptions produce lava volumes that fill up to 100,000 Olympic-sized swimming pools per day for months to years. These big eruptions occur on average every 200 to 500 years, whereas smaller-volume fissure eruptions occur every 40-50 years on average. In the early hours of 31 August 2014, the biggest fissure eruption in more than 200 years began about 45 km away from the Bárðarbunga volcano at the Holuhraun lava field (shown in Figure 1). This eruption was truly spectacular, lasting six months and presenting some great scientific opportunities. Yet not many people outside Iceland would have taken note because there was no disruption to air travel.


Figure 1. The top panel shows the locations of Icelandic towns and volcanoes, including the Bárðarbunga volcanic system. The red rectangle outlines the region shown in the bottom panel. The map in the bottom panel shows the Bárðarbunga caldera (dashed grey line) and the lava flow field and vents of the 2014-15 eruption at Holuhraun. Figure from Schmidt et al., (2015). Click image to enlarge.

No volcanic ash, but lots of lava and sulfur dioxide

During its first month, the eruption at Holuhraun was extremely powerful spewing fountains of lava up to 150 meters high (see Figure 2) along a 1.5 km long crack in the earth’s crust (which puts the “fissure” in “fissure eruption”). The eruption discharged lava at a rate of more than 200 m3/s, which is equivalent to filling five Olympic-sized swimming pools with lava each minute. Six months later, when the eruption ended, it had produced about 1.5 km3 of lava, covering an area of around 86 km2, which is the same area as Manhattan. In stark contrast to the eruptions of Eyjafjallajökull 2010 and Grímsvötn 2011, the eruption at Holuhraun produced negligible amounts of volcanic ash, but a lot of lava and sulfur dioxide. Sulfur dioxide is a toxic gas that was emitted into the lowermost atmosphere up to 6 kilometers high.

Credit: Michelle Parks

Figure 2. Lava fountaining above the volcanic fissure at Holuhraun (Iceland) in September 2014. Photo taken by Michelle Parks (University of Iceland). Click image to enlarge.

It became clear very quickly that the eruption was producing truly staggering amounts of sulfur dioxide, but continuous ground-based monitoring and measurement of the gas fluxes was very challenging due to the remoteness of the eruption site and the weather conditions in Iceland. This is where satellite data of the volcanic sulfur dioxide plume came to the rescue. In our new study we analysed such satellite data and combined these with computer modeling using the Met Office’s NAME model. This allowed us to track and compare the dispersion of volcanic sulfur dioxide (see Figure 3). It also enabled us to assess how much sulfur dioxide was emitted independent of the ground-based measurements in Iceland. We found that at its most powerful the eruption emitted about 120 kilotons of sulfur dioxide per day, which is eight times more than the daily amount of sulfur dioxide emitted from all man-made sources in Europe. We also calculated that during September 2014 a total of 2.0±0.6 million tons of sulfur dioxide was emitted by the eruption. This makes Holuhraun the largest volcanic sulfur pollution event in more than 200 years in Iceland. Its bigger sister, the Laki eruption took place in 1783-1784 CE and produced, over the course of 8 months, about ten times more lava and about 60 times more sulfur dioxide than Holuhraun did in September 2014. The Laki eruption is thought to have caused cooling of climate and substantial environmental stress across Europe in the mid-1780s.

The panel on the left shows the distribution of sulfur dixoide from the 2014-2015 eruption at Holuhraun on 6 September 2014 as seen by the satellite instrument called OMI which is onboard NASA's Aura spacecraft. The panel on the right shows the distribution of sulfur dioxide as predicted by the computer model simulation for the same day. Figure from Schmidt et al., (2015).

Figure 3. The panel on the left shows the distribution of sulfur dixoide from the 2014-2015 eruption at Holuhraun on 6 September 2014 as seen by the satellite instrument called OMI which is onboard NASA’s Aura spacecraft. The panel on the right shows the distribution of sulfur dioxide as predicted by the computer model simulation for the same day. Figure from Schmidt et al., (2015). Click image to enlarge.

Detection and monitoring of volcanic sulfur pollution

Over the course of the eruption, air quality monitoring stations in Iceland recorded unprecedented levels of sulfur dioxide, often significantly exceeding the current 10-minute mean air quality standard for sulfur dioxide set by the World Health Organization (WHO) to protect public health. However, volcanic pollution was not confined to Iceland: our study shows that volcanic sulfur dioxide was transported over large distances and detected by air quality monitoring stations up to 2750 km away from Iceland. For instance, on 6 September 2014 volcanic pollution reached Ireland where air quality monitoring stations recorded short-lived (up to 24 hours) spikes in surface sulfur dioxide concentrations. Air pollution regulations introduced in the 1980s mean that sulfur dioxide levels due to emissions from industrial sources are very low nowadays; hence the concentrations recorded on 6 September 2014 were really unusual. Other stations across Northern and Central Europe recorded similar pollution episodes during September 2014.

The air quality monitoring stations across Europe were essential for the detection and characterization of the air pollution events resulting from the eruption. These observations and our model simulations demonstrate that volcanic pollution from Icelandic fissure eruptions can easily reach Northern Europe and degrade air quality temporally. Away from Iceland there was no risk of long-term detrimental health effects because exposure to volcanic pollutants was brief. Right now the number of sulfur dioxide monitoring stations across Europe is in steady decline because sulfur dioxide concentrations are usually very low as a result of successfully legislated reductions of man-made emissions since the 1980s. We argue that existing air quality monitoring stations ought to be retained or extended to monitor volcanic pollutants from future eruptions in Iceland. This would facilitate the characterization and mitigation of volcanic gas and aerosol particle hazards, which could be severe in the event of a large-magnitude Icelandic eruption like a repeat of the 1783-1784 CE Laki eruption in Iceland. The Scottish Environment Protection Agency is going to extend their air quality monitoring of sulfur dioxide and volcanic ash in response to the eruptions in Iceland since 2010.

The next eruption…

Every eruption is different and what the eruptions of Holuhraun and Eyjafjallajökull have shown is that future Icelandic eruptions will pose new hazards and challenges for science and society. With each eruption we learn more about the volcanic processes involved and broaden our understanding of how to best utilize observations and computer models to understand these hazards and inform decision makers. We cannot predict the next eruption, but recent activity proves that in Europe we should prepare for the impacts of not only volcanic ash but also volcanic gases and airborne particles.

Further reading

Our paper describing the results is open access, which means that anyone can download and read it for free. The full reference is:

  • Schmidt A, Leadbetter S, Theys N, et al (2015) Satellite detection, long-range transport and air quality impacts of volcanic sulfur dioxide from the 2014–15 flood lava eruption at Bárðarbunga (Iceland). J Geophys Res Atmos 2015JD023638. doi: 10.1002/2015JD023638

I wrote a pair of blog posts describing fieldwork at the lava flow itself. You can can read them at:

There is also some background about even larger lava eruptions than at Bárðarbunga, including Laki 1783-84, in this post:

UPDATE: I was on the BBC Radio Scotland Drivetime show discussing the results of the study.  You can listen to the interview below:

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Installing Linux on Lenovo Thinkpad 11e

I recently updated my laptop to a Lenovo Thinkpad 11e.  The laptop meets military specifications for shock, humidity, temperature and dust and I swapped the hard drive for a solid state drive that has no moving parts.  It should be ideal for geological fieldwork.

I installed Linux Mint 17 XFCE, which is based on Ubuntu 14.04 Long Term Support.  Mostly things worked out of the box, but there were a few tweaks that I had to make to get everything as I wanted.  These notes are here to remind me and in case they are helpful to anyone else.

Enable brightness changing via function keys

Initially, the brightness function keys wouldn’t actually adjust the brightness.  The icon would appear but the brightness remained the same.  To fix:

  • Add the following to /etc/default/grub:
  • Then run:
    sudo update-grub

Swap End/Insert keys

By default, the function keys are set up to change brightness, volume etc.  Turning on FnLock sets them to the more useful F1 to F12 but has the unwelcome side effect of changing the End key into Insert.  I don’t know who thought that was a good idea.

There are a number of solutions described online that use xmodmap.  The downside with these is that they are forgotten when the laptop is suspended.  To make the change persistent:

  • Edit /usr/share/X11/xkb/symbols/pc so that it reads as follows:
    key  <INS> {    [  End        ]    };
    key  <END> {    [  Insert     ]    };
  • Then delete old keymaps:
    sudo rm /var/lib/xkb/server*.xkm

Turn CapsLock into another Ctrl key

This is more for personal preference, as is more comfortable when using Vim text editor, where the aim is to keep your fingers on the home row as much as possible.

  • Create a file /usr/share/X11/xorg.conf.d/10-keyboard.conf with the following contents:
    Section "InputClass"
            Identifier "system-keyboard"
            MatchIsKeyboard "on"
            Option "XkbOptions" "ctrl:nocaps"

Add myself to dialout group

This was necessary because my GPS connects via USB to serial adapter as /dev/ttyUSB0.  I needed to be part of the dialout group to have permissions to access it.

  • Run:
    sudo usermod -a -G dialout my_username
  • Then logout and back in.

Fix unstable wifi connection

Sometimes the wifi connection drops out intermittently, or is just slow.  The wifi card is an Intel Dual Band Wireless AC7260.  There seem to be a number of potential reasons and solutions for this online, ranging from hardware faults, problems with the router, power management and old drivers.  There are a number of questions about this on AskUbuntu, including here and here.  It isn’t too much of an issue for now, but when I find a solution I’ll update this post. <UPDATE: this went away when I upgraded to kernel 3.13.

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How big are the grains in a volcanic ash cloud?

Ever since European airspace was temporarily shut down during the 2010 eruption of Eyjafjallajökull, aircraft in the region have been allowed to fly through parts of volcanic ash clouds where the concentration is low. Satellite-based infrared sensors can be used to estimate ash cloud concentration, as well as other parameters such as grain size and height, in a process called a retrieval. Last month’s How do satellites map volcanic ash clouds? post explains how it works. The retrieved information, as well as data from computer simulations and other satellite- and ground-based sensors, is used to map the likely concentrations of airborne volcanic ash. The results inform decisions about where aircraft can fly.

There are few direct measurements from within ash clouds, so we must look for other information in their deposits on the ground. At large distances from a volcano (over 500 km) these exist as cryptotephra (hidden ashes) that are extracted from peat bogs and lakes. These grains were some of the largest particles within the cloud from which they fell, so are not representative of the average size at that point. However, they must be a significant component of the ash clouds closer to their source.

This week we published a paper showing that Icelandic cryptotephra deposited in the UK and northwest Europe are much bigger than satellite infrared retrievals would suggest. Investigating the difference in results between measurement types, we found that this is partly because current satellite retrieval algorithms are biased towards smaller sizes. Our results will help quantify the uncertainty in estimates of volcanic ash cloud properties. This is important because if aircraft are to fly through low concentrations of ash, you need to know how sure you are that the concentration really is low. They also suggest that incorporating the irregular shapes of volcanic ash grains in computer simulations and satellite infrared measurements will lead to improved results.

The main findings are explained below.

Understanding the discrepancy between tephrochronology and satellite infrared measurements of volcanic ash

The paper was divided into three sections, reflecting three areas of study: tephrochronology, ash transport computer models and satellite-based infrared retrievals. Each section reports a new result. Another aim of the paper was to increase understanding between these different fields.

Big grains go far…

The graphs below show the size distributions of Icelandic ash grains from across the UK, including samples collected by the public during the Eyjafjallajökull 2010 and Grímsvötn 2011 eruptions. The top graph has results obtained by measuring grains down a microscope, the bottom results were collected by a laser technique. The microscope method misses grains <10 microns diameter, but both methods give similar peaks showing that the smallest grains are a minor component.


The size of cryptotephra grains from the UK and northwest Europe. Image from Stevenson et al. (2015), Atmos. Meas. Tech. doi:10.5194/amt-8-2069-2015. See paper for full caption and details.

The median length of the cryptotephra grains ranges from 17-70 microns, with 5% of grains coarser than 45-125 microns (similar to the width of a human hair). These are similar to the lengths of individual grains reported elsewhere. In comparison, published median diameters retrieved from satellite infrared data are less than ~6 microns (similar to the size of a red blood cell) and represent size distributions that contain tiny numbers of grains larger than 30 microns.

…in agreement with model predictions…

Imagine dropping a handful of volcanic ash particles from 10 kilometers high. This is similar to the height of the 2010 Eyjafjallajökull plume, but a big eruption could carry material two or three times higher. The next graph shows how far ash grains of different sizes falling in a constant wind would be carried before they reached the ground. It represents a very simplified scenario, with no weather or particle-particle interaction, but gives an idea of the sizes of particles that can be expected at different distances. The different lines represent different particle shapes. Data from Eyjafjallajökull are shown for comparison; many of these particles were erupted when the plume was low.


Calculated travel distances for ash grains of different sizes and shapes. Image from Stevenson et al. (2015), Atmos. Meas. Tech. doi:10.5194/amt-8-2069-2015. See paper for full caption and details.

Even under the modest conditions simulated here, ash grains with simple,
spherical, shapes up to 41 microns diameter can be airborne for 24 hours, and those up to 29 microns could reach London. Using more complex and realistic shapes increases the travel distance, sometimes by up to a factor of three.

…and satellite infrared retrievals underestimate their size.

To understand how coarser-grained ash clouds would appear in satellite retrievals, we created a series of virtual clouds with different grain size distributions in the Met Office computers. Then we calculated simulated satellite images that showed how they would appear to satellite-based infrared sensors. Finally, we fed that information into the ash-detection software and ‘retrieved’ ash cloud properties such as grain size and concentration. The assumptions used to simulate the ash clouds are the same as those used in the retrieval, so this is a test of the algorithm and not of the physics of ash detection.

In the graph below, the x-axis is the size that we put in and the y-axis is the retrieved particle size. It is given in terms of the effective radius, which describes a distribution of particle sizes. The dotted line shows the answer that we would expect for a perfect retrieval. The diamonds mark the mean retrieved effective radius value.


Comparison between input and retrieved particle sizes from simulated satellite images. Image from Stevenson et al. (2015), Atmos. Meas. Tech. doi:10.5194/amt-8-2069-2015. See paper for full caption and details.

The retrieval algorithm uses various measurements, including the brightness temperature difference (BTD) caused by particles with a radius of less than 6 microns, to identify pixels in the images that contain volcanic ash. It then looks for the combination of ash cloud grainsize, concentration and height that would give the best match to the observations (including the BTD) based on the assumption that the grains are all dense spheres.

When the grainsize of the simulated ash cloud increases, the algorithm begins to miss more and more ash-containing pixels. This happens because the BTD effect gets weaker as the proportion of particles smaller than 6 microns decreases, so the cloud becomes harder to detect by this method. Trained forecasters may still be able to identify contaminated airspace by using other data, but retrievals of ash properties in these regions are not possible.

The graph shows that the mean effective radius is retrieved correctly for small particles, but reaches a maximum of around 9 microns for larger particles. This result is interesting because an ash cloud with an effective radius of 16 microns could contain a significant proportion of cryptotephra-sized grains. However, those values are rarely reported and published values are always 9 microns or less.

At larger grainsizes, it becomes harder to find a combination of parameters that give a good match to the observations. The retrieval becomes more strongly influenced by where you tell it to start looking. The influence of our chosen value of 3.5 microns, which is similar to airborne measurements of dilute ash clouds, is visible on the plot. Retrievals of the mass of volcanic ash also show deviations of over 40% from the input values.

Moving on from spherical grains?

Our results demonstrate that the grains in a volcanic ash cloud, hundreds of kilometers from the source, are up to several tens of microns in diameter. The proportion that these cryptotephra-sized grains represent within the airborne cloud is still unknown. Bias towards small grain sizes by retrieval algorithms may explain differences between cryptotephra- and satellite retrieval-based size distributions in such locations.


Retrievals have also been made of ash clouds close to their source volcano. Here, the deposits contain grains with diameters of hundreds of microns. Again, the retrieved sizes are very low. This suggests two things about satellite retrievals:

  • They are able to recognise a wider range of grain sizes than the current theory suggests should be possible.
  • The underestimation of grain size is even more significant close to the source volcano.

We suggest that this is a consequence of the assumption that volcanic ash grains are dense spheres. This forces any ash cloud exhibiting a BTD to be interpreted as being dominated by fine-grained particles.


Photographs of cryptotephra grains. The scale bar is 10 microns long. These are not dense spheres. Image from Stevenson et al. (2015), Atmos. Meas. Tech. doi:10.5194/amt-8-2069-2015. See paper for full caption and details.

Cryptotephra grains are extremely irregular and bubbly, as demonstrated in the image above. If it is possible for bubbly particles to cause BTD effect at larger particle sizes, as was suggested by a recent study, then ash cloud size distributions could be much coarser than current interpretations suggest. Developing alternatives to the dense spheres approximation is therefore likely to improve agreement between ash cloud retrievals and the deposits on the ground.

Further reading

Our study was published in Atmospheric Measurement
, which is an open access journal. This means that anyone can download and read the full article for free by clicking on the link below:

  • Stevenson JA, Millington SC, Beckett FM, Swindles GT, Thordarson T (2015) Big grains go far: understanding the discrepancy between tephrochronology and satellite infrared measurements of volcanic ash. Atmos Meas Tech 8:2069–2091. doi: 10.5194/amt-8-2069-2015

Atmospheric Measurement Techniques has an interactive peer review system, so the original version of the paper is also online, as a discussion manuscript, along with the reviewers’ comments. You can read those here:

  • Stevenson JA, Millington SC, Beckett FM, Swindles GT, Thordarson T (2015) Big grains go far: reconciling tephrochronology with atmospheric measurements of volcanic ash. Atmos Meas Tech Discuss 8:65–120. doi: 10.5194/amtd-8-65-2015

There are many other blog posts on volcan01010 about ash clouds and Icelandic eruptions, such as Ash cloud closes UK airports: what are the chances?, Grímsvötn 2011 UK ash deposition and A history of ash clouds and aviation. There is a full list on the Every post ever page.

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How do satellites map volcanic ash clouds?

Explosive eruptions can spread volcanic ash across continent-scale distances. Ash from the 2011 eruption of Chile’s Puyehue-Cordón Caulle volcano went right around the globe. The only realistic way of monitoring them is from space. Most people are familiar with the beautiful photos (visible light images) that satellite-based sensors can take but the most useful information for mapping volcanic ash clouds actually comes from invisible infrared light. This can be used to see ash clouds in the dark, to measure their altitude and even to give an indication of their grainsize and concentration. This post uses images from a spectacular eruption at Chile’s Calbuco volcano last week to explain how.

Visible light images

Imagine you are in charge of mapping the Calbuco ash cloud. Where is the atmosphere contaminated by ash? The middle of the cloud is obvious, as the tan-coloured ash is clear to see. But how can you define the edges? Do those wispy, white clouds in the southeast corner contain volcanic ash or are they just water droplets? Is there ash in that hazy region to the southwest?

Natural-colour image of the Calbuco ash cloud, taken at 18:35 UTC on 23 April by the MODIS instrument on NASA’s Aqua satellite. Source: NASA Earth Observatory. Click the image to enlarge.

These questions can be tricky, but they aren’t your biggest problem. The airline industry needs updated maps every six hours, but night is coming soon. Visible light images rely upon reflected sunlight so they are only useful during the day. You cannot map ash clouds using satellite photos alone.

Infrared images

Everything in the Universe emits energy as electromagnetic radiation (e.g. heat, light, x-rays). Hotter objects radiate more energy, and at shorter wavelengths. You can calculate the amount of energy radiated at different wavelengths, by an object of a given temperature, using Planck’s law, which is just a way of saying that white hot is hotter than red hot, but with maths.

Most of the radiation emitted by the things around us e.g. clouds, hills, trees, buildings, students, traffic cones, is in the infrared region of the electromagnetic spectrum. It’s invisible to our eyes, but satellites have sensors that are very sensitive to it. Radiation is emitted all the time, so infrared measurements can be made during the day or the night. Furthermore, the amount of energy can tell us about the temperature, because hot things radiate more.

Long wave infrared (11.45 µm) image of the Calbuco plume taken by the VIIRS instrument on the NOAA/NASA Suomi satellite on the morning of 23 April. Source: NASA Earth Observatory. Click the image to enlarge.

The image above looks like a black-and-white photograph. It isn’t. It is a map of long wave infrared energy emission. It has been drawn with a back-to-front colour scheme, where the objects emitting the most energy are darker. The ocean is dark because it is warmer than the land. The mountain tops are pale because they are high and cold, but only some of the have snow on them (compare the infrared with the visible image above). This type of colour scale, where colder is lighter, is typical for weather satellite data because it makes cold, high clouds appear white, just as they do in photographs.

Brightness temperatures

Planck’s law lets you estimate of the temperature of an object, based on the amount of energy that it radiates. This is known as the brightness temperature (BT). If you know how the temperature of the atmosphere changes with altitude, for example from weather balloon (radiosonde) data, and if you assume that the ash cloud is at the same temperature as the atmosphere around it, then you can convert the BT into an estimate of the altitude of the cloud. The BT image below has a pseudocolour scheme that has been set to highlight changes in temperatures in the highest, coldest, clouds. It shows the highest plume (coldest temperatures) directly above the volcano. Based on the BTs, the maximum height of the plume was estimated to be 18-20 km.

Brightness temperature image derived from data collected by the MODIS instrument on NASA’s Aqua satellite at 06:35 UTC on 23 April. Source: NOAA/CIMSS Volcanic Cloud Monitoring website. Temperatures are in Kelvin, which is the number of degrees Celsius above absolute zero e.g. 210 K = -63°C. Click to enlarge.

For nearly a decade, ash cloud heights have also been measured using a laser fired down from space by the CALIPSO satellite. It only measures along a line beneath the satellite track, so it cannot be used to make maps, but the data give precise measurements and have lots of vertical detail. They show that ash clouds are often made up of many thin layers at different levels, rather than just one single body.

Brightness temperature difference

Calculating the altitude of clouds from their BT is useful, but it doesn’t tell you whether the cloud that you are looking at is volcanic ash or just weather. The clouds that are highlighted in the lower part of the previous image are not volcanic. This is where the brightness temperature difference (BTD) method comes in. It is also known as the split window or reverse absorption technique and has been used for over 20 years. It works by comparing the brightness temperatures measured using two different wavelengths of infrared radiation, which behave differently as they pass through clouds of volcanic ash or water droplets. The diagram below shows how the difference arises.

The brightness temperature difference occurs because some radiation from the surface is absorbed as it passed through clouds. Volcanic ash absorbs more at shorter wavelengths than weather clouds.

The left hand diagram shows the radiation emitted by the ground (or ocean) and by thick clouds passing directly to the satellite sensor. The BTs calculated using the signals from the two wavelengths are the same. On the right hand side, the radiation from the surface passes through a semi-transparent cloud of water droplets (left) or volcanic ash (right) and is partly absorbed by the particles within it. The important factor here is that different wavelengths behave differently, depending on the composition of the cloud. Water droplets, water vapour and ice particles preferentially absorb infrared at longer wavelengths than volcanic ash. The BTD effect is strongest when the particle diameter is similar to, or slightly less than, the wavelength of the infrared light i.e. 10-12 µm. The BT recorded above an ash cloud on the 10.8 µm channel is lower than on the 12.0 µm channel. Subtracting one from the other gives the BTD; negative values are an indication that the cloud contains volcanic ash.

The BTD image below highlights the location of the volcanic ash cloud. The strength of the signal depends mainly on the ash concentration, the grainsize and the cloud height. The weather clouds to the south of the eruption are no longer highlighted. Notice also that there is no BTD very close to the volcano (e.g. the purple region in the BT image above). Here, the ash is so concentrated that the cloud is opaque and blocks all radiation from below. External factors such as the presence of weather clouds above or below the ash cloud, high atmospheric moisture levels or extremes of ground temperature also affect the signal and can lead to false alarms or undetected pixels. The detection limit of this method is around 200 µg/m³, which corresponds to “low concentration” in the post-Eyjafjallajökull European flight concentration zones.

Brightness temperature difference image derived from data collected by the MODIS instrument on NASA’s Aqua satellite at 06:35 UTC on 23 April. Source: NOAA/CIMSS Volcanic Cloud Monitoring website. Click to enlarge.

The BTD signal can also be incorporated into false colour images such as the RGB ‘dust’ scheme. These are designed to make ash clouds easy to identify. In regions of the world where aircraft have to Avoid All Ash, images such as this are very useful for mapping out no-fly zones.

False colour RGB ‘dust’ image derived from data collected by the MODIS instrument on NASA’s Aqua satellite at 06:35 UTC on 23 April. Source: NOAA/CIMSS Volcanic Cloud Monitoring website. Click to enlarge.

Retrieval of ash cloud properties

Retrieval algorithms use satellite infrared data to estimate the properties of ash clouds. This is where things get really interesting. Such data have become more important since the Eyjafjallajökull eruption of 2010, as flight restrictions in European airspace are now based on zones of varying concentration. They work because it is possible to estimate what the BTs at different wavelengths would be for ash clouds with varying properties. For example, if you assume that the ash particles are tiny little spheres, there are equations that can tell you how they absorb different wavelengths of infrared light. Other inputs to the calculations include particle size, ash concentration, ash cloud height and thickness, and ground temperature. The results of the calculations are compared with observations to see what gives the best match. Retrievals can be carried out on any pixel that is identified as ash.

The particle size and the concentration are the most important factors. Ash clouds contain particles of a range of different sizes. The effective radius is the particle size that has the equivalent optical properties to the ash cloud as a whole and is used to simplify calculations. The concentration is represented by the mass loading, which is the total mass of ash between the satellite and the ground. If the thickness of the cloud is known, this can be converted into a concentration. For example, for a 1 km thick cloud, 200 µg/m³ (low concentration zone) corresponds to a mass loading of 0.2 g/m², while 2 mg/m³ (medium) is 2 g/m² and 4 mg/m³ (high) is 4 g/m².

The simplest retrievals use a fixed cloud height and find the effective radius and mass loading that best fit the observations on the two channels of infrared data that were used to calculate the BTD. Others incorporate data from a third infrared channel and either retrieve the cloud height as well, or retrieve a series of other parameters from which the effective radius, mass loading and cloud height can be calculated.

Retrieved volcanic ash cloud parameters (effective radius, column loading, cloud height) derived from data collected by the MODIS instrument on NASA’s Aqua satellite at 06:35 UTC on 23 April. Source: NOAA/CIMSS Volcanic Cloud Monitoring website. Click to enlarge.

The figure above shows retrieval parameters for the Calbuco cloud. When interpreting the retrieved data it is important to bear a number of points in mind:

  • Retrieval quality is affected by the same external factors affecting the BTD (e.g. clouds below the ash).
  • The retrievals are made on the assumption that the particles are dense spheres.
  • The effective radius represents a size distribution, with diameters typically ranging from around 0.2 to 2 times the effective radius value.
  • Retrieval algorithms choose the best-fitting values to the observations, but they are not unique. Many combinations of effective radius, mass loading and cloud height can give the same BTD result.

The Calbuco data show the mass loading (ash cloud concentration) decreasing away from the volcano. They also show that the smallest particles are at the north eastern edge of the cloud.

More than just photos

This post showed how satellite infrared data can not only be used to map ash clouds but also to make estimates of their properties. There is a lot more to satellite data than just photos, and we didn’t even touch on satellite detection of volcanic sulphur dioxide, which can also be used to track volcanic ash (provided that the ash and the gas disperse together) and is important for understanding the effect of volcanic eruptions on climate. Neither did we mention computer models of ash dispersion, which are now being used alongside satellite data (e.g. by generating simulated satellite images) to improve the results from both techniques.

It’s amazing how much detailed information is now available during eruptions, online and in real time, for anyone to read. Hopefully this post will help you to get the most out of it.

Sources of satellite images

The visible and infrared images used in this post came from the NASA Earth Observatory page, as linked to by Calbuco calms down after explosions post on Eruptions Blog. All of the MODIS images are from the NOAA/CIMSS Volcanic Cloud Monitoring Web Portal, which provides access in near real time to data covering much of the globe, for the previous 35 days.

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How to watch the eclipse on the side of your car

There will be a solar eclipse visible in the UK on Friday (20 March), peaking at around 09:30-09:45 in the morning, depending on where you are.  This BBC News article describes when and where it will be visible.  It also links to the Royal Astronomical Society webpage, which has a How to Observe an Eclipse Safely guide.  The secret is not to look at the Sun.

This post describes a method that is a cross between the RSA’s pinhole and binocular methods.  We used it to watch an eclipse when we were doing geological fieldwork Tenerife in September 2005.

Project the eclipse onto the side of your car

You will need

  1. A piece of card or paper (to create a shadow)
  2. Binoculars (to magnify and focus the image)
  3. A car, or similar smooth surface (to project the image onto)

Rodrigo del Potro projecting an eclipse onto a car. Make the image larger by moving the binoculars further from the car.


Using the paper casts a shadow on the car that makes the image from the binoculars much easier to see.


  1. Make a hole in the middle of the card about the size of the eyepiece lens on the binoculars.
  2. Put the card against the binoculars so that one of the eyepieces is over the hole.
  3. Hold both in front of the car so that the paper casts a shadow.
  4. While looking at the car, angle the binoculars towards the Sun.

The final stage is the tricky one, as the Sun is a small target and it’s hard to know exactly where the binoculars are pointing.  The shadow cast by the binoculars onto the sunny side of the paper can help you to position them – try to make it as small as possible.  You can sharpen the image using the focus control on the binoculars.


There are obvious ways in which to improve this, such as putting the binoculars on a tripod and using tape to attach them to the card.  The appeal of this method is that it is quick, and it only needed things that we already had with us.


This should be totally obvious, but I’ll say it anyway:


If you want to know why, just try holding your finger where your eye would be.

The forecast is for clouds in Edinburgh on Friday morning, but maybe we’ll get lucky and there will be a break so we can see it.  Enjoy!

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Four years of volcan01010: Highlights of 2014

Four years!  Well, that’s gone by pretty quickly.  Check out this post for highlights from the last 12 months.  As always, expect Iceland, volcanoes, Python and open source software/GIS.

Iceland and volcanoes (volcan…)

This year on the blog has been dominated by basalt lava, and in particular the eruption of Bárðarbunga volcano at Holuhraun, Iceland.  I joined the team of University of Iceland scientists working there at the start of September and wrote two posts describing the eruption and particularly the effects of the sulphur dioxide gas that it was producing.  Part of the second post was quoted by the NASA Earth Observatory website, which also featured one of the videos, in an article about two great satellite images of the lava flow.

Photos, explanations of what the eruption was doing and descriptions of how it is to work there.

Videos of the crater and the lava flows including sampling and mapping the outline.

The other most popular Bárðarbunga post was a guest article by Ed Baines (@edwinbaynes).

The post is about the powerful floods that could result from a subglacial eruption at Bárðarbunga.  Ed describes how the current Jökulsá á Fjöllum canyon was produced by giant floods of the past.  His research into this was published this month.

Four other volcano-related posts you CANNOT AFFORD to miss:

  1. The distance this volcanic ash travelled to reach Ireland will amaze you!
  2. The secret is out about microbes’ new Eyjafjallajökull lava diet.
  3. You NEED to read this reliable information about Icelandic flood lavas.
  4. 20 journals that volcanologists just keep citing.

Open source software and GIS (…01010)

QGIS is an open source GIS package that’s especially great for putting together maps for printing.  It’s also really quick to import data from a csv file or spreadsheet.  The OpenLayers plugin loads maps from online sources e.g. Google Satellite, Open Street Map, Bing Aerial that you can use as a background.

This post outlines the routine that I follow at each sample site in the field.  It describes how to use handheld GPS (or smartphone GPS tracking app) alongside a normal camera to geotag your photos and logs, using the gpsbabel and GpsPrune software.

A handy script for anyone working with geochemical data for igneous rocks.  It adds fields with the names of different magma compositions to plots of Total Alkalis vs Silica.

Pretty picture

The sole purpose of the (Almost) 3D picture of Háifoss waterfall post was to share this picture/illusion, because I think it is pretty cool.

Haífoss, Iceland.  Click image for larger version.

Haífoss, Iceland. Click image for larger version.

 Highlights from 2011-2013

A list of all posts from 2014, and in fact since the blog began, can be found on the Every Post Ever page.  I’ve also picked out highlights from each of the previous years so far in the following posts.

Progress since last year

The Bárðarbunga eruption was a big help in bringing people to volcan01010.   Over 1,000 people visited the site during one day when the eruption began.  The blog had 58,000 page views in 12 months, compared to 28,000 last year.  Most of the traffic is still from the UK and USA and there is a steady flow to the software posts.  The Mail Online don’t need to worry about competition from me yet, but it is nice to see traffic increasing.  I’ve managed to keep posting about once a month.  The Twitter account now has 1902 followers (up from 881 last year), lots of whom joined back when the eruption began in September.

If you have enjoyed or found any of the posts useful this year, please spread the word.

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Easily plot magma compositions (TAS diagrams) in Python

I recently made a total alkali vs silica (TAS) plot to compare the magma of the Hekla 1947 eruption with the compositions of magmas from previous eruptions.  This post contains the code to draw the plot, including a module that draws the different compositional regions for you.

 Total alkali vs silica plots

Volcanic rocks have a range of compositions, and consequently a range of properties.  The most important measure is the proportion of silica (SiO2).  Low-silica magmas such as basalt are more dense, have high melting points and form less-viscous (i.e. more runny) melts than high-silica magmas such as rhyolite.  Eruptions of andesite magma or higher are more likely to explosive and pumice-forming as pressurised gases struggle to escape from the sticky magmas.  Magmas that are rich in alkali metals (Na, K) are typically less-viscous and crystallise slightly different minerals to the lower-alkali compositions.

TAS plots are a graphical representation of the silica and alkali contents of a magma.  The regions on this TAS plot are named with familiar (and unfamiliar) magma types and were defined in a report by Le Maitre et al. (2002).  The TAS classification page on Wikipedia has more information and links to their individual pages.

Example total alkali versus silica plot with the different compositional fields marked.  The plot compares tephra from the Hekla 1947 eruption found in the UK (Hall and Pilcher, Swindles) with in Iceland (Larsen et al) and other eruptions from Hekla volcano. Click to enlarge.

Example total alkali versus silica plot with the different compositional fields marked. The plot compares tephra from the Hekla 1947 eruption found in the UK (Hall and Pilcher 2002; Swindles 2006) with in Iceland (Larsen et al. 1999) and other eruptions from Hekla volcano. Click to enlarge.

The plot shows that the Hekla 1947 eruption was dacite-andesite in composition.  As you might expect for this composition, it began explosively (showering southern Iceland with pumice and ash for a few hours) before going on to produce lava flows for over a year.  The data were downloaded from Tephrabase and EarthChem databases, respectively.


The following code was used to draw the TAS plot above.  I wrote a module, called tasplot, with the code that draws and labels the fields via the add_LeMaitre_fields() function.  All the other commands are typical for plotting with Python and Matplotlib.

Follow the instructions on the BitBucket repository page at https://bitbucket.org/jsteven5/tasplot to install.  You can browse the source code of tasplot.py directly by clicking here.

# Import plotting modules
import matplotlib.pyplot as plt
import tasplot  # This imports the tasplot module

# Set up figure
fig = plt.figure()  # create figure
ax1 = plt.subplot(111)  # create axes and store as variable
tasplot.add_LeMaitre_fields(ax1)  # add TAS fields to plot

# Note that you can change the default colour and font size e.g.
# >>> tasplot.add_LeMaitre_Fields(ax1, color='red', fontsize=8)

# Plot the data (from pre-existing variables)
ax1.plot(hallpilcher_silica, hallpilcher_alkali, 'o', alpha=1,
         label='Hall and Pilcher (2002)')
ax1.plot(larsen_silica, larsen_alkali, 'o', alpha=1,
         label='Larsen et al. (1999)')
ax1.plot(swindles_silica, swindles_alkali, 'o', alpha=1,
         label='Swindles (2006)')
ax1.plot(earthchem_silica, earthchem_alkalis, 'o',
         color=(0.8, 0.8, 0.8), alpha=0.5, mec='white',
         label='EarthChem database', zorder=0)

# Decorate the plot
plt.xlabel(r'SiO$_2$ (wt%)')  # Use LaTeX notation for subscript
plt.ylabel(r'Na$_2$O + K$_2$O (wt%)')
plt.legend(loc='upper left', numpoints=1)
plt.title('Tephrabase: Hekla 1947 samples')
plt.savefig('Tephrabase_Hekla1947.png', dpi=150,

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Holuhraun fieldwork videos

When we sing Auld Lang Syne and raise a glass to 2015 on Wednesday, the eruption of Bárðarbunga volcanic system, Iceland, will have been going for four straight months. In that time, the eruption has covered over 80 km2 (1.3 Manhattan islands) of sandy outwash plain at Holuhraun, to the north of the Vatnajökull glacier, with a jumble of jagged black basaltic rock ranging in thickness from the height of a car to more than the height of a house.

By all measures, the eruption is slowing. The seismic energy released by earthquakes in the dyke (underground crack) that carries the magma to the surface is one ten-thousandth of what it was as the dyke was forming. The subsidence of Bárðarbunga caldera, probably caused by draining of magma away from the volcano and out to feed the lava, dropped from over 80 cm per day to less than 25 cm per day; the accompanying magnitude 5 or larger earthquakes there, which used to happen every day, can now be a week apart or more. Satellite measurements of heat flux show a decline from over 20 gigawatts in early September (to put this amount of energy in context, the average UK electricity demand in 2012 was 36 gigawatts), to fewer than 5 gigawatts by the end of November.

This doesn’t mean that the eruption will stop soon. Like the weakening spray from an aerosol can, the eruption rate declines exponentially.  The lower the flow, the more slowly it declines.  SSKKKOOOOSSSSSSHHHHH ..ssshhhssshhhssshhhssshhhssss ..sssssssss..ssssss ….sssss……sssss……sss! Icelandic scientists have predicted that it has ‘at least some months’ more to go. From initial rates of up to 1,000 m3/s, an average of 200 m3/s through September, the eruption rate in November was less than 100 m3/s.  This still corresponds to a healthy river of lava; the mean flow of the Thames through London is around 60 m3/s.  Effusion rates of less than 5 m3/s are typical of recent Hawaiian eruptions.

An interesting development is that the lava is now flowing through covered tubes. These are better insulated than open channels so the lava cools much more slowly (as little as 1°C per kilometre) and can travel further. I’m personally hoping that this means that the lava will extend further to the northeast, reaching Vaðalda and blocking the Svartá river that flows into Jökulsá á Fjöllum from the northwest (see map) to create a new lake.

Holuhraun lava flow field map, 24 December 2014 by Institute of Earth Sciences, University of Iceland modified to show the evolution of the flow field.  Source: http://en.vedur.is/earthquakes-and-volcanism/articles/nr/2947#des25 Click to enlarge.

Holuhraun lava flow field map, 24 December 2014 by Institute of Earth Sciences, University of Iceland modified to show the evolution of the flow field. Click to enlarge. Source: http://en.vedur.is/earthquakes-and-volcanism/articles/nr/2947#des25

Holuhraun Fieldwork Videos

The main purpose of this post is to share some videos that I recorded when I joined University of Iceland scientists studying the eruption in September. You can read about the trip and see photos in my Fieldwork at the Holuhraun post.

Fire fountains at the vent

This was filmed over a kilometre from the vent. The craters are over 50 m high, the fire fountains are 50 m above this. The lava sprays in the air powered by expanding gases, just like a Hogmanay champagne bottle. A few days after I filmed this, the craters filled with lava to become a churning lake of molten rock. There is spectacular drone footage of this on YouTube. A river of lava drains from the craters to feed the flow field. At the end of the clip you can see Suðri, a crater that was active at the beginning of the eruption.

In the foreground is the pale grey sand plains that extend north of the Vatnajökull glacier. All the darker material on top was produced by the eruption. The clouds above are mainly steam (+ sulphur dioxide, carbon dioxide, hydrogen chloride and other pollutants). The gas and the magma can easily separate, so there is no ash. This is why the eruption hasn’t affected aircraft or had nearly as much news coverage as it deserves for being Iceland’s largest eruption since 1783.

A ‘breakout’ on the lava flow

Every that you initially see here is lava. The coarse blocks formed as part of an early flow lobe that remains molten inside. Hot lava pours from a ‘breakout’ to form a smaller flow at the edge. The orange-yellow material has a temperature of over 800°C. Even standing 5 m away, the heat was intense. As the surface cools it solidifies to form a crust, like wax from a candle. This is broken by movement of the hot lava beneath and carried on the surface of the flow.

Sampling the lava

A University of Iceland scientist samples the active lava flow. The steel shovel has a melting point of around 1400°C so it doesn’t melt. Also, lava (rock) is a poor conductor of heat. The sample is dropped into a large metal saucepan and quenched by pouring water into it. Analysis of such samples has confirmed that the magma originates in Bárðarbunga volcanic system and estimated that it was last stored at a depth of 9-20 km beneath the surface. Future analysis of such samples will help explain why the eruption has produced so much sulphur dioxide pollution.

GPS mapping of the flow outline

One task at the eruption site was driving around the edge of the lava flow recording the tracks by GPS in order to map the extent of the flow. At the start of the eruption, the flow front advanced by hundreds of meters per day and the lava buried an area the size of a football pitch (soccer field) every 7 minutes. Now, the area of the flow field only increases slowly.  This is partly because the eruption rate has declined and partly because much of the new lava flows over or through the older material, contributing to the thickness of the flow instead.

Off road driving is illegal in Iceland, but the scientists obtained special permission to survey the lava. The tracks that you can see have since been buried beneath the extending flow. The most recent maps of the flow field are made from radar data collected by satellites, which has the advantages that it can cover the whole flow field instantaneously and that it works at night or in clouds. The downside is that it can be many days between measurements so it can miss rapid changes.

I like this video because it conveys the scale of the lava flow. You can see its height and the fridge-sized blocks on the original flow lobe. Reactions with the atmosphere have turned the surface pale. You can see all the fresh breakouts, dark and slabby, that still glow red inside in places. You can see how the wall of lava extends on and on into the distance. The NW margin of the flow was over 16 km long and it took 45 minutes to drive its full length.

Four months ago, this whole area was a flat and barren sandy plain. How will it look when we are toasting the arrival of 2016?

Happy New Year!

Further reading

As always, the best information on the eruption comes from the Icelandic scientists. Check out the Icelandic Met Office and the Institute of Earth Sciences pages for frequent updates from the Bárðarbunga Science Board. There are cool fieldwork photos on the Institute of Earth Sciences Facebook page.

The Fieldwork at Holuhraun post has more photos and explanations of the eruption.  You can find a list of my posts on Bárðarbunga and Iceland in general in the Every Post Ever page.

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Alaskan ash in Ireland: context, implications and media coverage

Long-range transport of volcanic ash was in the news last week, thanks to a recently published study by an international team of scientists, led by Britta Jensen and Sean Pyne-O’Donnell from Queen’s University in Belfast. They showed that volcanic ash grains found in Ireland had come from an ancient volcanic eruption 7000 km away in Alaska. This must have been an impressive ash cloud!

The study is a really nice example of scientific detective work, where different pieces of evidence are pieced together to support the conclusion. This post explains what they found and what it means.

How did they find the ash?

The team extracted tiny quantities of volcanic ash from a peat bog in Northern Ireland. Needless to say, the grains are extremely small (typically 20-125 microns). Such deposits are called ‘cryptotephra’ (Greek for ‘hidden ashes’; the word ‘tephra’ applies to everything ejected by an explosive volcanic eruption) and are useful as time markers in sediments and lake deposits. If you know that age of the eruption that produced a particular layer, you know the age of the sediment in which you found it.

Example of 860AD tephra shards from Sluggan Bog, Northern Ireland.  Modified from Jensen et al (2014).  Click image to view article.

Example of AD860B tephra shards from Sluggan Bog, Ireland. Modified from Jensen et al (2014). Click image to view article.

Cryptotephra are extracted by roasting samples, or digesting them in strong acid, to remove the organic material, then separating the cryptotephra from other sediments using magnets or by floating in them liquids of carefully chosen densities. The grains can then be analysed under a microscope. Databases such as Tephrabase record the layers that have been found. The layer in question, known as AD860B, had been recognised in Northern Ireland, Germany and even cores from the Greenland ice sheet.

The mystery was where it came from.

How do they know it came from Alaska?

Structurally, volcanic ash is a type of glass; all the atoms that were jiggling round freely in the molten magma were suddenly frozen in place when it quenched in the atmosphere. An electron microprobe fires a beam of electrons into the glass, exciting the atoms to give off x-rays. The x-rays from each element in the sample (O, Si, Al, K, Na, Fe etc.) have their own characteristic frequency so, by recording the energy released at each one, you can calculate how much of each element is present. Jensen’s team used electron microprobes to measure the chemical composition of the ash, looking for the clues about the source volcano.

Most European cryptotephras come from Iceland, but the compositions of AD860B didn’t match any known Icelandic eruptions, especially when comparing the aluminium and iron content. The data are a much better match to tephra from Alaska’s Wrangell volcanic field.

Major element geochemistry plot from Jensen et al (2014).  Click image to view article.

Major element geochemistry plot modified from Jensen et al (2014). Click image to view article.

Alaskan and Icelandic tephra have different compositions because of their different tectonic settings. Alaskan volcanoes are located on a subduction zone, where the Pacific tectonic plate is pushed underneath North America. Iceland is located on the mid-ocean ridge that runs through the Atlantic. The aluminium and iron contents of magma can be affected by crystallisation of a mineral called amphibole, which contains both elements and is commonly found in subduction settings but not mid-ocean ridges.

How did they know which eruption it was from?

Having identified a subduction zone volcano source, many candidates remain.  Alaska, British Columbia, the Cascades and even Kamchatka or Japan are all upwind of the sample sites. Three lines of evidence were used to link the cryptotephra to an eruption of the Bona-Churchill massif volcano in Alaska that produced a widespread deposit called the White River Ash.

  • There is excellent agreement on the age of each of the different deposits. Carbon dating of peat in Ireland gives a relatively wide age range of 776-887 AD. Carbon dating an Alaskan spruce tree killed by the eruption gave an age range of 833-850 AD. Ice-core tephras can be dated very precisely by counting annual layers in the ice. This gave an age of 846-848 AD.
  • The geochemistry of the tephrochronology and ice core samples all fall within the range of the White River Ash (red dots on figure).
Comparison of distal tephra geochemistry with White River Ash (red circles).  Modified from Jensen et al (2014).  Click image to view article.

Comparison of distal tephra geochemistry with White River Ash (red circles). Modified from Jensen et al (2014). Click image to view article.

  • The texture of the cryptotephra grains is similar to those examined from the White River Ash.

How does this compare to other findings?

Volcanic ash grains can travel a long way from their source. In 2011, satellites recorded ash from the eruption of Puyehue-Cordón Caulle, (Chile) circle the globe. Very distal deposits can also be identified. Last year a French team revealed that the source of a cryptotephra deposited in Greenland around 1257 AD was Samalas volcano, 13,500 km away in Indonesia. Impressively, the same layer has also been found in Antarctica. But these cryptotephra were found in ice cores. It’s much easier to find ash in ice cores because there is less contamination and because volcanic sulphate deposited at the same time forms an ‘acid spike’ that gives researchers clues where to look. Also, the grains are tiny (<5 microns), which means that they could have remained in the atmosphere for weeks before being deposited.

It is much harder to extract cryptotephras from soils or sediments, and the most distant of these typically have ranges of 2,000-3,000 km, e.g. Saksunarvatn (Grímsvötn, Iceland) ash in Slovenia or the Campanian Ignimbrite (Campi Flegrei, Italy) ash in Russia. The Queens University Belfast team already pushed that limit when they found the White River Ash and a number of tephras from volcanoes on America’s west coast in Newfoundland on the east, over 5,000 km away. Until now, there was only one example of cryptotephra found 7,000 km downwind after an intercontinental journey; ash from an eruption of Toba (Indonesia) 75,000 years ago had been found in Lake Malawi, Africa. But the Toba eruption was huge – 2,500 km3 tephra erupted – while the White River Ash is only 50 km3.

What are the implications?

Linking the AD860B ash to an Alaskan eruption raises the possibility that other unidentified cryptotephra layers also came from North America and not just from Iceland. The size of the ash reminds us that volcanic ash grains tens of microns in diameter can travel long distances. This agrees with our finding that ash deposited in the UK from Iceland’s 2011 Grímsvötn eruption came from the bottom 4 km of the plume.

The big grains also indicate deposition from a relatively concentrated ash cloud, rather than after weeks of dilute stratospheric drifting. This highlights the possibility of ash clouds from Alaska causing disruption not only in America, but across in Europe as well. It is important to be aware of this and to plan for it.

It is significant that the eruption producing the White River Ash was not huge. The size of volcanic eruptions is described by the Volcanic Explosivity Index. Small eruptions (VEI 3: up to 0.1 km3 tephra), such as the recent eruption of Pavlov volcano happen somewhere on the planet at least once per year, on average, causing only local aviation disruption. Eruptions like Toba (VEI 8: >1,000 km3 tephra) happen less than once in 10,000 years. This is a good thing.  If a similar eruption happens again, missing your holiday will be the least of your worries.

An eruption of 50 km3 (VEI 6) happens about once every 100 years on average. To evaluate the threat to Europe, you also have to consider the proportion of volcanoes capable of such an eruption that are located in Alaska and how often the wind blows in the right direction. The answer will be in the cryptotephra record.  We are probably talking about a couple of transatlantic ash events per millennium. If it was much more, there would be many more non-Icelandic layers in European sites. Ash from Alaska’s 28 km3 Katmai/Novarupta eruption in 1912 has been found in Greenland. Is it also in Europe, sitting beneath Katla 1918?

A few events per millennium is nothing to panic about, but if you think that it isn’t worth considering at all, bear in mind that Iceland’s ongoing Holuhraun eruption topped 1 km3 of lava last month.  It now qualifies as a large magnitude fissure eruption: a once in 270 years event.  These things can and do happen.

Media coverage

When I read the coverage of this story on the BBC website, I tweeted the following in dismay:

I took exception to the word imminent. Learning that transatlantic ash clouds have happened before doesn’t mean that one is about to happen now. Why would they say that, especially given the centuries-long gaps between such events? And why would the Telegraph and the Mail Online say the same, adding that such eruptions are scheduled to happen every 100 years?

Unfortunately, the wording can be traced back to the original press release, which contains the following line:

“With volcanoes like Mount Bona-Churchill – much more volatile than Eyjafjallajokull – scheduled to erupt on average every 100 years, another ash-cloud drama could be imminent, this time with consequences for trans-Atlantic as well as European travel.”

Universities are now judged on the ‘impact’ of their research, so getting widespread news coverage is important to them. However, exaggeration like this is really poor form by the Queen’s University Belfast press office. It’s also a pretty bad reflection on the churnalists who seem to have cut and pasted the story without reading the paper or getting a second opinion.

Further Reading

Original paper (Open Access):

Jensen BJL, Pyne-O’Donnell S, Plunkett G, et al (2014) Transatlantic distribution of the Alaskan White River Ash. Geology 42:875–878. doi: 10.1130/G35945.1

Other coverage:

Blog posts:

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