Grímsvötn eruption, Iceland, 2011

Figure 1. The steam-rich Grímsvötn ash plume on 22 May 2011 with the black-grey ash cloud spreading outwards from it. The ice cap around the vent is black because of the ash that has fallen on it. Photograph by Magnus Tumi Gudmundsson, University of Iceland.

 

Preliminary findings of the UK volcanic ash collection network

The Grímsvötn volcano began to erupt explosively during the evening of 21 May 2011.

The ash plume reached heights of 20 km for short periods at the start of the eruption, then the plume height decreased gradually over the following days.

According to the Icelandic Meteorological Office and the University of Iceland, the eruption ended on 28 May 2011.

Grímsvötn is the most frequently active volcano in Iceland, and lies beneath Europe's largest glacier, Vatnajökull. It last erupted in 2004 but that eruption was smaller and erupted less ash. This was the most powerful eruption in Iceland for over 50 years.

Volcanic ash is made up of tiny pieces of rock and glass, it is hard, abrasive and mildly corrosive, not at all like the soft ash that results from burning wood or paper.

Volcanic plumes contain volcanic ash, steam and other volcanic gases. Although most of the ash falls to the ground near the volcano (Figure 1), some fine ash can stay in the atmosphere for days and be carried great distances by the wind.

Ash fall in the UK

Figure 2. A map showing the locations of samples collected and sent to BGS by 10 June 2011

The BGS, the Met Office, Edinburgh University and other institutions in the UK coordinated sample collection during the Grímsvötn eruption for a number of reasons, including:

  1. to further research on volcanic eruptions and volcanic ash clouds
  2. to help inform Met Office volcanic ash cloud advice

The ash samples were collected for different types of analysis to show the extent of ash fall, the textures, chemical composition, sizes, shapes and other properties of the ash.

This information can help us understand how the ash forms, how it travels long distances and how it is removed from the atmosphere.

Knowing the properties of different types of ash will help to choose instruments for research aircraft and satellites with the best ability to detect and monitor ash.

The Grímsvötn eruption gave us the opportunity to test out new methods and involve members of the public in the effort to collect samples across the whole of the UK (Figure 2).

By 10 June 2011 we had received almost 200 samples.

The samples included rainwater, pollen filters, sticky tape on paper, uncoated sticky tape and ash collected on tissue paper and sponges.

We were particularly pleased to have received samples from primary and secondary schools, and the Met Office network of voluntary observers.

Preliminary analyses

Magma is molten or partially molten rock beneath the Earth's surface. When magma is cooled very quickly in an explosion into air or water it turns into tiny fragments of 'glass'.

Tiny, angular, glassy magma fragments in volcanic ash are called 'shards'. Sometimes the shards contain the remains of bubbles that once contained volcanic gas.

Microscope x500

Figure 3. Angular, glassy shards from Lerwick. The fragment at bottom right shows parts of what were once two bubbles in the magma.

Figure 3 shows ash grains wiped from the surface of a car in Lerwick in the Shetland Islands, where a lot of ash fell 23–24 May.

The ash was collected with a tissue. The grains were mixed with water, then a drop was put on a microscope slide.

The picture was taken through the microscope at 500x magnification.

The scale bar at the top is 100 micrometers (also called microns) long. This means that the scale bar could fit 10 times in a single millimetre.

The sharp-edged, glassy, brownish grains are volcanic ash. The grains are different sizes, from less than 10 microns to over 50 microns.

Pollen slide from the Met Office in Exeter, 24 May 2011.

Figure 4 shows a glass slide from the Met Office in Exeter used to collect pollen grains.

It was collected on 24 May and shows tiny brown glassy grains (5–10 microns) in rings.

The grains have the same colour and shape as volcanic ash, but electron microscope is needed to confirm their chemical composition.

The grains were washed down in rain and each ring is a fossil raindrop.

Reflected light microscopy

Figure 5 shows some of the different things that have been found so far on sticky tape samples.

The microscope that is used to look at the sticky tape samples is not as powerful as the one for looking at microscope slides, but the samples are so easy to collect that it allows us to look at many from all over the country.

The scale bar is 500 micrometers (microns) in (a) to (c) and 2.5 mm in (d).

Volcanic ash in a sample from Lerwick.
(b) Mineral grains. This is basically sand, the grains are blown around by strong winds and are found in most tape samples. They come in lots of shapes and sizes and are much bigger than the ash grains.

(c) Black, opaque grains, such as in the bottom right, were common in the tape samples, and are probably soot.
(d) Biological materials, such as pollen, bits of leaf and insects, were also found in most tape samples.

Scanning electron microscope (SEM)

Figures 6–9   The scanning electron microscope can produce beautiful images of ash grains.

It is very powerful and can show the detail of grains less than 10 microns long.

The following images are from a sample of ash sent in by Kirkwall Grammar School in Orkney, collected late on 24 May 2011.

The scale bars are in micrometers (microns).

Figure 6. Very sharp, angular glass shards stuck together in 'aggregates'. Note the very tiny 1 micron ash particles stuck onto the smooth surfaces of larger particles. These angular and blocky grains are typical of those formed by magma-water exposions.
Figure 7. Close up of a glass shard showing the outer wall of a bubble. The bubble in the magma would have contained volcanic gas.

Figure 8. Aggregates of angular and broad, flat ('platy') glass shards with smaller particles stuck to the surfaces of the larger particles. These platy fragments may be parts of bubble walls that have broken apart as volcanic gases explosively expanded during eruption.
Figure 9. Loose branching aggregate of platy and angular glass shards partly cemented together by cubic crystals of halite (rock salt). Halite is a cement that develops after interaction of the ash with sea spray in the atmosphere.

Preliminary results

Samples collected and analysed so far show that small amounts of ash fell in the UK, particularly the north, as the ash cloud passed overhead.

Samples that were collected between Monday and Wednesday (23–25 May 2011) are the most likely to contain ash.

The ash grains are between 1 and 50 microns in diameter, and many clumped together to form aggregates.

The images on this page show that some of the ash grains are very sharp and angular, some show smooth round bubble walls, some are flat and 'platy', others are more blocky. This variety of textures suggests that as the magma was erupted it was fragmented rapidly into tiny pieces by two types of explosion:

  1. very rapid bubble growth caused by decompression of the rising magma (just like shaking a bottle of of fizzy drink and taking the top off)

  2. by interaction with water (hot magma turns water instantly into steam causing explosions that break the magma into tiny pieces).

    This process is often called 'fuel-coolant interaction'.

We will continue to analyse the samples to get a fuller picture of where ash fell and when and the full range of particle sizes and textures. Another page will be produced when analysis is complete.

Acknowledgements

Thanks to all those who sent in samples.

In particular we'd like to thank the Met Office for enlisting the help of their network of voluntary observers and all schools, teachers and children who took part.


We would be unable to collect such good and widespread data without your efforts.

Contact

Contact Dr Sue Loughlin for further information.