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.
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:
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.
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.
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.
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.
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).
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).
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:
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.
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 Dr Sue Loughlin for further information.