Grímsvötn eruption: more questions and answers

As the eruption continues, here are some answers to some more common questions.

Flights are being cancelled.  Does that mean that ash is reaching the UK?

Yes.  It is falling in the Orkneys as I write this and the Met Office charts show the cloud above the UK on Tuesday and Wednesday at least.

Met Office predictions for ash dispersal on Tuesday and Wednesday (24th, 25th). The map shows ash being blown across the UK.

But I heard that the ash grains from this eruption were too big to reach us.

It is true ash being produced by the Grímsvötn eruption is generally coarser that that from Eyjafjallajökull last year, but that is not the whole story.  Volcanoes can eject magma at all sizes, from car-size blocks to tiny grains a millionth of a metre across.  There is not just one unique grainsize, but a grainsize distribution.  The fine ash (<64 microns) causes most problems, because it can travel further from the volcano before falling to the ground.  All explosive eruptions produce some fine ash, but the relative proportion can vary.

OK.  So is the problem that the Grímsvötn eruption is really big?

Exactly.  Even if the eruption was producing only 50% as much fine ash as Eyjafjallajökull, the material was being produced over 100 times more quickly, so we still end up with a lot of ash floating about.  You can see what I mean with this simple two part experiment.  (If anyone actually tries this out, can you post a video on youtube, please?  I’d love to see it!)

Part 1:

1. Mix together 2 parts icing sugar, 2 parts castor sugar, 2 parts coarse brown sugar and 2 parts sugar cubes in a coffee mug.
2. Go outside, and throw the mixture as high as you can up into the air.
3. The result is sugar cubes scattered around your feet and a puff of sweet-tasting dust drifting off in the wind.
4. Repeat steps 1+2 every half hour for a whole day. This is Eyjafjallajökull 2010.

Part 2:

1. Mix together 1 part icing sugar, 1 part castor sugar, 3 parts coarse brown sugar and 3 parts sugarcubes, this time filling up a really big sauce pan.  Proportionally, this has half as much fine ash as before.
2. Go up to the second floor and throw it out the window.  This is Grímsvötn 2011.

We’re flying to Austria on Saturday.  Do you think that we’ll get away?

As always, it depends on a combination of the weather and what the volcano is up to.  I think that the chances are good, though.  The low pressure system that was blowing the ash towards us will move past in the next day or so, and all indications are that the eruption is fizzling out.

How can you tell that the eruption is ending?

The height of the eruption column has been declining, from 20 km on Sunday down to ~5 km at the moment.  The height of the column is very sensitive to rate that material is erupted from the volcano, such that a halving in column height implies that the eruption rate has declined to a 16th of what it was before.

Are there any other signs?

Yes.  Seismometers at the volcano have measured continuous small earthquakes during the eruption.  This is called tremor, and is caused by movement of magma within the volcano.  The intensity of the tremor has been declining since the eruption began.

Also, GPS measurements on the volcano detected the surface gradually rising since the last eruption, as the magma chamber beneath it inflated.  Since the eruption began, the data show a subsidence of 25 cm, as the pressurised magma within the volcano is erupted and it deflates back down.  With the pressure in the magma chamber released, the eruption will only pick up again if fresh magma arrives from deeper in the system.

Therefore, unless another vent opens or new magma is injected, it is probable that the eruption will not pick up again and will end completely in the next few days.

So will the ash just disperse?

Yes.  The ash above us at the moment was erupted early in the eruption, and the current eruption rate is too low for newly-erupted ash to cause problems in Britain.  As the cloud disperses some of the finest grains will remain airborne for days, but their concentration will be too low to cause problems.  The new flight rules brought in during the Eyjafjallajökull eruption will get flights back in the air quickly, too.  This plot from the BBC website last year shows how the area of airspace that would be out of action is made much smaller by the threshold-based system.

Will we see ash falling here?

I hope so.  Last year some people saw ash falling, but the majority just saw blue skies.  This even led some people to say that there was no ash there at all.  They were the first ever Ash Cloud Sceptics.  They were wrong.  Ash grains fell through the air, and in the rain.  The figure below shows an example.  The ash cloud was measured by lasers on the ground (LiDAR), and sampled during special research flights.  The lasers and research planes will be in action again this week.

With the high winds and the rain, it may be very difficult to see the ash grains.  They will be very small, like dust.  If you do find some settling where you are, the British Geological Survey would like a sample!  Visit their ash collection website to find out how you can help.

The weather has been wild the past few days, and those scenes from Iceland are positively hellish.  Does this have anything to do with the Rapture?

No.  This has nothing to do with the Rapture.  Was the end of the world also foretold by the Grímsvötn eruption in 2004?  Or the one in 1998?  How about 1996, 1984, 1983, 1972, 1954, 1948, 1945, 1941, 1939, 1938, 1934, 1933, 1922, 1919, 1910, 1902, 1897, 1891, 1887, 1883, 1873, 1867, 1861, 1854, 1838, 1823, 1816 or any of the hundreds before that?  No.