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Anatomy of a volcano II

I’m going to take the liberty of bumping up another comment by Mike the geologist, posted in response to this post, explaining just what happens when a volcano erupts.

Just a quick recap of technical terms which might help for anyone following the news. Magma is the name for all molten rock when it is underground. Lava is molten rock at the surface. Tephra is anything solid thrown out of a volcano ranging from boulders to fine dust. Ash is a fine form of tephra.

Magma under Iceland is trying to reach the surface, but rarely manages it in one go. Instead, it will collect into huge underground reservoirs known as magma chambers. For a mountain like Hekla, the magma chamber is about 4km beneath the surface and can be anything up to 5km thick.

Before an eruption, the magma will make the journey from the chamber to the throat of a volcano extremely rapidly, Icelandic volcanoes are notorious for giving very little warning that they’re about to erupt – Hekla again has surprised people by going from near silent to full eruption in a few hours. It’s this upward movement of magma that triggers thousands of tiny earthquakes around the mountain and it is these that alert geologists that an eruption is imminent. In some cases, the rising magma also causes the mountain to inflate – it literally gets bigger by several metres.

At several kilometres beneath the ground, water actually is dissolved into the magma in the same way carbon dioxide is dissolved into soda water. Water changes the chemistry of the molten rock, making it stickier and more explosive.

The longer magma sits in a magma chamber the more it separates. Over tens if not thousands of years, the lightest stickiest part of the magma moves towards the top, taking with it most of the water and other gases. Left long enough, this magma erupts as a substance known as rhyolite – a very pale yellow or grey rock which is responsible for the beautiful colours in Landmannalaugar and Yellowstone in the US.

When any magma gets close to the surface, the water and other gases start to come out of solution (just like the carbon dioxide in your soda) and its these that force the eruption.

The amazing fire fountains at Fimmvörðuháls were being driven by gas coming out of extremely fluid basalt. The gas comes out nice and easily, the lava turns into a spray, but there aren’t any explosions.

If the magma is sticky, the gas can’t get out of solution so easily. Instead of fizzing, the magma literally explodes into tiny ash fragments.

If magma is close to the surface and encounters ice or water, the heat is enough to instantly turn them to steam and you have a huge so-called phraetic explosion. Craters like Ljotipollur are caused by these explosions and aren’t actually responsible for lava. In Iceland you also find explosion craters where lava has rolled over wet ground trapping steam.

What seems to be happening at the volcano right now is that new magma has been rising for a number of weeks and first found an outlet at Fimmvörðuháls. After a while, either this channel became blocked, or a new weakness appeared under Eyjafjallajökull, or this new activity was enough to wake up the main volcano.

It looks like a mixture the rising, hot, new basalt has triggered an eruption of older magma left over from the 1821-23 eruption which has had plenty of time to separate. There should be some detailed chemistry available tomorrow which could confirm this.

The old magma contains lots of gas which is responsible for pushing the ash to high altitudes. At the same time, the heat of the eruption is melting the glacier itself, the water is coming into contact with the magma and creating smaller, but still atom-bomb sized phraetic explosions which are adding to the ash fall.

The eruption is now being unofficially rated as a 3 on the Volcanic Explosivity Index. This is an eight point scale in which each point upwards marks a ten-fold increase in power. If it is a 3 that would put it in the same region as the formation of Surtsey in 1963, the eruption of Eldfell ten years later and Hekla in 2000. That would still only make it a tenth the size of Hekla’s 1947 eruption and ONE THOUSANDTH the size of Laki.

The latest is that some ash has now fallen in Göteborg, Western Sweden accompanied by a strong sulfur smell. Ash has also fallen in Skye, Scotland and there is a possibility much of the north of England will get a fine dusting overnight.

From what I’m reading there is some confusion exactly what is going on – the visibility in the area is lousy and not just because of the eruption, but they’re hoping conditions will clear enough that geologists can get a good, close look at the eruption – lucky devils.

One thing is certain, this volcano is much more dangerous than Fimmvörðuháls and there’s no prospect of the public getting close for some time.

Thank you Mike!



Comments on this entry are closed.

  • Lissa April 16, 2010, 1:57 pm

    Thank you, Mike. Clearing explaining something complex to laypeople is a gift I admire.

  • Simon Brooke April 16, 2010, 2:07 pm

    There were little bits of Iceland on my car in Strathaven, south of Glasgow, this morning, if that’s a useful datapoint. At least, I can’t be certain that the dust came from Iceland but it was pale grey, fine and gritty – very abrasive.

  • Skúli Pálsson April 16, 2010, 2:14 pm

    Fróðlegt (líka fyrir Íslendinga), takk.

  • Jen April 16, 2010, 2:17 pm

    Brilliant post. Thanks for the explanation Mike!

  • Mike Richards April 16, 2010, 2:56 pm

    Thanks folks. I just wish I could be there to see it.

    If Alda is happy, I’ll try and digest the geological stuff and keep you up to date.

  • alda April 16, 2010, 2:59 pm

    Very happy! 🙂

  • Amy Clifton April 16, 2010, 3:38 pm

    Mike, minor correction regarding Hekla’s magma chamber. All geodetic, seismic and strain data points to it being around 8 km depth, with a possible lower chamber at 12-15 km.

  • TomThumb April 16, 2010, 4:07 pm

    Thanks Alda. I sympathize with those who worry that this is the end of the world, what with the 2000 paged Report arriving simultaneously with this eruption. I say, “This will pass.” Better times are coming.

  • Mike Richards April 16, 2010, 5:09 pm

    Thanks Amy,

    You’ve probably got more modern numbers than me – my stuff on Hekla is 20 years old.

  • The Fred from the forums April 16, 2010, 5:40 pm

    I hope Mike Richards has students and that they appreciate what they’re getting.

    You know someone *really* understands a subject when they can explain it to non-specialists and make it sound simple.

  • Joerg April 16, 2010, 5:47 pm

    Thanks for this information.

    “there’s no prospect of the public getting close for some time”

    🙁 But fortunately, there is still the webcam on Valahnjúkur, which has been redirected towards the new scene and is showing the eruption close up – for the first time after several days of cloudy weather.

    There are several videos of the fallen ash – like this one:


    To me it looks pretty scary, particularly as the ash is apparently pretty poisonous. And I saw footage of horses and sheep covered in ash – definitely no easy time for the farmers in this area.

  • Mike Richards April 16, 2010, 5:49 pm

    @The Fred from the forums

    Yes I have students, sadly I don’t teach geology and no they don’t appreciate it 😉

    Thanks though.

  • Mike Richards April 16, 2010, 5:57 pm

    Not much to add to today’s news. Apart from some early chemical work has been done on the ash from the eruption. Health officials are most concerned with the concentration of fluoride compounds in the ash.

    Whilst small amounts of fluoride are good for your teeth – you do brush don’t you? In large doses it causes fluorosis which affects bones and teeth. Amongst other things it can wreak havoc with your joints and produce agonising pain. In large doses, fluoride simply poisons you.

    Livestock are at particular risk because they can’t help but eat particles of ash if they are grazing outside. Humans are at risk if they eat large quantities of food or drink water that’s been contaminated with ash.

    The good news is that the ash has only 1/3 as much fluoride as the stuff that comes out of Hekla and is much less dangerous to animals. I’d imagine though that all livestock in the region is safely indoors and being fed with prepared feed. Obviously the health authorities in Iceland will also be monitoring the water supply to the locals.

    The risk passes quite quickly as fluoride is soluble and washes into the soil almost as soon as some of Iceland’s inevitable rain arrives.

    Over here in the UK, there’s been quite widespread reports of a sulfurous smell in the air. The health authorities here are saying there is no immediate risk to peoples’ health, but anyone who uses an inhaler or other medication for lung conditions should carry it with them just in case.

  • joeinvegas April 16, 2010, 6:18 pm

    First time Iceland has been in our news for a while, as the cause for airports closing across Europe. congratulations

  • Ethan April 16, 2010, 7:55 pm

    If the shiat really hits the fan in Iceland, you’re all welcome to come to Canada…

  • Joerg April 16, 2010, 8:21 pm

    Just another link to the vodafone webcam, which has also been relocated and is now directed at Gigjökull.


    Apparently, the former glacier lagoon has vanished completely and is all filled up with debris.

  • Joerg April 16, 2010, 9:24 pm

    An absolutely awesome video about the correct – and wrong – pronunciation of “Eyjafjallajökull”:


    Coming just in time before it gets completely out of hand. In Germany it often sounds like some – previously unknown – delicacy with eggs.

  • Mike April 16, 2010, 10:19 pm

    Thanks for the link Joerg.

    There’s a collection of images here from the webcam; the eruption is especially pretty at sunset:


    By the looks of it, the eruption has already filled in the small crater at the foot of the Gígjökull.

    And this one taken today from Hvolsvelli is just awesome:


    If you watch, someone had a fabulous view from their horse ride.


  • alda April 16, 2010, 11:19 pm

    Thanks, all, for the input.

    Joerg – I saw that today and was about to post it to the front page. Thanks! 🙂