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!