The Icelandic National Broadcasting Service [RÚV] recently had an excellent and easy to understand explanation of the forces at work in Bárðarbunga, the caldera beneath Vatnajökull glacier that has been repeatedly mentioned in connection with the current eruption in Holuhraun. I’ve taken the liberty of translating it for the edification of those who don’t speak Icelandic and perhaps don’t have access to this sort of information. Please keep in mind that this was written on September 3rd, and since then two new fissures have opened up.
The translation is taken more or less verbatim from this link. Please keep in mind that I may not have all the scientific terms down pat, but I believe I’ve managed to get the meaning across:
Some 4,500 million years ago, when our solar system had just been created, the earth was a single ball of fire – a smouldering cauldron of melted rock. Gradually its surface cooled and the earth’s crust was formed. Yet deep in the centre of the earth there is still a vast amount of heat, and material is constantly seeking to find a way to the surface. As it gets near the surface it begins to melt and forms lava fountains that find their way out. This happens in two places: where the heat deep in the earth is especially great, in so-called hot spots, and where the earth’s tectonic plates either meet or pull apart. Iceland sits on top of such a division of plates, and it is also on top of a hot spot. Consequently volcanic eruptions here are more common than in most parts of the world.
The centre of [Iceland's] hot spot is beneath the area around Bárðabunga, a region that seems quite innocuous as it is covered by Vatnajökull glacier. However, if the glacier were peeled back it would reveal one of the country’s largest and most active volcanoes. There are around seven kilometres from one edge of the caldera to the other, which is approximately the distance from the old city centre of Reykajvík to the Grafarvogur suburb.
Underneath the volcano, the molten lava flows from the centre of the earth and collects in a huge chamber believed to be under the Bárðarbunga mountain.
In recent years, the pressure beneath the mountain has grown. The land has risen, but in the end, something has to give. Like water, the lava finds the easiest route along which to flow, and instead of breaking through the magma chamber it has flowed to the side, into a dyke, or tunnel. This flow of magma has now lengthened and currently extends some 40 km to the north.
The flow came to a halt beneath Holuhraun [where the current eruption is] and the magma broke its way to the surface. However, the eruption of lava out of the fissure in Holuhraun is less than what is believed to flow into the dyke, and so the current eruption is not sufficient to release the pressure.
What will happen next is uncertain. There have been many large earthquakes in Bárðabunga itself and scientists say they cannot exclude the possibility that it will erupt. If that happens, it will melt its way through a layer of ice 850 metres thick, which means that flooding and ash emissions will follow. As yet, however, there are no indications that this will happen.