That ash cloud has risen to over 55,000 feet in height. 55,000 feet! That happens to be where the Jet Stream is also located. The Jet Stream is a narrow ribbon of fast, upper level winds that circle the globe and divide the cold polar air to the north from the relatively warmer air to the south. These winds flow generally from west to east but tend to meander much like a river. The picture above shows the latest path of the "Jet" over Iceland and you can see just how far it is pushing that ash cloud south and east. This has brought ash as far east as continental Europe!
So why the cancellations? Well, if a plane were to fly into that ash cloud it may lose power. An ash plume is made up ash, glass and tiny bits of rock. The glass is capable of hurting a plane's electronics and the rocks can damage its turbines. Those tiny bits of rock can heat up and stick to the engine's components resulting in a loss of power. Not to mention a lack of visibility for the pilot and harmful gases could be pumped into the cabin. As for how long this problem may persist...this could be a frequent issue. The Eyjafjallajokull Volcano has been very active as of late and shows no signs of calming down. Iceland sits right over a divergent plate boundary where two tectonic plates are in a constant state of seperation allowing magma and volcanic gas to reach the surface. Every now and then, as pressure from below builds, this volcano literally "burps" spewing ash high into the atmosphere. Let's hope for those stranded passengers that this ash clears in the next day or so but another plume may not be far behind.