Vatnajökull and the volcanoes under the glacier in Iceland

Over on Jon Frimann's Iceland Volcano and Earthquake Blog, there has been a lot of talk about the activity under Vatnajökull (see map below), the largest glacier on Iceland and specifically near Grímsfjall/Grímsvötn. I thought I'd take a closer look at the great glacier of the island nation and specifically, the volcanic activity that occurs around and under the icecap (and then take my speculative stab at what might be going on near Grímsvötn).