North and South Iceland seem to be quite different, as there are many volcanic zones active in the south, but only one in the north. The North Volcanic Zone (NVZ) in the north-eastern part of the country replaced the volcanic zone in Húnaflói, extending from Snæfellsnes Peninsula. But volcanic activity has been found in between these two zones in the past, mainly 1-2 million years ago. The Skagafjörður Volcanic Belt was a rift zone, widening the fjord by about 10 kilometers until activity came to an end. Also, activity did occur north of Eyjafjörður, and created for instance Grímsey, where the rocks are known to be around 1 million years old. https://www.visindavefur.is/svar.php?id=4435
This map shows how the transition has taken place in steps of 1.5° from west to east. The map base is from Wikipedia, the lines represent the pattern of division lines resulting from convection within the mantle.
Magic Magma 