The map shows Islands largest volcanos.
Iceland has a very high number of active volcanoes due to its unique geological conditions. The island has about 130 volcanic mountains, of which 18 have erupted since the settlement of Iceland. Over the past 500 years, Iceland's volcanoes have erupted a third of the total global Lava output, and the Laki eruption of 1783 gave rise to the largest single lava flow in recorded world history.
Geologists explain this intense activity as being due to a combination of the island's position on the Mid-Atlantic Ridge and a volcanic hotspot underneath the island. The island sits astride the boundary between the Eurasian and North American Plates, and most volcanic activity is concentrated along the plate boundary, which runs across the island from the south-west to the north-east of the island.
http://walasa.com/icelands-volcano-millions-of-tons-of-ash-from-a-volcano/