The bedrock of the moors west of Lake Mývatn is made up of interglacial lava flows. Most of the mountains in the vicinity of the lake were formed by eruptions under the ice sheet in the glacial periods of the Ice Age. Eruptions that melted their way up through the ice formed table mountains (Mounts Bláfjall, Sellandafjall, Búrfell, Gæsafjöll), those which didn't formed hyaloclastite ridges (Mounts Vindbelgjarfjall, Námafjall, Dalfjall, Hvannfell).