During the Miocene Epoch, 5.3–23 million years ago, the landscape within Iceland’s active volcanic zones was characterised by volcanism. Lava erupted from fissure vents, shield volcanoes, and other types of volcanoes. Outside of these volcanic zones, forces of erosion (water erosion and weathering) shaped the lava, forming V-shaped valleys and coastal cliffs. Generally speaking, the Icelandic landscape would have been much flatter than today.
During the Pliocene Epoch, 2.6–5.3 million years ago, the climate gradually became colder and glaciers began to form. During the Ice Age (the Pleistocene Epoch), 0.001–2.6 million years ago, glacial periods alternated with warmer interglacial periods. The Ice-Age glaciers that once covered Iceland have had the greatest impact in shaping the modern landscape. They dug U-shaped valleys and deep fjords faced by steep mountains. The Ice-Age glaciers were also responsible for the formation of striking hyaloclastite mountains within Iceland’s active volcanic zones. These include the hyaloclastite ridges by the highland lake Langisjór and the hyaloclastite tuya Herðubreið, which formed during a subglacial eruption.