Hrafntinnuhryggur (Obsidian Ridge) is composed of rhyolite with remnants of obsidian from plugs and dikes. It is 2 km long and 100 m high, formed on a north-south fissure south of Krafla. It has been dated at 20,000 years. It thus formed under ice around the last glacial maximum. Obsidian is a rhyolitic glass formed by rapid cooling of magma. At this location the obsidian is completely clear and free of the crystals and gas bubbles which form as the rock progressively changes from obsidian to light gray rhyolite. A row of explosion craters is situated to the west of Hrafntinnuhryggur. The largest one contains a pond where there is geothermal clay and there are steam vents on the ridge to the north. There is another smaller ridge southwest of Hrafntinnuhryggur. It is clear that rhyolitic magma has been present in the east of Krafla's magma chamber for more than 20,000 years. During geothermal drilling operations in Krafla, the magma has been drilled into at just over 2000 m depth.