Kerið in Grímsnes, Iceland.
Vulcanologists used to class Kerið as an explosion crater. Explosion craters are formed in explosive eruptions, which sometimes leave deep craters. However, deeper studies of the Grímsnes area have not revealed the existence of any ash deposits that could be traced to an explosive eruption in Kerið andi t is now believed that it was originally a large scoria crater. It is clear that as much as half of the Tjarnarhólahraun lava flowed from kerið. In its present form, the crater was probably formed by a small magma chamber beneath the crater being emptied towards the end of the eruption, resulting in a collapse. Beneath a certain level, cavities and fissures in the rock are filled with groundwater, the surface of which is called the water table. The water in Kerið does not drain out, but rises and falls according to changes in the water table. Thus, the crater is like a window on the groundwater.