Although volcanologists originally believed Kerið was formed by a huge volcanic explosion, as is the accepted norm with volcanic craters, more thorough studies of the Grímsnes region failed to find any evidence of such an explosion in Kerið. It is now believed that Kerið was a cone volcano which erupted and emptied its magma reserve. Once the magma was depleted, the weight of the cone collapsed into the empty magma chamber. The current pool of water at the bottom of the crater is at the same level as the water table and is not caused by rainfall.