The crater Kerið was formed 6,500 years ago and lies at the northern end of a row craters known as Tjamarhólar. It’s oval, about 270m long, 170m wide and 55 m deep; the depth of the water at the bottom varies between 7 and 14 m. According to an old tradition, a rise of a water level here is accompanied be a corresponding fall in the pontoon Búrfell in Grímsnes and rise versa. ...
Kerið lies in the Iceland’s Western Volcanic Zone, which run-through the Reykjanes peninsula and the glacier Langjökull. traces of vulcanism are not very obvious, since most of the eruption sites are low-lying and many craters are hidden be vegetation, but 3 craters in the Grímsnes area are clearly discernible and well-known: Kerið, Seyðishólar, and Kerhóll. ...
Volcanologists used to class Kerið as an explosion crater. Explosion crater are formed in explosive eruptions, which sometimes leaves deep craters. However, deeper studies have not revealed the existence of any ash deposits that could be traced to an explosive eruption of Kerið, and it is now believed that to was originally a large scoria crater (1). 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 (2) resulting in a collapse (3). 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 (4). Thus, the crater is like a window on the groundwater.