Hrossaborg is a scoria and tephra crater lying on an eruptive fissure which extends with several widely separated craters from Veggir at Mývatnsöræfi north to Hólssel. Hrossaborg was originally almost circular and about 500 m in diameter, but is now distorted due to floods of the Jökulsá river, which eroded it on the east and west and opened a gateway through the eastern wall. During the floods, sediments carried by the river, were deposited on the lee side and to a lesser extent on the south side of the crater wall. What remains of the crater is about 40 m in height. The formation of the crater occurred soon after the last deglaciation. It began erupting scoria which forms the lower part of the crater rim. The nature of the eruption then changed dramatically as water invaded the eruptive vent causing steam explosions which added thick, laminated tephra layers and lava bombs on top of the scoria.