The second volcanic cycle, the Hverfjall cycle, began 2500 years ago with a gigantic but brief eruption, which formed the explosion crater (tephra ring ) Hverfjall (also named Hverfell). An eruption in Jardbadshólar followed, producing the lavafield between Reykjahlíð and Vogar. Approximately 200 years later a vast lava flow, the Younger Laxá-lava, was erupted (see above). The lava dammed up the present Lake Mývatn and also the lakes Sandvatn, Grænavatn and Arnarvatn.