The second largest shield volcano in Iceland, aptly named Skjaldbreiður (broad shield). The summit at more than 1km and the crater is roughly 300 metres in diameter. It was formed in one huge and protracted eruption roughly 9,500 years ago. he extensive lava fields which were produced by this eruption, flowed southwards, and formed the basin of Þingvallavatn, our largest lake, and Þingvellir, the "Parliament Plains" where the Icelandic national assembly, the Alþing was founded in 930. View from the Þingvellir visitor center on Haki.