Skjaldbreiður is a lava shield volcano in Iceland that formed around 9,500 years ago. Its massive eruption created the basin of Þingvallavatn, Iceland's largest lake, and Þingvellir, where the national assembly was founded in 930. The volcano's summit reaches 1,066 meters, with a crater about 300 meters in diameter. The lava shield covers 170 square kilometers and is part of the Oddnýjarhnjúkur-Langjökull volcanic system. Over time, the lava fields have twisted and torn, forming fissures and rifts within Þingvellir National Park.