Skjaldbreiður, literally meaning the "Broad Shield", is an Icelandic shield volcano formed in a huge and protracted eruption roughly 9,000 years ago. The extensive lava fields which were produced by this eruption, flowed southwards, and formed the basin of the Lake Þingvallavatn , Iceland's biggest lake, and Þingvellir, the "Parliament Plains" where the Icelandic National Assembly, the Althing was founded in the year 930.

The volcano culminates at 1,060 meters, and its crater measures roughly 300 meters in diameter.

Straddling the Mid-Atlantic ridge, the lava fields from Skjaldbreiður have been torn and twisted over the millennia, forming a multitude of fissures and rifts inside the Þingvellir National Park , the best knowns of which are the Almannagjá , Hrafnagjá and the Flosagjás .