Skjaldbreiður (, "broad shield") is an Icelandic lava shield formed in an eruption series roughly 9,500 years ago. The extensive lava fields produced by this eruption flowed southwards and formed the basin of Þingvallavatn, Iceland's largest lake, and Þingvellir, the "Parliament Plains", where the Icelandic national assembly, the Alþing, was founded in 930.
The volcano summit is at , and its crater measures roughly in diameter. The Skjaldbreiður lava shield covers with a volume of about . and is sometimes considered a separate southern part of the Oddnýjarhnjúkur-Langjökull volcanic system. In this context, its most recent eruption was 3,600 years ago, with the earliest eruption after the last ice age 10,200 years ago. There are at least three lava units deposited between 6,000 and 9,000 years BP.
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 known of which are Silfra, Almannagjá , Hrafnagjá and Flosagjá .
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