Skjaldbreiður

Skjaldbreiður (, "broad shield") is an Icelandic lava shield formed in a huge and quite protracted eruption series from about roughly 9,500 years ago. The extensive lava fields which were 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 1066 m, and its crater measures roughly 300 metres in diameter. The Skjaldbreiður lava shield covers 170 km2 with a volume of about 13 km3. and is sometimes considered as a separate southern part of the Oddnýjarhnjúkur-Langjökull volcanic system which it is usually classified as being within. In this context its most recent eruption would be 3600 years ago, and the earliest eruption after the last ice age 10,200 years ago. There are at least three lava units deposited between 6000 and 9000 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.