Eldgjá is a volcano and a canyon in Iceland. Eldgjá and the Katla volcano are part of the same volcanic system in the south of the country.
Situated between Landmannalaugar and Kirkjubæjarklaustur, Eldgjá is the largest volcanic canyon in the world, approx. 40 km long, 270 m deep and 600 m wide at its greatest.
The first documented eruption in 939 was the largest flood basalt in historic time. The areal extent of the lava is around 800 km2. An estimated 18 km3 of magma poured out of the earth. Evidence from tree rings from around the Northern Hemisphere indicated that the eruption in 939 caused the summer of 940 to be one of the coolest summers in 1500 years. Summer average temperatures in places as disparate as Central Europe, Scandinavia, Canada, Alaska, and Central Asia were 2°C lower than normal.
In March 2018, a team of medieval historians and scientists from the University of Cambridge suggested that a famous medieval Icelandic poem, Vǫluspá, estimated to date from 961, was a roughly contemporary chronicle of Eldgjá's eruption in 939. The researchers suggested that the dramatic imagery of Eldgiá's eruption was purposefully invoked in order to accelerate the Christianization of Iceland.
There is a waterfall named Ófærufoss within the canyon. A natural bridge across the waterfall vanished in 1993, reportedly due to excess water from melting ice.
The northern part of Eldgjá, including Ófærufoss, and surrounding areas, have been a part of Vatnajökull National Park since 2011.