East of the bridge over the river Jokulsa á Fjollum on road 1, take a short detour onto gravel road F88 to Hrossaborg (horse fortress), an explosion crater. The 500m in diameter crater is thought to have formed around 10,000 years ago.
The initial stage of the eruption built up a scoria wall, but then water flooded the vent, causing steam explosions, which deposited tephra and lava bombs over the scoria.
Then the crater wall was broken through on its east side by glacial meltwater flooding from the Jokulsa River.
Take the path up the crater wall for a fine view over the highlands of Iceland.
Hrossaborg was used as the football stadium scene in the 2013 Tom Cruise film, Oblivion.