Vatnafjöll (Icelandic pronunciation: [ˈvahtnaˌfjœtl̥] ⓘ) is a 40 km (25 mi) long, 9 km (6 mi) wide basaltic fissure vent system that is south-east of Hekla, Iceland.

[4] More than two dozen eruptions have occurred at Vatnafjöll during the Holocene Epoch.

[5]: 4313  While these eruptions were predominantly effusive some basaltic tephra deposits have been found up to 40–50 km (25–31 mi) away.

A Mw5.9 with an epicenter near the main Vatnafjöll edifice occurred on 25th May, 1987,[5]: 4313  where the transform South Iceland seismic zone meets the East volcanic zone of Iceland.

Later analysis has suggested that initiation was from strain relief but a subsurface dyke intrusion then occurred over the next 3 days.