Silfra

thumb|Silfra fissure thumb|The Mid-Atlantic Ridge passing through Þingvellir thumb|Rocks and boulders that have piled up in the fissure due to earthquakes. Silfra () is a rift formed in the Mid-Atlantic Ridge – the divergent tectonic boundary between the North American and Eurasian plates – and is located in the Þingvallavatn Lake in the Þingvellir National Park in Iceland.

== Formation == Silfra lies in the Þingvellir valley and within the Þingvellir National Park. The valley, and Silfra itself, were formed by the divergent tectonic drift of the Eurasian and North American plates. The plates drift about farther apart every year, building up tension between the plates and the earth mass above. This tension is relieved through periodic major earthquakes at approximately ten-year intervals, which have caused cracks and fissures to form in Þingvellir valley; Silfra lies at the rim of the Þingvallavatn Lake and is one of the largest and deepest of these fissures. The Silfra fissure intercepts a major aquifer, which feeds multiple springs at its base. Boulders and rocks falling into the widening cracks have formed caves within the fissures.