Located in the Þingvallavatn Lake in the Þingvellir National Park in Iceland, Silfra is a rift that is part of the divergent tectonic boundarybetween the North American and Eurasian plates.
The formation of Silfra and the Þingvellir valley is a consequence of the tectonic drift of the Eurasian and the North American plates. Every year, the plates drift about 2 cm farther apart, which builds up tension between the plates and the earth mass above. This tension is released through a major earthquake approximately every ten years. In these earthquakes, cracks and fissures are formed in Þingvellir. Silfra is one of the largest cracks and started with a deep cave where most of the underwater wells feed it. The site lies at the rim of the Þingvallavatn Lake.
Caves in Silfra were also formed through earthquakes. With each earthquake, boulders and rocks fall into cracks in Silfra, deepening and widening the base of Silfra over time.
About 50 kilometers north of the Þingvallavatn Lake lies Iceland’s second largest glacier, Langjökull. In the past, melting ice from the glacier would run through a river directly into the Þingvallavatn Lake. A few thousand years ago, the volcano Skjaldbreiður erupted masses of lava, which blocked the river. Due to this event, right after having melted from Langjökull, water trickles underground into porous lava rock. From this point, the water takes 30 to 100 years to travel 50 kilometers to the Þingvallavatn Lake in the ÞingvellirNational Park. This groundwater is potable.