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The geology of Silfra and the Þingvellir Valley are connected to the tectonic drift of the Eurasian and the North American plates. Every year, the plates drift about 2 cm 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 nourish it. The site lies at the rim of the Þingvallavatn Lake.
About 50 kilometers north of the Þingvallavatn Lake lies the home of Iceland's second largest glacier Lángjökull. 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 Skjáldbreiður erupted masses of lava, which blocked the river. Due to this event, right after having melted from the glacier Lángjökull, water trickles underground into porous lava rock. From this point, it takes a drop of water between 30 and 100 years to travel 50 kilometers to the Þingvallavatn Lake in the Þingvellir National Park. The water is 50 to 100 years old once it reaches the lake from the melting glacier through the lava field, and is potable.
Silfra, by virtue of its location in the Þingvallavatn Lake, contains clear, cold water that attracts scuba divers drawn to its high visibility and geological importance; divers are literally swimming between continents. The rift claims a shallow depth nearest to the bank, but deepens and widens further out.
Scuba diving professionals have divided Silfra into three sections: Silfra hall, Silfra cathedral and Silfra lagoon. At its deepest, Silfra is 63 metres deep but diving this depth is seldom done and only by appropriately qualified advanced divers. In any case the most wondrous sights are in Silfra cathedral which is a 100 meter long fissure where you can see from end to end.