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.
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 Þingvellir National Park. This groundwater is potable.
Silfra, by virtue of its location in the Þingvallavatn Lake, contains clear, cold water that attracts scuba divers due to its high visibility and geological importance; divers are literally swimming between continental plates. The rift is shallow nearest to the bank, but deepens and widens further out.
Silfra can be divided into three sections for diving: Silfra hall, Silfra cathedral and Silfra lagoon. Silfra has a maximum depth of 63 metres but diving to this depth is seldom done as it requires technical diving skills. The most spectacular section is Silfra cathedral which is a 100 metres long fissure where a diver can see almost from end to end.
The rift offers exceptional visibility and has been placed on the world's top 50 diving destinations by at least one travel site. The visibility reaches 70–80 metres. Diving tour operators however make exaggerated claims of 150 to 300 metres.
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- Geology of Silfra, Silfra.org – website
- Water in Silfra, Silfra.org – website
- Silfra Caves, Silfra.org – website
- Top Diving Destination, Dive Iceland
- Video of Diving Silfra, Diving Video of Silfra
- Detailed information and photos of Silfra, Silfra.org – website
- Diving, Þingvellir National Park website
- Detailed description of Silfra, DIVE.IS website
- Three sections of Silfra, Total Iceland
- Into the deep: World’s 50 best dive sites, CNN Travel