A crack between worlds
The Silfra fissure is a tear in the Earth's crust, located directly on the Mid-Atlantic Ridge within Þingvellir National Park. This is the divergent boundary where the North American and Eurasian tectonic plates are pulling away from each other. The process is slow but relentless; the plates drift apart at a rate of about 2 centimeters each year. This constant tension is released through periodic earthquakes, which have shaped the valley for millennia. The current fissure opened dramatically during a series of earthquakes in 1789.
This location is geologically significant. Þingvellir is a UNESCO World Heritage site because it was the meeting place of the Althing, Iceland's parliament, starting in 930 AD. For over 800 years, leaders would gather in this rift valley, surrounded by stone and turf booths, to set laws and settle disputes. The very ground they met on is evidence of the planet's deep, geologic time.
Water filtered for a century
The water in Silfra is astonishingly clear, with underwater visibility that often exceeds 100 meters (328 feet). This is not seawater, but ancient glacial meltwater. It originates from the Langjökull glacier, about 50 kilometers away. This water begins a slow, 30 to 100-year flow, percolating through kilometers of porous underground lava rock. This extensive filtration process removes nearly all impurities, resulting in some of the purest water on the planet.
The water temperature remains a constant 2°C to 4°C (36°F to 39°F) all year. Because the spring water is constantly flowing into the fissure, it never freezes. Life in these cold, clear water is minimal. The fissure hosts unique forms of bright green algae, sometimes called "troll hair," and colorful mineral deposits on the basalt rock. While Þingvallavatn lake has a population of Arctic char, only the dwarf char, a subspecies, lives in the fissure year-round, mostly hidden in deep recesses. Another rare inhabitant is the endemic amphipod crustacean, Crymostygius thingvallensis, found only in the groundwater systems of the lake.