Industrial runoff, geological wonder
The Blue Lagoon exists because of the Svartsengi Geothermal Power Plant, which began operating in the 1970s. The lagoon is a human-made basin filled with the mineral-rich byproduct of generating electricity and hot water. The process starts 2,000 meters below the surface, where extreme heat and pressure create geothermal seawater. This fluid is a mixture of about two-thirds seawater and one-third freshwater.
At these depths, the water reaches a scorching 240°C (464°F). It is drawn to the surface and used to run turbines for electricity generation. Afterward, it passes through a heat exchanger to warm fresh water for a municipal heating system. The remaining brine, now cooled but still laden with dissolved minerals, is released into the surrounding Illahraun lava field. This lava field is geologically young, having formed during an eruption in 1226. The porous volcanic rock was not naturally sealed, so as the silica-rich water flowed into it, the minerals precipitated and coated the rock, forming the lagoon's basin.
A colloidal sky in water
The lagoon's famous milky-blue color is not a pigment; it is an optical effect created by chemistry and light physics. The main ingredient is silica (silicon dioxide), which is dissolved in the superheated water deep underground. As the water is brought to the surface, it cools and depressurizes rapidly. This change causes the dissolved silica to precipitate out of the solution, forming a colloidal suspension of minuscule amorphous silica particles.
These silica particles have the perfect size—averaging around 40 nanometers—to scatter visible light, a phenomenon known as Rayleigh scattering. This is the same physical principle that makes the Earth's sky appear blue. The tiny particles scatter shorter-wavelength blue light more effectively than longer-wavelength red light. When sunlight penetrates the lagoon, the blue light is scattered and reflected back to the observer's eye from the suspended silica and the white silica mud lining the bottom. The water itself is colorless. The final bathing temperature of the water averages a comfortable 37–39°C (99–102°F).