Why is the Blue Lagoon blue?

A wonder of the world 

The Blue Lagoon water is one of the world’s wonders, found nowhere else in the world. This unique resource originates deep within the bedrock, where ground water and seawater combine to form something completely new. Merged in darkness, under immense pressure, aided by geothermal heat, the geothermal seawater is rich in silica, algae, and minerals. But why is it blue?  

The interplay of silica and sunlight 

The Blue Lagoon water contains a considerable amount of the chemical compound of silicon (represented by the symbol Si) and oxygen (represented by the symbol O). The compound contains twice as many units of O as Si and has the chemical symbol SiO2. This is the white, clay-like substance that our visitors are so familiar with. Commonly referred to as silica, it reflects light in a way that gives the lagoon its characteristic blue color—similar to how the sky appears blue. 

Sunlight contains all visible colors, and each color has its own wavelength. Red light has the longest wavelength, and blue the shortest, with other colors falling in between. When the Blue Lagoon water cools, tiny silica particles (SiO2) precipitate. These particles are much smaller than the wavelengths of visible light, making it difficult for them to reflect light. 

However, the silica particles do reflect relatively more of the shorter wavelengths, which are closer to their size. Since blue light has the shortest wavelength of the visible colors, it is the color that reflects the best when sunlight hits the silica in the water. As a result, the lagoon takes on its distinctive blue hue.  

The color is always in flux—during sunny days, the blue color is more apparent. When it is overcast or dark outside, the quality of the color changes. It is a living body of water in so many ways; always reacting to nature, and always worth exploring.