A little atlas of thermal water colours

Colour as a signature

A spring’s hue tells the story of its chemistry and geology. It comes from three main causes: dissolved minerals, the micro-organisms that thrive in hot water, and the way light is scattered by fine suspended particles.

Milky blue

That opal blue, like some geothermal lagoons, often comes from dissolved silica: tiny particles scatter the light and give this almost unreal milky colour. The water looks dense, luminous, milky.

Greens and vivid colours

In the hottest pools, heat-loving micro-organisms sometimes colour the edges green, orange or brown — the Grand Prismatic Spring in the United States is the most famous example. The hotter the centre, the more the palette spreads outward.

The white of travertine

When calcium-rich water deposits its carbonate, it sculpts dazzling white basins and terraces, as at Pamukkale. Here the colour isn’t in the water but in the rock it builds as it flows.

The ochre of iron

Iron-rich springs leave rusty streaks: dissolved iron oxidises on contact with air and stains stones and channels. Each colour is a clue — learning to read it means understanding a spring before you even step in.