A chemical clock in a crater
Each November, the water in Blue Lake / Warwar undergoes a remarkable transformation. From a muted steel-grey color in the winter months, it turns into a brilliant cobalt blue. This predictable annual change is not caused by algae, but by a chemical reaction tied to the seasons. The lake, which occupies a volcanic maar, shows the local limestone-rich aquifer. This geology provides an ample supply of dissolved calcium carbonate in the water.
The primary trigger for the color change is water temperature. As summer approaches and the sun warms the surface layers to around 20° Celsius (68° Fahrenheit), the solubility of calcium carbonate decreases. Simultaneously, dissolved carbon dioxide is released from the warming water, which increases the pH. This change causes microscopic crystals of calcite (a form of calcium carbonate) to precipitate and become suspended in the upper layers of the lake. These tiny, white crystals are exceptionally good at scattering the blue wavelengths of sunlight, making the entire lake appear intensely blue. In winter, the cooler water dissolves the calcite crystals, and the lake returns to its darker, less bright state. ## The aquifer