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
The Blue Lake is geologically young, situated within the Mount Gambier Volcanic Complex, which may have last erupted as recently as 6,000 years ago. The eruption was a phreatic eruption—an explosive event caused by magma superheating groundwater. This created the deep crater, or maar, that the lake now fills. The complex includes four crater lakes, but two have since dried up due to a falling water table.
The lake's physical dimensions are impressive. It has a surface area of about 70 hectares (170 acres) and an average depth of 72 meters (236 feet), with the deepest point measured at 77 meters (253 feet). The lake's water source is not from rivers or streams, but from the slow seepage of groundwater through the porous Gambier Limestone that forms the regional unconfined aquifer. This natural filtration process makes the water exceptionally clear, a necessary condition for the light-scattering effect of the calcite crystals to be so dramatic. The lake itself is the primary drinking water source for the city of Mount Gambier.