Moraine Lake, nestled within Banff National Park in Alberta’s Valley of the Ten Peaks, is renowned for its breathtaking scenery and intense turquoise-blue hue. This striking coloration sets the lake apart from typical bodies of water, suggesting a unique natural phenomenon is at play. This distinctive appearance is caused by the geological processes that shaped the Canadian Rockies, not by artificial dyes or plant life.
Glacial Erosion and Silt Generation
The process begins high above the lake, as Moraine Lake is directly fed by meltwater from the Wenkchemna Glacier and other ice field remnants. As the glacial ice moves across the landscape, it acts like giant pieces of sandpaper against the underlying bedrock.
The glacier’s immense weight and movement grind the solid rock beneath it into an incredibly fine powder. This mechanical weathering process, known as abrasion, is most pronounced during the warmer summer months when glacial meltwater is abundant. The resulting pulverized debris is continuously flushed out from beneath the ice by streams of meltwater.
The bedrock in this part of the Rockies is primarily composed of sedimentary rocks, including limestone and dolomite, which are susceptible to this grinding action. This finely ground material is then carried in suspension by the glacial runoff, transforming the clear meltwater into a milky, cloudy stream. This sediment-laden water eventually flows into Moraine Lake, supplying the lake with the material responsible for its brilliant color.
The Unique Composition of Rock Flour
The material created by the glacial grinding is known as “rock flour” or “glacial milk.” Rock flour consists of finely ground silt and clay-sized particles, typically less than 2 microns in diameter, making them too small to settle quickly. These minuscule mineral fragments remain suspended within the water column for long periods, creating a colloidal suspension.
The composition of this suspended material, derived from the local sedimentary rock, often includes calcium carbonate and other silicates. The extreme fineness of the particles is paramount; they are similar in size to the wavelengths of visible light. This small size allows them to interact with light in a way that larger, heavier sediment particles cannot.
The concentration of this rock flour is highest during the peak melt season in late spring and summer, which is why the lake’s color intensifies during this time. The higher the concentration of these suspended particles, the more turbid the water becomes, giving it a distinctive milky appearance before the optical effect takes place.
The Physics of Light Interaction
The vibrant turquoise color is an optical illusion caused by the interaction of sunlight with the suspended rock flour particles. When sunlight, composed of all colors of the visible spectrum, penetrates the lake’s surface, the rock flour acts as a selective light scatterer.
The water itself absorbs the longer wavelengths of light, such as red, orange, and yellow, causing them to disappear quickly beneath the surface. However, the suspended rock flour particles preferentially scatter the shorter wavelengths, specifically blue and green light, back toward the observer.
This phenomenon, related to Mie scattering due to the particle size, results in the perceived brilliant turquoise hue. The intensity of the color can vary dramatically with the time of day and the angle of the sun, as the viewing angle changes how the scattered blue and green wavelengths are reflected. The sheer volume of these suspended, light-scattering particles is what gives Moraine Lake its opaque blue color.