Why is Cracker Lake Turquoise?

Have you ever been or seen photos of turquoise lakes? If you’ve been following me for awhile then you have seen my pictures of Cracker Lake, which is one such lake. Back in 2017 I was lucky enough to visit Cracker Lake, which is located around six miles into the backcountry of Glacier National Park. As I was setting up my tent, I remember looking out at this turquoise lake and thinking how it looked even better than the many pictures I had seen of it when preparing for this trip. Which is why I was surprised the next morning when I woke up to a very different scene, a dark blue lake. Where had the turquoise color gone? As I got out and looked over the lake I watched how the lake slowly turned back to turquoise, it was fascinating to see.

Since then I still kept thinking about that moment and wondering about the science behind it. Eventually I just decided to email the National Park Service and ask them. They were kind enough to respond and provide me with not only the answer I had been looking for but also some article links. The answer is what geologists call “rock flour.”

Rock Flour, also called glacial flour, is created by glaciers. As the ice slowly moves downhill, it traps the rock beneath it and pulverizes it into a fine powder. When the glaciers melt, the runoff takes the rock flour into the lake. The powder is so fine that it doesn't immediately sink and it stays suspended within the lake. That explains why the lake looked so opaque but how does this affect its color? The answer to that question is wavelengths. The color we see is formed from light bouncing off an object to our eyes and each color has its own wavelength. A good visual is to think of waves formed in the water and the space between each wave. Sometimes the waves are fast and have less distance between each other and other times it’s the opposite. The shorter wavelengths are the cooler colors of purples, blues and greens and longer wavelengths are the reds, yellows and oranges. The water itself absorbs the longer wavelengths, which is why we see water as blue and the rock flour absorbs purples and indigo wavelengths leaving our eyes to only see green with some blue. So cool! Also, the color of the lake will actually shift depending on factors like the angle of light, amount of rock flour and depth of the lake. This explains why at Cracker Lake I woke up to a blue lake that morning and as the sun rose above the mountains the light changed it back too turquoise.

Gotta love the natural sciences, a little geology and and a dash of color theory thrown in to jazz it up! Because of the power of glaciers, these lakes are truly something special in our great world. It does make me wonder though, with the glaciers melting in places like Glacier National Park, how much longer will these lakes be able to hold their turquoise color?

https://youtu.be/dBQoGdUfm1s

Sources:

https://earthobservatory.nasa.gov/images/145055/how-glaciers-turn-lakes-turquoise

https://indianapublicmedia.org/amomentofscience/rock-flour.php

https://www.livephysics.com/physical-constants/optics-pc/wavelength-colors/

https://www.nps.gov/glac/learn/nature/melting-glaciers.htm