August 30, 2016
The photo above shows Cracker Lake in the east-central part of Glacier National Park, Montana. This one mi (1.6 km) long lake, referred to as a tarn, was formed in a basin carved by a glacier during the last ice age. After the glaciers from the last ice age melted completely new valley glaciers, such as Siyeh Glacier, formed during a recent cold period that ended around 1850. Tarns or cirque lakes are quite common in Glacier Park.
The beautiful turquoise color shown is the true color of the water. Sometimes called glacial milk, the unusual color is due to the presence of rock flour, which consists of tiny clay particles formed as rocks stuck to the bottom and sides of a glacier grind against bedrock. This abrasion reduces some of the bedrock to a fine powder that looks like the flour used to make bread. These minute clay particles are just the right size to reflect more of the blues and some of the greens than any of the other wavelengths, thus the water appears distinctly blue. As the ice melts, the rock flour is exposed and transported downstream by meltwater, often ending up in a nearby tarn, such as Cracker Lake.
In addition to the rock flour, meltwater also carries larger pieces of rock material, including pebbles, sand and silt. These larger rock particles reach the lake and quickly settle to the bottom as long as the water is flowing (spring/summer). In contrast, the much smaller rock flour remains suspended in the water until the fall and winter when the meltwater stops flowing or the lake freezes over. Only then does the water become calm enough to allow the rock flour settle to the bottom. Photo taken on August 20, 2016, at 3:44 p.m.Photo Details: Camera Model: Canon PowerShot ELPH 360 HS; Lens: 4.5-54.0 mm; Focal Length: 8.17mm; Aperture: ƒ/4.5; Exposure Time: 0.0020 s (1/500); ISO equiv: 100; Software: QuickTime 7.6.6.
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