Montana's Earth Science Picture of the Week

Do not adjust your set!

Photo by Rod Benson 2015

This photo was taken from near the base of Siyeh Glacier in the east-central part of Glacier National Park. The view shows a 1-mile long lake named Cracker Lake, which was formed in a basin carved by a glacier during the last ice age. All of the glaciers from the last ice age melted completely, and then new valley glaciers such as Siyeh Glacier (photo) formed during a recent cold period that ended around 1850 ("The Little Ice Age"). Such lakes, referred to as "tarns" or "cirque lakes," are common in Glacier Park. Here is a photo taken from the opposite end of the lake.

To access my blog post about the day I hiked above Cracker Lake (lots of photos), click here: The was my best day of hiking ever!

Got Milk?. . .
The beautiful turquoise color shown in the photo is the true color of the water. Sometimes called "glacial milk" (photo), 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. As the ice melts this rock flour is exposed and transported away by melt-water (photo), often into a nearby tarn such as Cracker Lake.

They won't settle down! . . . .
In addition to the rock flour, melt-water also carries larger pieces of rock material - 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 melt-water stops flowing or the lake freezes over. Only then does the water become calm enough to let rock flour settle to the bottom.

Why so blue? . . .
Sunlight includes many different wavelengths of light ranging from the longer "reds" to the shorter "violets" (ROYGBIV). A white T-shirt is white because it reflects all of the wavelengths, a black shirt is colorless because it absorbs all of the wavelengths, and a red shirt is red because it absorbs the OYGBIV and reflects the R (red wavelengths). Apparently the tiny particles of rock flour suspended in the lake are just the right size to reflect more of the blues and some of the greens than any of the other wavelengths.

Seriously? "Cracker" Lake? . . .
Before the park was established, there was a copper mine at the head of the lake. According to legend the mine received its name when two prospectors, L. C. Emmonds and Hank Norris, after staking their claim, had a lunch of cheese and crackers on the site. Eventually the lake was named after the mine.

Below: In 2011 a team of researchers from Middlebury College in Vermont extracted a core sample from sediment in Cracker Lake. The core, which measured 3.19 meters, was analyzed along with cores from several other lakes in Glacier Park to better understand the advance and retreat of glaciers since the last glacial period ended about 10,000 years ago. Glaciers in the park completely melted as the last glacial period (sometimes referred to as the last "ice age") ended. Evidence from the lake sediment cores and along with other types of evidence indicate that glaciers reappeared in this area about 6,500 years ago, and then experienced several periods of advance and retreat. Periods of advance occurred from 6,900 to 5,700 years before present (BP), from 3,700 to 1,900 BP, from 1,600 to 1,300 BP, and then again during a recent cold period referred to as The Little Ice Age that ended around 1850 AD. Perhaps more importantly, evidence shows that the time period since the end of Little Ice Age is the most dramatic episode of ice retreat in the last 10,000 years.

Photo courtesy of Jeff Munroe, Chairman of the Geology Dept. at Middlebury

Term: tarn (cirque lake) - Be sure to tell how it is formed?


*The blue won't last forever

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