A cache by STNolan Message this owner

Hidden : 6/1/2023

Difficulty:

Terrain:

Size: Size: other (other)

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Geocache Description:

Mendenhall Rock Flour - Juneau Earthcache #5

You are standing in front of the Mendenhall Lake, take some time to enjoy the scenery around you. Depending on what time of the year you're visiting, you might see some ice chunks floating along the waterline, in the fall the fog may sock you into your own little world, and in the summer you may see one of the most gorgeous sights in all Southeast Alaska - the sun. As you take in the views I hope you learn a thing or two about a colorful natural phenomenon! As a reminder, this is an Earthcache - there is no container here, rather you must visit the two sets of coordinates to collect answers to a series of questions and send the answers to the CO to claim the cache.

Glacial Erosion

Glaciers work kind of like gigantic sheets of sandpaper. You know how, when you rub the coarse side of sandpaper over a wooden surface, you create a lot of fine sawdust? Well, glaciers do pretty much the same thing, except they work on mountain rock rather than wood. Glacial ice moves very slowly downhill. While it moves, bits of rock and gravel get stuck between the ice and the mountain, forming a coarse surface similar to the sand on sandpaper. The rocks grind together, creating the glacier equivalent of sawdust. Geologists call this dust “rock flour,” and as glaciers melt during spring and summer, they transport it down into mountain lakes.

Here’s where things get colorful. The rock flour is so fine that it doesn’t sink to the bottom of the lake. Instead, it remains suspended throughout the water. When sunlight hits, the water absorbs the long wave colors of the spectrum: the reds, oranges, and yellows. At the same time, the rock flour absorbs some of the shortest light waves, the purples and indigos, then scatters the remaining light back to our eyes. Thanks to these processes, the reflected light is mostly green, with a dash of blue. And voilà: turquoise water!

Each glacier produces its own flavor of rock flour and as such, no two rivers or lakes sport the same hue. The intensity of the color of the water is dependent on several factors: the amount of glacial runoff, the composition of the rock flour, how much rain the region has seen recently (a lot of rain can dilute the runoff), and the angle at which the water is viewed. I highly recommend taking a further look at some of my listed sources for more information, particularly if you are interested in how light refraction plays into our perception of color in glacial waters!

Questions

In order to log this earthcache please send me the answers to the following questions via the message center or email. Please make sure you take good notes when you're on scene! Think critically and have fun! Please note that this cache might not be able to be completed during winter months when the lake is frozen over.

1. What color does the water appear to be as you view it at GZ? Review a recent weather report for the past 3-5 days. How might that weather have impacted the color of the water? (Hint: Think about the potential effects of aggitation and dillution)

2. Is the water color more or less intense as you look straight down versus looking out at a distance? Why do you suppose the color looks different?

3. Proceed to the second set of coordinates at WP2. This is Steep Creek. What differences does the location have compared to the original coordinates that might explain the difference in water coloration.