5 July 2011

Varves from Yellowstone Lake

Posted by Callan Bentley

The other day, driving south through Yellowstone on our way to the Tetons, I stopped to give my students the obligatory look over Yellowstone Lake’s West Thumb, looking east towards the snowcapped Absaroka Range in the distance. But in addition to a lovely view and a few birds, we found something really cool…

Those are varves, seasonally-laminated couplets of sediment (light = summer; dark = winter) that are made of soft, semi-coherent mud from the bottom of Yellowstone Lake. However, they are not at the bottom of Yellowstone Lake, but are raised about 6 feet above the lake level. What the heck?, you may be asking yourself…

How did these lake-bottom sediments get above lake-top?

Uplift, my friends, uplift. The Yellowstone volcanic center is a dynamic place, and parts of it, like the nearby Mallard Lake dome, are uplifting. Yellowstone Lake in general is tilting, with the northwestern part (inside the caldera) going up, making things like Le Hardy Rapids, and flooding is occurring along the southern arms of the lake. As the dynamics of the magma chamber below deform the ground above, the lake flows away from some areas and drowns others. We were lucky enough to encounter this great evidence of the former lake bottom high and dry and available for inspection.