A Question of Balance

Probably the least popular blogging topic around here is when WC indulges in geology. Rocks are admittedly not for everybody, but this is seriously cool.

The Snake River plain is the path of the North American plate over the Yellowstone hotspot, which now lies under Yellowstone National Park. The plain is a vast, shallow trench that cuts across Basin and Range mountain structures, roughly parallel to movement of the North American plate. It is underlain almost entirely by basalt erupted from large shield volcanoes. Beneath the basalts are rhyolite lavas and ignimbrites that erupted as North America passed over the hotspot. The central Snake River plain is similar to the eastern plain, but differs by having thick sections of interbedded lake and stream sediments, including the Hagerman fossil beds.

WC was at the Snake River plain, where the the middle zone blends into the eastern zone. At Balanced Rock Park, Salmon Falls Creek has cut through the basalts and into the partially cemented ash and lake layers.

Salmon Falls Creek Canyon, near Castleford, Idaho

It’s a wonderful picnicking spot, with Canyon Wrens to serenade you and cooler from the creek and rock walls. You may ask yourself how that tiny creek carved a 1,200 foot deep canyon. The answer is partly the dam upstream that diverts most of the water to agriculture. And partly the current absence of an ice age, when much greater precipitation fell in the arid drainage than does now.

The creek has carved down into the ash layers, displaying the detailed bedding.

Rhyolite and Ignimbrite Bedding

The lighter stuff is Rhyolite; the darker is Ignimbrite, which flows as a burning hot ash flow, and acquires a lot of carbon as a result, making it darker. The ash fell in level layers. The subsequent lava eruptions caused both pressure downward, from the weight of the overlaying lava layers – more than 1,200 feet at Salmon Falls Creek – and the pressure of the molten rock pressing up from underneath. The result is the curving in the formerly level bedding you see here.

As the lava on top cooled, it fractured into characteristic columns. Water and wind erosion then carved those columns into fantastic shapes. These are on the canyon wall above Salmon Falls Creek.

Eroded Basalt Columns, Salmon Falls Creek Canyon

And the most famous eroded column of them all, Balanced Rock, at 55 foot wide, 40 ton rock balanced on 1.5 foot by 3 foot base.

Balanced Rock, with Mrs. WC for Scale

The narrow base is softer, partially metamorphosed Rhyolite, a layer of ash supporting the remains of a column of basalt. Here’s another view from the road, again with Mrs. WC climbing up.

Balanced Rock, Seen from the Road

It’s worth the drive.

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