Saturday, August 9, 2014

Geo 730: August 9, Day 586: Volcanic Neck

Volcanic necks (aka, chimneys, throats, plugs, etc.- I don't know if there's a "correct" term) such as this are pretty common in the Western Cascades, where thousands of feet of overlying volcanic and volcanic-derived debris has been removed by millions of years of erosion. For the most part, these "old Cascades" have not seen eruptive activity since the Pliocene, about 5 million years ago. The roughly cylindrical conduits that carried magma toward the surface, once cooled, are often more resistant to weathering and erosion than the surrounding, often fragmented, rock. Exacerbating this fact, the lava conduits themselves often create localized hydrothermal systems, which as we've seen in the series recently, can render resistant rock into piles of gloppy, utterly incompetent clays. As a result, the difference between the resistant core and less resistant host rock becomes even greater. So while tooling around in the Western Cascades watching the landscape, one often sees spires such as this sticking up. They're not all necessarily volcanic necks, and one should try to look for contacts to make sure it's not an isolated bit of, say, a dike or vein, but generally, in drive by, I'm generally comfortable identifying examples such as this as the conduit of an old volcano.

By the way, if you look at the FlashEarth Location, you should be able to pick out the spire above the curve in the road, to ENE of the crosshairs.

Photo unaltered. June 14, 2014. FlashEarth Location.

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