Thursday, September 3, 2009

Wut?

There was an interesting article in OregonLive last night on a successful campaign to clean up the mess and misuse along a 21-mile stretch of the Molalla River near Portland. It was such a success, in fact, that the run is being seriously considered as an addition to the streams recognized and protected under the National Wild and Scenic Rivers system.

What drew my attention, though, wasn't the story as much as the lead photo. Any geology types have an idea what we're looking at? I'm guessing it was a lava tube or flow; those radial columnar joints suggest a circular outline, at least on the bottom. The bedrock is somewhat obscured by moss and plants above the midline of the structure, and I can't tell from the picture whether the radiating joints continue to make a full circle. Whatever it is, I like.
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4 comments:

  1. Silver FoxSeptember 5, 2009 at 7:38 AM

    Cool pic! The columnar jointing in basalt sometimes baffles me - maybe it is a filled-in lava tube.

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  2. Chris RSeptember 5, 2009 at 8:02 AM

    Prepare to make myself look foolish

    Is it certain we're looking at basalts? The fracture patterns look more like cleavage in a shale than jointing to me - perhaps it could be a fold hinge?

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  3. Anne JeffersonSeptember 5, 2009 at 10:29 AM

    No, Chris, it's definitely basalt - probably ~17 Ma Columbia River Basalt. I think Silver Fox's idea is a possibility. If I recall correctly, the CRB flows were quite thin and inflated with lava tubes, which allowed them to travel such long distances from their sources. But I'm not a petrologist...

    Anyways, it's a beautiful picture, Lockwood.

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  4. LockwoodSeptember 5, 2009 at 12:32 PM

    I'm confident it's basalt. There are some marine seds interbedded with CRB in the W. Cascades (the Willamette Valley was an embayment at the time), but they tend to be nearshore, and strongly bedded. No massive shales that I know of. There is some gentle folding in the region, but I don't think you could get that feature from deformation. The sharp cutoff on the lower left suggests a minor fault to me.

    As I noted in the post, this is from the news... I don't really have any idea where it is. But I agree with Anne: it's strikingly beautiful.

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