Friday, September 24, 2010

Friday Field Photos: Mystery Cracks

I posted this photo elsewhere just recently, and got a little feedback as to what the irregular to polygonal cracks on the surface of these columnar-jointed basalt faces might be. Here are a few more photos. At this point, MOH and I had turned around on our Winnemucca-to-the-Sea Highway journey (turned from going toward Winnemucca to going away from Winnemucca), in order to spot a pullout to view the cracks better, and also to get a better look at the nearby Deep Creek Falls. (Okay, we really turned around to look at the falls.)
From the south side of Highway 140 right below the falls and a little less than 3 miles west of Adel, Oregon, here's the best roadcut I saw of the basalt: other roadcuts are in tight, narrow turns with no nearby pullouts.
A closer view.
The columns are weathering into somewhat rounded, rectangular to irregular shapes, which at times look like stacks of flattened pancakes and other times look like hoodooish sentinels watching over the road. Outcrops up the hill show the same patterns, exposures at the falls look merely columnar. So far, the consensus is that the basalt is undergoing incipient spheroidal weathering.

Additional ideas are welcome!


View Bend OR to Winnemucca NV in a larger map

Trip report to be continued...


Dan McShane said...

They look like a combination of pillow basalt and degassing of the underlying water bearing sediments. Might be a flow into a fresh water lake or stream that drove all the water out and then cooled into columns. There is defintely some sort of preferential weathering taking place. Good eye spotting this. I will have to keep an eye out for this sort of thing while travelling through eastern Washington.

Silver Fox said...

I've seen similar patterns on a much smaller scale in rhyolite and ash-flow tuffs that have flowed into water, and that was the first thing I thought of. I didn't think these were really pillows, but am intrigued by the degassing from below idea. Most geos on twitter think it's spheroidal weathering not related to the flow's cooling properties.

I remain unsure. (It would be time for more study if I lived up there.)