30 December 2010

A whopper of a mystery rock from the Golden State

Posted by Callan Bentley

When I was out in California in October 2009, about the same time that Garry Hayes passed me on the highway heading up to Yosemite while I was headed down towards the Modesto Airport, I pulled over to check out some rock exposures on the side of the road. This was near the Don Pedro Reservoir (right about here). While standing there, my attention was caught by a strange looking “rock” in a ~2 m wide hole at the side of the road.

Here’s what it looks like:

As you can see, it shows a “malted milk ball” (Whopper) texture, with dark spherules full of vesicles sitting in a light-colored, poorly-sorted matrix. The “orbs” have brown rims that are far less vesicular.

Here’s a close-up of the edge of the big central orb, shown with my patented* “animated focal plane” GIF technology.

Field of view is about 1 cm. The dark little vesicular orbs and ellipsoids vary in size between 1 and 15 mm (long axis) are distributed in a matrix that looks suspiciously like concrete. These “Whoppers” have orangey brown (oxidized?) rinds.

What’s going on here?

Is this even natural? If  you flip it over, you see this:

That looks a lot like an external mold of the flexible spiral-bound housing used to run cables through. So I guess it is concrete …of a kind I’ve never seen before. Where do the little orbs come from? Are they natural, only to be incorporated into an artificial “rock” (urbanite)? Or are they also manufactured?

Has anyone run into these vesicular “malt balls” in concrete before? Do tell!


* Not really.