AW #28: Deskcrop!

Or, in my case, a shelf crop photographed on the floor. Matt at Research at a Snail's Pace is hosting this month's Accretionary Wedge with the theme of Deskcrops- in other words, rocks that "crop out" indoors; I've been thinking of them as domesticated rocks. My contribution is a vaguely heart-shaped pillow basalt that I picked up for Callan, who sent out a request a year and a half ago, and is hearing about this acquisition only now.  This was obtained on Marys Peak on a geoexcursion with Dana a bit more than a month ago... and I really need to get back to this series and finish it up.  So as it turns out, this isn't really my deskcrop, after all. It just happens to be residing in my apartment at the moment. (All the pics should get much bigger if you click on them.)

To highlight some of the distinguishing features of pillow basalt, let's start with the exterior: characteristically, if fairly fresh, the outside will have a glassy rind with and "alligator-skin" texture- sort of pebbly, where the "pebbles" are a couple of cm. in diameter and flattened.  

The scale is in inches on the outer edge and centimeters on the inner side.  Accessory minerals are zeolites and calcite, and you're almost guaranteed to see some combination of these in pillows in my experience.  This particular piece is about a quarter of a single pillow, and the overall shape of the whole pillow is roughly sphereoidal, generally with some flattening along the original vertical axis- oblate sphereoids.  Somebody thought they looked like pillows, though they're not flattened that much, hence the name.

But there are a number of ways rocks can come to have round shapes- erosional transport and sphereoidal weathering are two that come to mind, and there are plenty of ringers for pillows in other basalts around the PNW.  The glassy rind is very susceptible to weathering, and is often not very distinctive in older outcrops. If I had to pick one feature as diagnostic, I'd pick radial jointing, shown nicely in the photo above. Note the coating of white secondary minerals- again, calcite and zeolites- along the joints. Some of the zeolites are pretty resistant to chemical weathering, and I've seen quite a number of "outcrops" where the spiderwebs of white minerals allowed the original pillows to be identified, even though the entire pile had weathered to clay, or saprolite.

I generally get out the door pretty quickly after I wake up in the morning- I generally pick Ozma up and give her a little scratching, but I don't spend a lot of time sitting around.  She was pretty jealous that I was paying so much attention to a rock this morning, and despite repeatedly moving her, she managed to get into nearly half the pictures as she tried to figure out the appeal.

This is a photo of our quarry at the quarry... a fair portion of this fell apart in the walk back to the car.

And above is the exposure it came from. As I said, I'll get back to this and write it up in more detail sometime, but it's nearly 6:00, I still have a bunch of stuff I want to read, and that's all for now.