Friday Fragment: Blueschist

Today's fragment is one of my all time favorites, blueschist. This rock type is pretty easily recognizable, strangely enough, by it's blue color and schisty texture. The main mineral responsible for the blue color is glaucophane. Glaucophane forms at very high pressures, an equivalent of a minimum of 15-18 km (about 10 miles or more), yet relatively low temperatures of 200 to 500 Celsius. At those depths, the temperature should be much higher. So what kind of environment could have created this beastie?

It turns out this is a rock we couldn't have explained before the advent of plate tectonics. When cold oceanic crust subducts below the edge of a neighboring plate, there is some lag time before it can heat up to the ambient temperature of the depths it now occupies. Furthermore, since subduction consists basically of wedging surface material down into the earth, material along the upper surface of the subducting plate can be carried to great depths, detach, and be wedged back up toward the surface. In other words, material can be carried down to substantial depths (and the associated pressures), then fairly rapidly- in geologic terms- get shoved back toward the earth's surface, before it has time to heat up.

Of course, that's a pretty uncommon set of events. In consequence, blueschist is a pretty uncommon rock.

The first two pictures are looking parallel to the foliation. You can think of it as looking at a book edge-on; it's the view that shows the structure best.
Click for bigger.
Click for bigger. And the third is looking perpendicular to the foliation- again, you can think of this as looking at the cover, or open page, of a book. You can't see the internal structure and contortions this material went through during metamorphosis as clearly, but (if you look at the full size picture) you can see the felty texture formed when a foliated metamorphic rock is dominated by acicular (needle-like) mineral grains. The term for this texture that I vaguely remembered was nematoblastic, but like so much I have learned, this term is now obsolete. Another rock type that I associate with this felty texture is amphibolite, a product of fairly high-grade metamorphism of basalt. (Followup: this surface is dominated by what is probably a mixture of chlorite and muscovite- not acicular minerals- but the felty texture that dominates the rock shows through pretty well, I think)

This rock is a boulder that sits at the southeast corner of Wilkinson Hall, the geosciences building here at OSU. I'm not certain of its source, but I'd be willing to bet it's from Bandon, Oregon.
(full-sized GE image)

There is a large pod of blueschist along the terrace just inland from southern shore of the Coquille River, and this rock was quarried to build the south jetty. If you're ever driving through, it's about a mile off Route 101, and very much worth the time it takes, to go look at the jetty. The quarry is somewhat overgrown, but collecting should be done in that area, not on the jetty itself.

One last thing: those of you who have had the oppportunity and pleasure of studying thin sections know how stunningly beautiful even mundane rocks can become under a petrographic microscope. Those who haven't, well, take a look at some of these pictures. There are quite a number of pleochroic minerals, those that change color depending on the angle and polarization of light passing through them, but there are very few as stunning as glaucophane. I think these photos are copyright, so I'm just posting the link. If gemstones looked as nice macroscopically as many minerals do microscopically, no one would have ever heard of diamonds.