Why is blueschist significant?

There was a time when geologists thought the Earth's crust was more or less stable. We take plate tectonics so much for granted these days that it's hard to remember that until about 60 years ago just about the only action geologists accepted in the Earth's crust was the bubbling up of masses of igneous material in orogenic events, to form batholiths, or in volcanic events. There is a (ahem) mountain of (cough) rock-solid evidence for plate tectonics, which, quibbling about details aside, is as well accepted a theory as that of evolution or relativity. Blueschist was there in the beginning, however, and was an important datum in establishing the behavior of subduction zones.

What is schist in general?

Schist is a widespread metamorphic rock. It has been subjected to extremes of pressure, temperature, or both, within the earth. These extremes can do several things to rocks; in the case of schist, they cause a recrystallization of the minerals within the rock, which tend to reform in planes perpendicular to the direction of compression. In addition, schists are usually micaceous, exhibiting the luster characteristic of flakes of mica, and often also contain needle-like amphibole crystals (like actinolite). Blueschist forms in a very special environment, however, and is for that reason a strong evidence of plate tectonics.

How does blueschist form?

The oceanic crust forms out of long cracks from which mafic (manganese and iron-rich) rock extrudes; the extruded material moves away from the central ocean fissures something like a conveyor belt (this is the metaphor I was taught in college). Junk from the ocean and sediments from the land rain down more or less thickly on this oceanic crust as it moves toward the continents. Now, the continents float, because they are marginally lighter than the mafic material, while the oceanic crust hits the edge of the continents and bends down, back toward the mantle.

As it sinks, it heats up and big "bubbles" of melted oceanic crust boil up to erupt in andesite-producing volcanoes along the coast like the Cascades in North America. I was taught that these volcanoes can be fairly violent in their eruptions because the oceanic material has a lot of water locked up in it, and when pressure is released . . . .

But none of this was clear to geologists when plate tectonics was being debated, because there was no technology to image sinking oceanic crust. (Now it can be done, and the sonic image of subducting oceanic crust is one of the triumphant justifications of the theory.) Blueschist, on the other hand, develops as the crud on top of the oceanic crust literally gets scraped off and smashed by the "impact" with the continent.

Blueschist has as its signature mineral glaucophane, a beautiful blue amphibole which forms in high pressure/comparatively low temperature environments, and it pops up in outcrops as a highly metamorphosed rock in seas of somewhat less metamorphosed greenschists and serpentinites along (for example) the west coast of the United States, particularly in the coastal ranges of California and Oregon. (The areas which have experienced just the right high/pressure low temperature metamorphism are called "blueschist facies" precisely because of the characteristic presence of this rock there.)

Proponents of the plate tectonic theory pointed out that blueschists couldn't form in very deep (hot) places, because it is intense pressure, but not heat, that produces blueschists (high heat would cause different minerals to form in the schist). Occam's razor pointed to plate tectonics and the relentless pressure of the scraping of crustal plates of an entire planet--near the surface--as the most economical model to explain the widespread presence of heat-sensitive metamorphic rocks like the blueschists along the California coast (and here and there elsewhere on the planet where conditions have been right).

Blueschists are somewhat harder than the rocks they are normally associated with and often weather out of them as outcroppings. In the Bay Area they are called "knockers." I picked up a few pieces--which I treasure as being somehow significant in the history of science--on the Oregon coast in Bandon (they dynamited a nearby blueschist outcrop to provide hard stone for their jetty).


URLs.

Blueschist photos: http://geology.about.com/library/bl/images/blblueschist.htm. (Including some beautifully colored specimens.)
Greenschist: http://geology.about.com/library/bl/images/blgreenschist.htm.
Course lecture by a pro at Tulane: http://www.tulane.edu/~sanelson/eens211/contact&regional_metamorph.htm.
Tomography of the Japanese subduction zone: http://epsc.wustl.edu/classwork/classwork_210a/transparencies/wysession/w06.jpg.

References.

Alt, David, and Hyndman, Donald. 1978. Roadside Geology of Oregon. (See pp. 97-98 on the blueschist specimens in Bandon, Or.)
----------. 1984. Roadside Geology of Northern California. (See pp. 13-24 on the California coastal range and its oddities.)
Press, Frank, and Siever, Raymond. 1982. Earth (third edition). (See pp. 375-391 on metamorphism, 386 on blueschists in particular.)

I am not a professional geologist. I checked this information, but I may have gotten something wrong, alas!

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