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Our transcription: For many years geologists have been able to relate individual facies to the pressure and temperature conditions of metamorphism. But they had no satisfactory explanation for the geologic processes that form metamorphic rocks, that is, until the theory of plate tectonics emerged. One good example is this relatively rare metamorphic rock called "blue schist." Experimental work had shown that the minerals in blue schist form only under very unusual metamorphic conditions. These conditions are a pressure range equivalent to a depth of 15 to 30 kilometers in the crust and a very cool temperature, only 200 to 400 degrees centigrade. That's the approximate cooking temperature of a kitchen oven or toaster. At a depth of 15 to 30 kilometers, however, the temperature is normally about twice as hot, 500 to 750 degrees centigrade. So the only way that rocks can be metamorphosed to blue schist facies , is to be quickly shoved down to those extreme depths and then rapidly brought back up before the rocks have time to heat up completely. And that's exactly what happens where two tectonic plates are colliding in a subduction zone. In fact, blue schist bearing rocks normally occur in long linear zones that mark ancient plate subduction boundaries.
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