Eclogite

Click the photo to see the larger version. When basaltic rocks are brought to depths of more than 40 kilometers and temperatures above 500°C, the mineral assemblage of olivine and feldspar changes to garnet and pyroxene. It is rare for rocks at that depth to be brought back to the surface, and rarer still for those rocks to avoid being altered again (by retrograde metamorphism) on the way up.

While eclogite is rare at the Earth's surface, it is very common at depth and has an important role in plate tectonics. When oceanic plates are subducted, their great thicknesses of basalt and gabbro change to eclogite and gain nearly 10 percent in density. This change in density makes subducted plates even more eager to descend.

This eclogite specimen from Jenner, California, was part of a subducting plate during Jurassic times, about 170 million years ago, when it formed. Much more recently, during the last few million years, it was raised and mixed into younger subducted rocks of the Franciscan complex. The body of eclogite is no more than 100 meters across today. It consists of high-magnesium pyrope garnet, green omphacite (a high-sodium/aluminum pyroxene), and deep-blue glaucophane (a sodium-rich amphibole).

For more photos see the Metamorphic Rocks Gallery.

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