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.
Eclogite consists primarily of sodium-rich pyroxene and garnet rich in aluminum and magnesium (pyrope-almandine). It may also include rutile, kyanite and quartz. Retrograde metamorphism yields glaucophane and serpentine.
Images 1-8 of 8
- Graphic Index
- Text Index
