Gneiss
Written by Boss Tambang Tuesday, 01 December 2009 23:06
This boulder consists of gneiss (pronounced “nice”). Gneiss is a typical metamorphic rock type, in which a sedimentary or igneous rock has been deeply buried and subjected to high temperatures and pressures. Nearly all traces of the original structures (including fossils) and fabric (such as layering and ripple marks) are wiped out as the minerals migrate and recrystallize. The streaks are composed of minerals, like hornblende, that do not occur in sedimentary rocks.
Gneiss is a tough stone, more strongly metamorphosed than schist. You can see that unlike schist, gneiss doesn’t fracture along the planes of the mineral streaks. And thicker veins of large-grained minerals form in it, unlike the more evenly layered appearance of schist. With still more metamorphism, gneisses can totally recrystallize into granite.
Despite the highly altered nature of gneiss, it can preserve geochemical evidence of its history, especially in minerals like zircon which resist metamorphism. The oldest crustal rocks known are gneisses from western Greenland. Their carbon isotopes show that life existed there at that time, nearly four billion years ago. Gneiss is an old German word meaning bright or sparkling.
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