By Andrew Alden
In gneiss, less than 50 percent of the minerals are aligned in thin, foliated layers. You can see that unlike schist, which is more strongly aligned, 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 turn to migmatite and then totally recrystallize into granite.
Despite its highly altered nature, gneiss can preserve chemical evidence of its history, especially in minerals like zircon which resist metamorphism. The oldest Earth rocks known are gneisses from Acasta, in northern Cananda, that are more than 4 billion years old.
Gneiss makes up the largest part of the Earth's lower crust. Pretty much everywhere on the continents, you will drill straight down and eventually strike gneiss.
Gneiss is an old German word meaning bright or sparkling.
For more photos see the Metamorphic Rocks Gallery.