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Gneiss ("nice") is foliated metamorphic rock that has a banded
appearance and is made up of granular mineral grains. It typically
contains abundant quartz or feldspar minerals.
Gneiss is a typical rock type formed by regional metamorphism,
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.
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
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 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.
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