Gneiss from the Precambrian of Michigan, USA. (Cranbrook Institute of Science collection, Bloomfield Hills, Michigan, USA)

Gneiss is a high-grade, foliated metamorphic rock. It is readily recognized by the presence of alternating bands of light and dark minerals. It forms by regional or burial metamorphism of schist, granite, or granite-like intrusive igneous rocks.

This gneiss is a sample of the oldest known rock in Michigan. The Watersmeet Gneiss has zircons that have been dated to 3.56 billion years old (early Paleoarchean). The date apparently corresponds with the age of the protolith - the original rock before metamorphism. Very old rocks are common on the Moon and in the Asteroid Belt. Very old rocks are scarce on Earth - this is the result of erosion by running water and plate tectonic recycling and deformation. The oldest reported Earth rocks are all Canadian - the Acasta Gneiss (Eoarchean, 4.03 Ga), rocks in the Nuvvuagittuq Greenstone Belt (Eoarchean, 4.28 Ga), and subsurface Baffin Island rocks (Eoarchean, 4.45 to 4.55 Ga). The latter rocks are the ~same age as Planet Earth (4.55 Ga).