Acasta formation

Acasta gneiss

Orthogneisses, about 4 billion years old, with older enclaves of mafic rock, Slave Craton, Canada. Credit: Yuichiro Ueno, Tokyo Institute of Technology.

The Acasta formation is the oldest known outcrop of rock on Earth. It occurs about 350 kilometers north of Yellowknife in Canada's Northwest Territories and consists of gneiss (a type of coarse-grained metamorphic rock) that dates back just under four billion years – to around the beginning of the period of late heavy bombardment.