Orthogneisses, about 4 billion years old, with older
enclaves of mafic rock, Slave Craton, Canada. Credit: Yuichiro Ueno,
Tokyo Institute of Technology
The oldest known outcrop of rock on Earth. It occurs about 350 km 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.