Acasta Gneiss

Acasta Gneiss is an ancient rock complex in Canada’s Northwest Territories, about 300 kilometers north of Yellowknife, that preserves some of the oldest known pieces of Earth’s crust across roughly 13,000 square kilometers. Made chiefly of tonalitic and granodioritic gneisses, it includes the Idiwhaa tonalitic gneiss, whose igneous zircons have a U–Pb crystallization age of 4.02 billion years, making it, as of 2019, the oldest widely accepted known felsic rock on Earth. More than a single formation event, it records over a billion years of repeated melting, crystallization, metamorphism, and intrusion, making it a key site for understanding the early Earth.

Northwest Territories · Canada