Acasta Gneiss

A tonalitic gneiss complex in Canada’s Northwest Territories that contains rocks dated to about 4.0 billion years ago, among Earth’s oldest known crustal rocks.

U-Pb dating: A geochronological method that dates minerals by measuring uranium isotopes and their lead decay products. Uranium–lead measurements on zircon establish the ages of its oldest rocks.
Gneiss: A coarse-grained metamorphic rock with compositional or mineral bands formed under high temperature and pressure. The complex’s name describes its banded metamorphic texture.
Nuvvuagittuq Greenstone Belt: An Archean rock assemblage in Quebec whose age has been debated, with proposed dates older than 3.8 billion years. Its disputed older age complicates claims about Earth’s oldest preserved rocks.
Early Earth: Earth’s first billion years, when its surface, atmosphere, and interior evolved under conditions unlike those of the modern planet. The complex offers rare direct evidence about crust formed during this interval.
Zircon: A durable zirconium silicate mineral that can preserve trace elements and radioactive isotopes from its formation. Zircon crystals retain the isotopic evidence used to date the complex.
Tonalite: A coarse-grained igneous rock dominated by plagioclase feldspar and quartz, with little alkali feldspar. Tonalitic compositions make up much of the ancient crustal material.
Isua Greenstone Belt: A Greenland rock belt containing some of the oldest widely accepted evidence of early life and ancient crust. It preserves comparably ancient rocks but also records early sedimentary and biological evidence.
Continental crust: The relatively low-density, silica-rich outer layer that forms Earth’s continents. Its ancient tonalitic rocks bear on when and how continental crust first developed.
Hadean: The earliest division of Earth’s history, extending from its formation about 4.54 billion years ago to roughly 4.0 billion years ago. Its oldest rocks approach the end of this poorly preserved eon.
Metamorphism: The transformation of rock through heat, pressure, and chemically active fluids without complete melting. Metamorphism reshaped the complex after its protoliths formed.

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