126. Acasta Gneiss (1983)
The Canadian Shield, the largest expanse of Precambrian rocks on Earth, is host to Earth’s oldest known rock – the Acasta Gneiss of the Slave Province of the Shield, located about 300 kilometres north of Yellowknife, Northwest Territories.
Gneiss is a type of rock that has been metamorphosed (affected by intense heat and pressure) at depth in Earth’s crust. Geological Survey of Canada geologist Janet King discovered the Acasta Gneiss during fieldwork in 1983.
The composition and texture of the Acasta Gneiss suggest that, before metamorphism, the gneiss was an igneous rock that formed about 4.03 billion years ago. We know it is that old because the Survey’s Geochronology Laboratory determined it by isotopic dating. This analytical laboratory process measures the ratios between parent and daughter atoms in mineral grains such as zircon that contain traces of radioactive uranium. Essentially, this is counting how many uranium atoms have transformed to lead, and permits calculation of an age. Isotopic dating gives absolute ages in years or millions of years. In contrast, the layered successions of rocks give only relative ages – older than, or younger than.
Category: Rocks, Fossils, Minerals and Meteorites
Stern, R. and Bleeker, W., 1998. Age of the World’s Oldest Rocks Refined Using Canada’s SHRIMP: The Acasta Gneiss Complex, Northwest Territories, Canada; Geoscience Canada, v. 25, no. 1, p. 27–31. https://journals.lib.unb.ca/index.php/GC/article/view/3966/4480
Mojzsis, S.J., Cates, N.L., Caro, G., Trail, D., Abramov, O., Guitreau, M., Blichert-Toft, J., Hopkins, M.D., and Bleeker, W., 2014. Component geochronology in the polyphase ca. 3920 Ma Acasta Gneiss; Geochimica et Cosmochimica Acta, v. 133, p. 68–96. doi:10.1016/j.gca.2014.02.019
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