Sunday, April 3, 2011

Rock of Ages

A sample of the Acasta Gneiss. The Acasta Gneiss is dated at 4.2 billion years old. Specimen size: 5 cm. Specimen and picture: A. Fraser

It is generally accepted that the age of the Earth and the rest of the solar system is about 4.55 billion years (plus or minus about 1%). This value is derived from several different lines of evidence. Unfortunately, the age of the Earth cannot be determined directly from material that is solely from the Earth. Meteorites have been used to date the age of the Earth as these and the other planets were formed at the same time. The Earth is a dynamic planet and the processes of erosion and crustal recycling as a result of plate tectonics have destroyed the entire earliest surface of the Earth. The oldest known exposed rock outcrop found so far is that of the Acasta Gneiss in Canada. The Acasta Gneiss was dated by radiometric means at 4.2 billion years (4.2 Ga) in age (Tsuyoshi, 2007). And, I have a specimen of the Acasta gneiss!

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