The Acasta Gneiss - long famous for representing the oldest known rocks on Earth.  It's relatively ordinary-looking metamorphic rock, but it's among the oldest on Earth - 4.031 billion years old!  Before September 2008, Acasta Gneiss was the oldest known preserved Earth rock.  Some metamorphic rocks from the eastern margin of Hudson Bay are now known to be 4.28 billion years old.  Zircons from the Jack Hills Conglomerate in Western Australia date to 4.404 billion years.  In August 2010, published research announced the discovery of subsurface rocks at Baffin Island in the Canadian Arctic are 4.45-4.55 billion years old.


The Acasta Gneiss Complex (Acasta Formation) consists of various specific rock types.  The specimen shown below is an orthogneiss (a metagranodiorite, or granodiorite gneiss).  Zircons from this material have been dated to 4.031 billion years old - one of the oldest dates for any Earth material.


Locality: The site is in the southern part of a hook-shaped peninsula along the Acasta River, northeast of Exmouth Lake, along the western margin of the Slave Province (Slave Craton), about 210 to 220 miles north of the city of Yellowknife in far-northern Canada (barely north of the 65 10' North latitude line and barely east of the 115 35' West longitude line - see the Exmouth Lake 1:50,000 topographic quadrangle - Canadian Dept. of Mines, Energy & Resources map sheet 86G/4).  This is the same locality documented in Bowring & Williams (1999) (Contributions to Mineralogy and Petrology 134: 3-16) (locality SAB94-134).


Acasta Gneiss (cut surface; 5.9 cm from top to bottom) - 4.031 b.y. orthogneiss (metagranodiorite) from the Acasta Gneiss Complex along the Acasta River, northern Canada.



Satellite photo of 4.031 b.y. Acasta Gneiss locality, along the Acasta River, in the southern part of the hook-shaped peninsula along that river - see center of photo.  (satellite photo provided by TerraMetrics-Google Earth)



Sample collected & generously donated by Sam Bowring.




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