ACASTA
GNEISS
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 (see oblique aerial
photo) 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.