Southern Illinois Mega-erratic Revisited: a 2.66 Billion Year-old Find

ISGS staff member Dick Berg stands atop the erratic. In December 2010, the ISGS reported on one of the largest glacial erratics (~22×10×11 feet with an estimated weight of ~100 tons) ever found in Illinois. But what made this finding unique was its location in southern Illinois (near Texico in Jefferson County), about 50 miles north of the southernmost extent of continental glaciation in the northern hemisphere. We reported that this erratic is an exotic rock that was brought to Illinois by continental glaciers about 150,000 years ago as ice sheets scoured, scraped, and plucked pieces of bedrock during their southward advance. The erratic was identified as being pink granite that most likely originated from the Canadian Shield north of Lakes Huron and Superior, and was transported to its far southern location.

So that we could more definitively trace the erratic's origin and path to southern Illinois, small pieces of the erratic were removed and sent to the Arizona Laserchron Center at the University of Arizona, so that the age of the rock could be determined. The chosen technique was uranium-lead dating performed on the mineral zircon. This technique dates zircon ranging from ~ 1 million to 4.5 billion years old with a precision usually within 1%. An age with this method indicates when the granite crystallized as it cooled from molten rock deep in the earth (long prior to being transported to Illinois). The age determination is based on the fact that there are of two uranium isotopes contained in the mineral that decay and form two lead isotopes, with half-lives of 4.47 billion years and 704 million years respectively. By calculating the ratios of uranium and lead isotopes from more than 25 extracted zircon crystals, the age of the southern Illinois granite erratic was calculated to be 2.66 +/- .03 billion years old, more than half the age of the earth (~ 4.54 billion years old)! This places the rock's birth in the Archean Eon on the geologic time scale, the eon during which the oldest rocks on earth formed. Based on the known ages of granites in Canada, it can be concluded that our rock definitely originated from the Superior Province of the Canadian Shield, north of Lakes Huron and Superior, as previously suspected, and not from the Grenville Province to the east and northeast. We now believe that the rock travelled about 700 miles or so by glacial transport, from rural Ontario (northeast of Sault Ste. Marie) to its present location in Jefferson County, Illinois. This result confirms that glacial transport was most likely via the Lake Michigan basin.

Map of the likely path of the erratic