Scientists Date Rocks at Record 3.96 Billion Years Old
The oldest rocks ever discovered have been found in northern Canada, according to U.S. and Australian scientists who announced Wednesday that they have recovered two rocks that are 3.96 billion years old.
The rocks were part of an ancient geological formation in the Northwest Territories, and they are about 100 million years older than the previous record holders found in Greenland and Antarctica.
Samuel A. Bowring of Washington University in St. Louis, senior scientist on the project, said the rocks will help scientists delve “into a period of Earth’s history about which we know almost nothing.”
“These rocks are really tantalizingly close (in age) to what we think is the origin of our part of the universe. These are incredibly old rocks,” said Thomas O. Wright, program manager at the National Science Foundation, which announced the discovery Wednesday along with Washington University, the Australian National University and the Northwest Territories Geology Division of Indian and Northern Affairs.
Could Clear Up Mysteries
Scientists are interested in rocks of that age because they could help clear up a number of mysteries concerning the early Earth. The Earth formed along with the other planets out of a cloud of gas and dust orbiting around the equator of an infant sun.
But in the beginning, the Earth itself was a ball of gas, and at some point it began to solidify and form a thin, hard film of material on its surface, called the crust. Scientists would like to know precisely what materials made up the early crust, when it formed, and whether the dynamic processes seen today were active then as well.
Several different lines of evidence establish the age of the solar system at 4.5 billion years, so the rocks recovered by Bowring’s team formed on Earth when the planet itself was only about half a billion years old.
Furthermore, Bowring said in a telephone interview, the rocks are granite, “which is an evolved rock, so they had to be derived from an even older rock.”
That suggests that older rocks should be found in the area, and Bowring said he plans to lead his scientific team back to the same region next summer to collect more rocks. Last summer they picked up a total of 4,000 pounds of rocks.
He said he knew they were old, but he did not know how old until he took samples from two rocks, one weighing five pounds and the other 50, to Canberra, Australia. In Canberra, Bowring studied his rocks with the Sensitive High Mass-Resolution Ion Microprobe (SHRIMP), one of the most sensitive isotope-dating instruments in the world.
SHRIMP can date any rock by bombarding a tiny sample with a 10,000-volt beam of charged particles, which knocks atoms and molecules out of the sample. The freed atoms and molecules are then separated by the mass spectrometer, which counts and measures them.
The ratio of certain types of atoms to others gives the age of the sample, which in this case turned out to be 3.962 billion years, give or take 3 million years.
Some scientists have theorized that the Earth’s first crust did not last long and was broken up when it was bombarded by debris from the early solar system crashing into the planet. But the rocks collected by the Bowring team suggest that at least parts of the Earth’s crust date back to the planet’s infancy.
“This stuff strongly implies that we had a real continental crust as long ago as 3.9 billion years,” Wright said.
