U.S. and Australian scientists have discovered the world's oldest known rocks, chunks of material 3.96 billion years old that date to the very infancy of the Earth.
The rocks are about 100 million years older than any previously found on Earth.The National Science Foundation, which partially funded research on the project said the rocks could help to resolve questions about one of the earliest and least-understood periods in Earth's history.
Samuel A. Bowring, of Washington University in St. Louis, head of the research group, said scientists know "almost nothing" about this period.
Scientists agree the Earth and other planets of the inner solar system formed about 4.5 billion years ago, perhaps from a huge nebula, a cloud of cosmic gas and dust.
Bowring, his students and Janet E. King of the Geological Survey of Canada collected the rocks in a remote region north of Great Slave Lake in Canada's Northwest Territories. One sample weighs about 5 pounds, the other about 50 pounds.
A conventional radioactive isotope analysis of the rocks indicated they were extraordinarily old. But the complexity of the samples made it impossible to establish a specific age.
So Bowring took samples to a special isotope dating device located at the Australian National University in Canberra.
The device, the Sensitive High Mass-Resolution Ion Microprobe, or SHRIMP, is the only one of its type in the world. SHRIMP uses a beam of electrically changed particles to dislodge atoms and molecules from a sample. The sample's age then is determined by further separations and measurements of the ejected material.
SHRIMP showed that the rock originally was formed or crystallized 3.962 billion years ago. Bowring said the measurement is accurate to plus or minus 3 million years.
Before the measurements, geologists recognized two different rock samples as the oldest on Earth. One sample, collected on the southwestern shore of Greenland, was 3.82 billion years old. The other, found in Antarctica, was 3.87 billion years old.
Bowring believes the discovery and dating of additional samples of ancient rock may be important in improving man's understanding of the ancient Earth. Although scientists agree on the approximate time of Earth's formation, there is uncertainty about how the planet evolved or differentiated.
These processes transformed the planet from a liquid sphere of molten iron and silicate material into a solid with the distinctive layered landmarks recognized today. These are the thin (5-20 miles thick) surface crust; the mantle, an underlying layer of rock about 1,800 miles thick; and an outer and inner core.
Geologists estimate it took about 700 million years for differentiation of the Earth to occur. The newly identified rock samples thus could provide glimpses of the final 100 million years of the process. Geologists have theorized about how differentiation occurred, but there's been little hard evidence to support the theories.