Rocks in Canada’s Yukon Territory have revealed evidence that “snowball earth” may not be a misnomer for our planet more than 716 million years ago. Harvard University geologist Francis Macdonald and other geologists determined that the rocks were once located at sea level in the tropics. Their work, reported in the journal Science, “Calibrating the Cryogenian,” supports a theory that glaciers covered the earth at all latitudes during the Sturtian ice age.

In Harvard Gazette coverage of the study, Macdonald, an assistant professor in earth and planetary sciences, says, “This is the first time that the Sturtian glaciation has been shown to have occurred at tropical latitudes, providing direct evidence that this particular glaciation was a ‘snowball Earth’ event.” He adds that the team’s data suggest that the Sturtian glaciation lasted a minimum of 5 million years.

The Sturtian glaciation and the Marinoan glaciation, which occurred 850 to 630 million years ago during the Cryogenian period, are the names for the greatest ice ages in earth’s history. Geologists have questioned whether the entire planet including its oceans was covered with ice or a slushy mix of ice and water.

The rocks that Macdonald and his colleagues analyzed showed glacial deposits and other signs of glaciation. The scientists were able to determine, based on the magnetism and composition of these rocks, that 716.5 million years ago they were located at about 10 degrees latitude. Macdonald points out that climate modeling predicts the oceans would freeze over if sea ice developed within 30 degrees latitude of the equator.

Because fossil records indicate eukaryotes survived during this period and the early animals arose about the same time, some scientists have wondered how life could have continued if earth was a solid ball of ice. Macdonald says there would have been temperature changes and patches of open water somewhere on the planet.

This work was supported by the Polar Continental Shelf Project , which provides ground and air services to scientific groups from universities and government agencies working in isolated areas in the Canadian Arctic, and the National Science Foundation.