In the study, Mungall, his graduate student Jacob Hanley and Geological Survey researcher Doreen Ames concluded Sudbury Igneous Complex is predominantly derived from shock-melted lower crust rather than the average of the whole crust as has been previously supposed. The researchers discovered a subtle but significant enrichment of iridium, an extremely rare metal found mainly in the Earth’s mantle and in meteorites. Due to the low magnesium and nickel content found in the samples they concluded that the iridium came from the meteorite itself rather than the Earth’s mantle.
The discovery of the iridium allowed the researchers to paint a picture of what happened billions of years ago, when a meteorite collided with the earth at a velocity exceeding 40 kilometres per second and caused a shock melting of 27,000 cubic kilometres of the crust. “The impact punched a hole to the very base of the crust and the meteorite itself was probably vaporized,” says Mungall. This collision, he says, caused a plume of iridium-enriched vaporized rock to surge up and recondense on top of the impact site. Simultaneously, the cavity collapsed within minutes or hours to form a multi-ring basin 200 to 300 kilometres in diameter and one to six kilometres deep.
“Picture a drop falling into a cup of milk, thus producing a bowl-shaped depression for a moment before the milk outside rushes back in to fill the hole,” says Mungall. “Now imagine that the falling drop of milk is a rock 10 kilometres in diameter, and the resulting depression is 30 to 40
kilometres deep.”
The Sudbury Basin is the second oldest very large impact crater site in the world but is one of the most accessible and well preserved. The oldest one, South Africa’s two-billion year-old Vredefort Crater, has eroded over time and only the basement remains. Another impact site, the Chicxulub Crater in Yucatan Peninsula, believed to be responsible for the extinction of the dinosaurs, lies buried under beds of limestone.
The study was funded in part by the Geoscience Laboratories of the Ontario Geological Survey and the Geological Survey of Canada.