Cosmic Collision

Almost two billion years ago, a meteorite collision caused part of the Earth’s crust to flip inside out and left a dusting of a rare metal on top of the crater, according to new U of T research.

The study, published in a June issue of Nature, examines the effects of meteorite collisions on the Earth’s evolution. Researchers from U of T and the Geological Survey of Canada studied a 250-kilometre-wide crater in Sudbury, Ont. Known as the Sudbury Structure, the crater was caused by a Mount Everest-sized meteorite 1.8 billion years ago. The meteorite blasted a large hole in the Earth, causing the planet’s upper crust to be buried under several kilometres of melted rock derived from the lower crust.

Until now, there has been little hard evidence to prove that a meteorite can pierce through the Earth’s upper crust and alter its compositional makeup. “It had not really been appreciated that large impacts would selectively move material from the bottom of the crust up to the top,” says James Mungall, the lead researcher and a U of T geology professor. “This has been suggested for the moon at times in the past, but ours is the first observational evidence that this process has operated on Earth.”

Researchers found a subtle but significant amount of iridium, an extremely rare metal found mainly in the Earth’s mantle and in meteorites. Due to the low magnesium and nickel content in the samples, they concluded that the iridium came from the meteorite rather than the Earth’s mantle.

The discovery of iridium allowed the researchers to paint a picture of what happened when the meteorite collided with Earth at a velocity exceeding 40 kilometres per second, and caused a shock melting of 27,000 cubic kilometres of the crust.