7 June 2004

Ancient Meteorite Turned Earth Inside Out

by Kate Melville

A meteorite collision nearly two billion years ago caused part of the Earth's crust to flip inside out and left a dusting of a rare metal scattered on the top of the crater, say University of Toronto researchers.

The study, appearing in Nature, examines the devastating effects of meteorite impacts on the Earth's evolution. Researchers from the University of Toronto and the Geological Survey of Canada studied the remains of a 250-kilometre wide crater in Sudbury, Ontario, known as the Sudbury Igneous Complex, caused by a collision with a Mount Everest-sized meteorite 1.8 billion years ago. They discovered that the meteorite burrowed deep into the Earth's upper crust - which measures an average of 35 kilometres thick - and caused the upper crust to be buried under several kilometres of melted rock derived from the lower crust.

The dynamics of meteorite impacts remain a source of debate among researchers and, until now, there has been little hard evidence to prove a meteorite could 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 lead researcher James Mungall. "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."

Mungall and his co-researchers 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 km/sec 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."