Meteorite Impact Reformulated Earth's Crust, Study Shows

John Roach
for National Geographic News
January 12, 2006
About 1.8 billion years ago, a meteorite or comet the size of Mount Everest slammed into what is now Canada.

According to James Mungall, a University of Toronto geologist, the impact turned part of the Earth's crust inside out and dusted the surface with a rare metal.

Mungall and other experts studying impact craters, such as this one in Sudbury, Ontario, hope to understand how a period of continual bombardment about four billion years ago shaped the planet.

Until now researchers had found scant evidence that a meteorite could pierce through Earth's upper crust and alter its compositional makeup.

"Over a few hundred million years when this was going on, there must have been a lot of mixing going on in the upper crust," said Mungall, who studies the Sudbury impact site.

David Kring is a planetary scientist at the University of Arizona in Tucson and an authority on impact craters. He said the findings from Sudbury are similar to those he and his colleagues have reached from studying a crater in Chicxulub, Mexico.

"I don't think it is yet widely appreciated, but impact cratering has the capacity to redistribute the chemical elements in the Earth's crust," Kring said.

As well, Kring adds, an emerging theory in the field of impact crater research is that the largest of these impact events early in Earth's history may have created the conditions needed for the evolution of life.

The impacts, he explains, would have heated water in the Earth's crust and created vast hydrothermal vent systems. Many scientists believe these unusual underwater ecosystems helped give rise to early life.

Researchers assumed volcanic activity mostly created hydrothermal vent systems. "But four billion years ago a dominant source was impact-generated hydrothermal systems," Kling said.

Impact Evidence

The field of impact crater research is just coming into prominence in the scientific community. Mungall says that 15 years ago scientists couldn't even agree that the Sudbury crater resulted from a meteorite impact.

The signs of the impact are vague, because most of the crater has eroded. Geological processes, such as plate tectonics and volcanism, have almost completely eroded Earth's oldest impact craters.

But the Sudbury and Chicxulub craters, along with a third in Vredefort, South Africa, are still visible enough to provide clues to the planet's formative years.

Today the Sudbury impact basin is about 37 miles (60 kilometers) long and 19 miles (30 kilometers) wide. Mungall and his colleagues believe the crater was originally about 155 miles (250 kilometers) in diameter.

Scientists looking for signs of the impact must cover a large area of ground, and much of the evidence they look for is small.

According to Mungall, the most convincing pieces of evidence are shatter cones—coned-shaped fractures in the rock ranging in size from inches to tens of feet across.

"The only way you can get shatter cones is when extremely strong shock waves are passing through material. They don't form any other way," he said. "The only other places you see them on Earth are around nuclear test sites."

Other bits of evidence include microscopic, flaky diamonds formed by the passage of shock waves through carbon-rich rocks. The shock waves also transform tiny mineral crystals into glass.

Explosive Impact

To make the Sudbury impact crater, the meteorite would have to have been about 6 miles (10 kilometers) in diameter traveling at 89,000 miles per hour (143,232 kilometers per hour), Mungall says.

Shock waves from the meteorite as it plunged into Earth likely caused up to 6,500 cubic miles (27,000 cubic kilometers) of crust to melt, he says.

A plume of superheated rock from the deepest part of the 19- to 25-mile-thick (30- to 40-kilometer-thick) crust then flew upward and landed on top of the impact site, essentially turning the crust there inside out, Mungall explains.

Mungall also suggests that the meteorite vaporized on impact. Its components then condensed and rained back down.

This, he says, would account for the increased concentrations of iridium—a rare metal found mainly in the Earth's mantle and in meteorites—he and his colleagues found in the upper layers of the crater's crust.

The Sudbury site also has relatively low concentrations of magnesium and nickel, two elements that are common in Earth's mantle. The researchers therefore concluded that the iridium originated from the meteorite.

According to Kring, of the University of Arizona, events like those at Sudbury 1.8 billion years ago and Chicxulub 65 million years ago were tiny compared to those during the period of heavy bombardment in Earth's formative years.

His calculations suggest there were perhaps as many 40 impact events that produced craters at least 620 miles (1,000 kilometers) in diameter during that time.

"That would have redistributed the chemical elements in Earth's crust to a great extent," he said.

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