[meteorite-list] Meteorite Impact Reformulated Earth's Crust, Study Shows (Sudbury Crater)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Jan 12 15:48:08 2006
Message-ID: <200601122032.k0CKWvm17503_at_zagami.jpl.nasa.gov>

http://news.nationalgeographic.com/news/2006/01/0112_060112_meteorite.html

Meteorite Impact Reformulated Earth's Crust, Study Shows
John Roach
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.
Received on Thu 12 Jan 2006 03:32:56 PM PST