[meteorite-list] Scientists Reconstruct Ancient, Massive Impact

[Ron Baalke]

http://news.agu.org/press-release/scientists-reconstruct-ancient-impact-that-dwarfs-dinosaur-extinction-blast/

Scientists reconstruct ancient impact that dwarfs dinosaur-extinction blast
American Geophysical Union
Press Release
9 April 2014

WASHINGTON, D.C. - Picture this: A massive asteroid almost as wide as 
Rhode Island and about three to five times larger than the rock thought 
to have wiped out the dinosaurs slams into Earth. The collision punches 
a crater into the planet's crust that's nearly 500 kilometers (about 300 
miles) across: greater than the distance from Washington, D.C. to New 
York City, and up to two and a half times larger in diameter than the 
hole formed by the dinosaur-killing asteroid. Seismic waves bigger than 
any recorded earthquakes shake the planet for about half an hour at any 
one location - about six times longer than the huge earthquake that struck 
Japan three years ago. The impact also sets off tsunamis many times deeper 
than the one that followed the Japanese quake.

Although scientists had previously hypothesized enormous ancient impacts, 
much greater than the one that may have eliminated the dinosaurs 65 million 
years ago, now a new study reveals the power and scale of a cataclysmic 
event some 3.26 billion years ago which is thought to have created geological 
features found in a South African region known as the Barberton greenstone 
belt.  The research has been accepted for publication in Geochemistry, 
Geophysics, Geosystems, a journal of the American Geophysical Union.

The huge impactor - between 37 and 58 kilometers (23 to 36 miles) wide 
- collided with the planet at 20 kilometers per second (12 miles per second). 
The jolt, bigger than a 10.8 magnitude earthquake, propelled seismic waves 
hundreds of kilometers through the Earth, breaking rocks and setting off 
other large earthquakes. Tsunamis thousands of meters deep - far bigger 
than recent tsunamis generated by earthquakes - swept across the oceans 
that covered most of the Earth at that time.

"We knew it was big, but we didn't know how big," Donald Lowe, a geologist 
at Stanford University and a co-author of the study, said of the asteroid.

[Graphic]
A graphical representation of the size of the asteroid thought to have 
killed the dinosaurs, and the crater it created, compared to an asteroid 
thought to have hit the Earth 3.26 billion years ago and the size of the 
crater it may have generated. A new study reveals the power and scale 
of the event some 3.26 billion years ago which scientists think created 
geological features found in a South African region known as the Barberton 
greenstone belt.
Credit: American Geophysical Union

Lowe, who discovered telltale rock formations in the Barberton greenstone 
a decade ago, thought their structure smacked of an asteroid impact. The 
new research models for the first time how big the asteroid was and the 
effect it had on the planet, including the possible initiation of a more 
modern plate tectonic system that is seen in the region, according to 
Lowe.

The study marks the first time scientists have mapped in this way an impact 
that occurred more than 3 billion years ago, Lowe added, and is likely 
one of the first times anyone has modeled any impact that occurred during 
this period of the Earth's evolution.

The impact would have been catastrophic to the surface environment. The 
smaller, dino-killing asteroid crash is estimated to have released more 
than a billion times more energy than the bombs that destroyed Hiroshima 
and Nagasaki. The more ancient hit now coming to light would have released 
much more energy, experts said.

The sky would have become red hot, the atmosphere would have been filled 
with dust and the tops of oceans would have boiled, the researchers said. 
The impact sent vaporized rock into the atmosphere, which encircled the 
globe and condensed into liquid droplets before solidifying and falling 
to the surface, according to the researchers.

The impact may have been one of dozens of huge asteroids that scientists 
think hit the Earth during the tail end of the Late Heavy Bombardment 
period, a major period of impacts that occurred early in the Earth's history 
- around 3 billion to 4 billion years ago.

Many of the sites where these asteroids landed were destroyed by erosion, 
movement of the Earth's crust and other forces as the Earth evolved, but 
geologists have found a handful of areas in South Africa, and Western 
Australia that still harbor evidence of these impacts that occurred between 
3.23 billion and 3.47 billion years ago. The study's co-authors think 
the asteroid hit the Earth thousands of kilometers away from the Barberton 
Greenstone Belt, although they can't pinpoint the exact location.

"We can't go to the impact sites. In order to better understand how big 
it was and its effect we need studies like this,' said Lowe. Scientists 
must use the geological evidence of these impacts to piece together what 
happened to the Earth during this time, he said.

The study's findings have important implications for understanding the 
early Earth and how the planet formed. The impact may have disrupted the 
Earth's crust and the tectonic regime that characterized the early planet, 
leading to the start of a more modern plate tectonic system, according 
to the paper's co-authors.

The pummeling the planet endured was 'much larger than any ordinary earthquake,"
said Norman Sleep, a physicist at Stanford University and co-author of 
the study. He used physics, models, and knowledge about the formations 
in the Barberton greenstone belt, other earthquakes and other asteroid 
impact sites on the Earth and the moon to calculate the strength and duration 
of the shaking that the asteroid produced. Using this information, Sleep 
recreated how waves traveled from the impact site to the Barberton greenstone 
belt and caused the geological formations.

The geological evidence found in the Barberton that the paper investigates 
indicates that the asteroid was "far larger than anything in the last 
billion years," said Jay Melosh, a professor at Purdue University in West 
Lafayette, Indiana, who was not involved in the research.

The Barberton greenstone belt is an area 100 kilometers (62 miles) long 
and 60 kilometers (37 miles) wide that sits east of Johannesburg near 
the border with Swaziland. It contains some of the oldest rocks on the 
planet.

The model provides evidence for the rock formations and crustal fractures 
that scientists have discovered in the Barberton greenstone belt, said 
Frank Kyte, a geologist at UCLA who was not involved in the study.

"This is providing significant support for the idea that the impact may 
have been responsible for this major shift in tectonics," he said.

Reconstructing the asteroid's impact could also help scientists better 
understand the conditions under which early life on the planet evolved, 
the paper's authors said. Along with altering the Earth itself, the environmental 
changes triggered by the impact may have wiped out many microscopic organisms 
living on the developing planet, allowing other organisms to evolve, they 
said.

"We are trying to understand the forces that shaped our planet early in 
its evolution and the environments in which life evolved," Lowe said.

Notes for Journalists

Journalists and public information officers (PIOs) of educational and 
scientific institutions who have registered with AGU can download a PDF 
copy of this article by clicking on this link: 
http://onlinelibrary.wiley.com/doi/10.1002/2014GC005229/abstract

Or, you may order a copy of the final paper by emailing your request to 
Nanci Bompey at nbompey at agu.org. Please provide your name, the name of 
your publication, and your phone number.

Neither the paper nor this press release is under embargo.

Title
"Physics of crustal fracturing and chert dike formation triggered by asteroid 
impact, ~3.26 Ga, Barberton greenstone belt, South Africa"

Authors:
Norman H. Sleep: Department of Geophysics, Stanford University, Stanford, 
CA, USA;

Donald R. Lowe: Department of Geological and Environmental Sciences, Stanford 
University, Stanford, CA, USA.

Contact information for the authors:
Norman Sleep: +1 (650) 723-0882, norm at stanford.edu

AGU Contact:
Nanci Bompey
+1 (202) 777-7524
nbompey at agu.org