[meteorite-list] Scientists Reconstruct Ancient, Massive Impact

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 9 Apr 2014 16:05:03 -0700 (PDT)
Message-ID: <201404092305.s39N53Gu017401_at_zagami.jpl.nasa.gov>

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
Received on Wed 09 Apr 2014 07:05:03 PM PDT