[meteorite-list] Oldest Known Impact Crater Found in Greenland

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 28 Jun 2012 11:55:50 -0700 (PDT)
Message-ID: <201206281855.q5SItow9015238_at_zagami.jpl.nasa.gov>

http://www.cardiff.ac.uk/news/articles/oldest-known-impact-crater-found-9091.html

Oldest known impact crater found
Cardiff University
28 June 2012

A 100 kilometre-wide crater has been found in Greenland, the result of a
massive asteroid or comet impact a billion years before any other known
collision on Earth.

The spectacular craters on the Moon formed from impacts with asteroids
and comets between 3 and 4 billion years ago. The early Earth, with its
far greater gravitational mass, must have experienced even more
collisions at this time - but the evidence has been eroded away or
covered by younger rocks. The previously oldest known crater on Earth
formed 2 billion years ago and the chances of finding an even older
impact were thought to be, literally, astronomically low.

Now, a team of scientists from Cardiff, the Geological Survey of Denmark
and Greenland (GEUS) in Copenhagen, Lund University in Sweden and the
Institute of Planetary Science in Moscow has upset these odds. Following
a detailed programme of fieldwork, funded by GEUS and the Danish
'Carlsbergfondet' (Carlsberg Foundation), the team have discovered the
remains of a giant 3 billion year old impact near the Maniitsoq region
of West Greenland.

"This single discovery means that we can study the effects of cratering
on the Earth nearly a billion years further back in time than was
possible before," according to Dr Iain McDonald of the School of Earth
and Ocean Sciences, who was part of the team.

Finding the evidence was made all the harder because there is no obvious
bowl-shaped crater left to find. Over the 3 billion years since the
impact, the land has been eroded down to expose deeper crust 25 km below
the original surface. All external parts of the impact structure have
been removed, but the effects of the intense impact shock wave
penetrated deep into the crust - far deeper than at any other known
crater - and these remain visible.

However, because the effects of impact at these depths have never been
observed before it has taken nearly three years of painstaking work to
assemble all the key evidence. "The process was rather like a Sherlock
Holmes story," said Dr McDonald. "We eliminated the impossible in terms
of any conventional terrestrial processes, and were left with a giant
impact as the only explanation for all of the facts."

Only around 180 impact craters have ever been discovered on Earth and
around 30% of them contain important natural resources of minerals or
oil and gas. The largest and oldest known crater prior to this study,
the 300 kilometre wide Vredefort crater in South Africa, is 2 billion
years in age and heavily eroded.

Dr McDonald added that "It has taken us nearly three years to convince
our peers in the scientific community of this but the mining industry
was far more receptive. A Canadian exploration company has been using
the impact model to explore for deposits of nickel and platinum metals
at Maniitsoq since the autumn of 2011."

The international team was led by Adam A. Garde, senior research
scientist at GEUS. The first scientific paper documenting the discovery
has just been published in the journal "Earth and Planetary Science
Letters".
Received on Thu 28 Jun 2012 02:55:50 PM PDT