Earth's oldest known impact crater found in Greenland

Jun 28, 2012
Earth's oldest known impact crater found in Greenland
An artistic expression of how a large meteorite impact into the sea might have looked. Credit: Carsten Egestal Thuesen, GEUS.

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 , 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 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 the Geological Survey of Denmark and (GEUS) in Copenhagen, Cardiff University in Wales, 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.

Earth's oldest known impact crater found in Greenland
View southwest from Fossilik Hill looking into the centre of the impact structure.

"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 Cardiff University's 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 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 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.

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More information: For further information about the Maniitsoq structure see: www.geus.dk/cgi-bin/webbasen_n… 2%7Ccgifunction=form

Link to the paper in Earth & Planetary Science Letters: dx.doi.org/10.1016/j.epsl.2012.04.026

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Jeddy_Mctedder
3.4 / 5 (9) Jun 28, 2012
i wish the author detailed a few sentences about WHAT evidence the scientists ultimately relied upon.

most likely---from the tone of the article it sounds like some of the evidence came from gravitometric models and data from the sattelites they put up. but more likely a whole slew of isotopes that showed that the crust in this area COULD not have aged normally without something from outer space introduced. I like it.
Torbjorn_Larsson_OM
3 / 5 (2) Jun 29, 2012
From the linked abstract:

"The presently exposed features of the Maniitsoq structure were buried 2025 km below the surface when this event occurred at c. 3 Ga, during waning convergent orogeny. These features include: a large aeromagnetic anomaly; a central 35×50 km2 large area of comminuted quartzo-feldspathic material; regional-scale circular deformation; widespread random fractures with featherlike textures; intense fracture cleavage; amphibolitegranite-matrix breccias unrelated to faulting or intrusions; formation and common fluidisation of microbreccias; abundant evidence of direct K-feldspar and plagioclase melting superimposed on already migmatised rocks; deformation of quartz by slip; formation of planar elements in quartz and plagioclase; and, emplacement of crustally contaminated ultramafic intrusions and regional scale hydrothermal alteration under amphibolite-facies conditions.

... rule out a terrestrial origin of the structure."
Torbjorn_Larsson_OM
3 / 5 (2) Jun 29, 2012
The editor won't let me edit my C&P. It is "20-25 km below" of course.
docmordin
5 / 5 (4) Jun 29, 2012
I wish the author detailed a few sentences about WHAT evidence the scientists ultimately relied upon.


A. A. Garde, et al. (2012) obtained Anhedral, concave zircon grains from the study area, which yielded three poorly-defined 207Pb/206Pb age groups at 3.016 ± 0.023 Ga, 3.006 ± 0.0013 Ga and 2.976 ± 0.013 Ga (SHRIMP-1 ion probe data; see: A. A. Garde, et al., "Rapid maturation and stabilisation of middle Archaean continental crust: The Akia terrane, Southern West Greenland", Bull. Geol. Soc. Den., 47, 1-27, 2000). That data provided a minimum age of the Maniitsoq structure at approximately 3 Ga.