Earth's oldest known impact crater found

Earth's oldest known impact crater found An international team of scientists is reporting the discovery in Greenland of Earth's oldest known impact crater, dating to 3 billion years ago. The 60-mile-wide crater is the result of a massive asteroid or comet impact a billion years before any other known collision on Earth, they said.
Earth must have experienced many collisions between 3 billion and 4 billion years ago, researchers said, but evidence for most of them has been eroded away or covered by younger rocks.
The previously oldest known crater discovered on Earth -- the 180-mile-wide Vredefort crater in South Africa -- formed 2 billion years ago.
"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," Iain McDonald of Cardiff University in Wales, who was part of the study team, said in a University release Thursday.
In the 3 billion years since the impact, the land has been eroded down to expose deeper crust 15 miles below the original surface, and all external parts of the impact structure, including any obvious bowl-shaped crater, have been erased.
However, 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, the scientists said.
Three years of work were needed to confirm all the key evidence of a cosmic impact.