World News Oldest Impact Structure on Earth

By: Johnathan Wiggins

Scientists have identified the oldest known impact crater on Earth, and ancient structure could tell us how our planet emerged from a long-ago frozen phrase. Yarrabubba Crater, a 43-mile-wide geological feature in Western Australia, is 2.229 billion years old, plus or minus 5 million years, a new study reports. That's about half the age of the Earth itself and 200 million years older than the previous record holder, the 190-mile-wide Vredefort Dome in South Africa. 

Yarabubba’s age is intriguing, because a lot was going on 2.229 years ago. Photosynthesizing cyanobacteria had just started pumping large amounts of oxygen into Earth's atmosphere, initiating a dramatic process known as the Great Oxidation Event. The Yarrabubba impact appears to have occurred just as our planet started coming out of a “Snowball Earth” period, when much of the planet was covered by ice. And that may not be a coincidence. The age of the Yarrabubba impact matches the demise of a series of ancient glaciations. After the impact, glacial deposits are absent in the rock record for 400 million years. Ancient craters such as Yarrabubba are difficult to find on our Earth. Many get buried when crustal plates dive beneath each other, and most others are worn away by wind and water over the eons. 

There were researchers who studied grains of monazite and zircon that were recrystallized by the impact, measuring the amounts of uranium, thorium and lead contained in each. Monazite and zircon readily take up uranium but not lead when they crystallize, and uranium and thorium radioactively decay into lead at known rates. The planet also came out of a deep freeze, which is one of the multiple snowball phases Earth has experienced during its 4.5-billion-year history, and this was around the time of the Yarrabubba impact. 

Although debris from craters older than the Yarrabubba crater has been found and dated in Australia and Africa previously, in those cases, scientists couldn't identify the craters that corresponded with the debris they found.