Geophysicists conducting drilling as part of geothermal research claim to have stumbled across the largest asteroid impact zone ever found on Earth. Covering a 400 km (249 mi) wide area in Central Australia, the two ancient craters are believed to be the result of a single meteorite that split in two moments before crashing into the Earth.

While the crater caused by the impact has long since disappeared, the team of geophysicists uncovered the twin scars of the impacts while drilling more than 2 km (1.2 mi) into the Earth’s crust as part of a geothermal research project. The drill core they retrieved contained traces of rocks that has been turned to glass as a result of the extreme temperature and pressure of the impact.

"The two asteroids must each have been over 10 kilometers (6.2 mi) across – it would have been curtains for many life species on the planet at the time," says lead researcher Dr Glikson from the Australian National University (ANU) School of Archaeology and Anthropology. "Large impacts like these may have had a far more significant role in the Earth’s evolution than previously thought."

The researchers say it is difficult to deduce the exact date of the impacts as there is a lack of evidence like that left by other meteorite impacts, such as the layer of ash found in rock sediments resulting from a plume that is thought to be responsible for the extinction of much of the life on Earth at the time, including a large number of dinosaur species.

While the surrounding rocks range from 300 to 600 million years old, Dr Glikson says that a corresponding ash layer has not been found in sediments around 300 million years old.

"It’s a mystery," Dr Gikson says. "We can’t find an extinction event that matches these collisions. I have a suspicion the impact could be older than 300 million years."

Magnetic modeling revealed hidden bulges in the Earth's crust that are rich in iron and magnesium, indicating they came from the Earth's mantle.

"There are two huge deep domes in the crust, formed by the Earth’s crust rebounding after the huge impacts, and bringing up rock from the mantle below," Dr Glikson says.

The two impact zones are located in the Warburton Basin in Central Australia, near the borders of the Northern Territory, Queensland and South Australia. Measuring more than 400 km (249 mi) across, the zones extend through the Earth's crust, which the team says is around 30 km (18.6 mi) thick in the area.

The team's findings have been published in the journal Tectonophysics.

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