Thursday, April 10, 2014

Ancient Asteroid Impact Dwarfs The One That Wiped Out Dinosaurs

The asteroid that annihilated the dinosaurs 65.5 million years ago caused a pretty massive collision. However, that asteroid is nothing compared to one that hit South Africa 3.26 billion years ago. Norman Sleep and Donald Lowe, both from Stanford University, were able to reconstruct the impact, discovering the asteroid’s true size for the first time. The results have been accepted for publication in the American Geophysical Union’s journal, Geochemistry, Geophysics, Geosystems (G-cubed).

The Chicxulub crater, formed by an asteroid impact that would ultimately wipe out the dinosaurs, is about 150 kilometers (93 miles) wide. This is completely dwarfed the crater that is likely about 478 kilometers (297 miles) in diameter in South Africa. Sleep and Lowe were able to analyze the rocks in the crater and estimate the size of of the asteroid that would have caused it.

The conclusion? An asteroid about 37 kilometers (23 miles) in diameter (nearly four times larger than the one that caused the Chicxulub) impacted Earth’s surface at an incredible rate of 20 kilometers per second (12 miles per second). Upon impact, seismic waves shook the Earth for over half an hour and creative massive tsunamis that are many times larger than the one that devastated Japan in 2011. It is likely that this collision is one of many early impacts that actually fractured our plate tectonic system into what we know today. Yes, it was an asteroid so big, it actually broke the Earth.

The biggest problem with studying this impact and others like it during the Late Heavy Bombardment period early in Earth’s history is that a lot has happened since then. Between erosion and 3.26 billion years of plate movement, the impact site isn’t a clearly-defined area anymore. Lowe had to analyze rocks over a very large region in order to discover the evidence of such a massive impact.

All of Lowe’s geological data were combined with Sleep’s knowledge of physics in order to create computer models and extrapolate the original size of the crater and asteroid. The impact is believed to have happened a few thousand kilometers away from an area known as the Baberton Greenstone Belt in South Africa, which has some of the most ancient rocks on the planet. There is evidence that the seismic activity from the asteroid’s impact actually influenced some of those rock formations, even at such a great distance.

When the Earth (and the solar system) was still young, about 3-4 billion years ago, large impacts are believed to have occurred somewhat regularly as larger chunks of debris were still somewhat common in the inner solar system. There has also been evidence of similar impacts in South Africa and Western Australia, but this is the first time that one of those massive impacts has been so painstakingly reconstructed. Asteroid impact as large as this likely changed the microbial biodiversity on the planet, which could have allowed other species to evolve and fill niche positions.
A graphical representation of the size of the asteroid thought to have killed the dinosaurs, and the crater it created, compared to an asteroid thought to have hit the Earth 3.26 billion years ago and the size of the crater it may have generated. A new study reveals the power and scale of the event some 3.26 billion years ago which scientists think created geological features found in a South African region known as the Barberton greenstone belt. Credit: American Geophysical Union


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