Why are there no craters on Earth
During the past 3.5 billion years, it is estimated that more than 80 bodies, larger than the dinosaur-killing asteroid that struck the Yucatan Peninsula 66 million years ago, have bombarded Earth.
However, tectonic processes, weathering, and burial quickly obscure or destroy craters. For example, if Earth weren't so dynamic, its surface would be heavily cratered like the Moon or Mercury.
Work by B.C. Johnson and T.J. Bowling predicts that only about four of the craters produced by these impacts could persist until today, and geologists have already found three such craters (larger than 170 km in diameter).
Their study indicates that craters on Earth cannot be used to understand Earth's bombardment history.
Johnson and Bowling write, however, that layers of molten rock blasted out early in the impact process may act as better records of impacts-even after the active Earth has destroyed the source craters. The authors suggest that searches for these impact ejecta layers will be more fruitful for determining how many times Earth was hit by big asteroids than searches for large craters.
The three craters are:
1. A meteor impact that hit Earth 3.24 billion years ago, remnants of which are preserved in South Africa.
2. The late Miocene Messinian Salinity Crisis (MSC)-This extraordinary geologic event is marked by massive salt accumulation, basin desiccation, and possible major sea-level drop as a consequence of the reduced water exchange with the Atlantic Ocean.
3. Provenance of volcanic ash found in Maya pottery from El Pilar, an archaeological site on the Yucatan Peninsula.
The study has been published online in the journal GEOLOGY.
(Posted on 28-05-2014)