HOW EARTH REMAINS CRATERLESS

During the past 3.5 billion years, it is estimateed that more than 80 bodies, larger than the dinosaur-killing asteroid
that struck the Yucatan Peninsula 66-million years ago, have bombarded the Earth.
     However, tectoniuc 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 geologist have already found three such craters (larger than 170 kms in diameters).

     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 impact-even after 
 the active Earth has destroyed the source craters. The autherrs suggest that searches for these impact eject layers will be more fruitful 
 for determining how many times Earth was hit by big asteroids than searches for large craters. If you want to read more about this topic, 
 you can find the study in the journal Geology. 
   
         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.