- © 2006 by the Mineralogical Society of America
At the beginning of the solar system, impacts and collisions were dominant processes. After an early collision that may have led to the formation of the Moon, both Earth and Moon suffered intense post-accretionary bombardment between about 4.5 and 3.9 billion years before present. There is evidence from lunar rocks for an intense “Late Heavy Bombardment” at about 3.85-3.9 Ga, which must have had severe consequences for Earth as well, even though no terrestrial record has yet been found. Several 3.4 to 2.5 Ga old spherule layers in South Africa and Australia and two impact craters near 2 Ga represent the oldest terrestrial impact records found to date. Thus, the impact record for more than half of Earth's geological history is incomplete, and there is only indirect evidence for impact processes during the first 2.5 billion years of Earth history.