RSAA Colloquia / Seminars / Feast-of-Facts: Thursday, 16 April 2015, 11:00-12:00; Duffield Lecture Theatre

Andrew Glikson

"Evidence of early heavy asteroid bombardment of Earth during 3.5-2.5 billion years ago"

One of the major unknowns in the history of Earth is concerned with the effects on the Earth of the Late Heavy Bombardment, documented on the moon during ~3.95-3.85 billion years. To date pre-3.6 billion years-old terrains disclose little evidence for asteroid impacts, likely due to the masking of their textural, mineralogical and geochemical signatures by high grade metamorphism. However, since 1986 the progressive discovery of asteroid ejecta and associated tsunami events in Archaean sediments indicates ongoing bombardment by asteroids on the scale of tens of kilometre in diameter. To date at least 15 such events are recorded in South Africa and Western Australia. It is estimated that, due to erosion and removal of asteroid ejecta units across unconformities and the difficulty in identifying such units in the field, no more than one in ten asteroid impact units have been found. The seminar will report a number of new discoveries and consider their implications for the nature and evolution of the early Earth crust and mantle.