[X] My Assistant

[alan]

At least three massive asteroids smashed into the Earth more than 3.2 billion years ago, and caused such destruction, they dramatically changed the structure and composition of the Earth's surface. This is according to new research from scientists at the Australian National University. The team uncovered evidence of major earthquakes, faulting, and volcanic eruptions that were so violent they dramatically changed the way the Earth's surface was forming. This happened during a period that the Moon also suffered heavy bombardment.

“Our findings are further evidence that the seismic aftershocks of these massive impacts resulted in the abrupt termination of an over 300 million years-long evolutionary stage dominated by basaltic volcanic activity and protracted accretion of granitic plutons,” Dr Glikson said.

“The precise coincidence of the faulting and igneous activity with the impact deposits, coupled with the sharp break between basaltic crust and continental formations, throws a new light on the role of asteroid impacts in terrestrial evolution,” Dr Glikson said.

http://www.universetoday.com/am/publish/ea...ids.html?582005

Could one of the geologist interpret this please.

[tty]

These three impact layers have been known for quite a while from both Pilbarra in Australia and the Barberton mountain land in South Africa. The new thing is the suggestion that they had large scale tectonic effects. Personally I feel rather skeptic. I think impacts large enough to have large-scale (or even global) tectonic effects would have left much more dramatic traces. The impression one gets from these layers are that the impacts were certainly larger than Chicxulub, but not by orders of magnitude.

Incidentally there is quite a lot of rock older than 3 bya, there is some in nearly every precambrian shield. There is not much supracrustal rock though, which is probably the reason there are rather few traces of life. The "cutoff point" is 3.8 bya, from before which there is virtually nothing, except some detrital zircon crystals.

tty

[Bob Shaw]

These three impact layers have been known for quite a while from both Pilbarra in Australia and the Barberton mountain land in South Africa. The new thing is the suggestion that they had large scale tectonic effects.

Well, the copious basaltic outflow known as the Deccan Traps have been implicated with an impact - not local to India, either, but due to globally transmitted shockwaves (as seen on the Moon).