[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.

[Guest_Richard Trigaux_*]

Thanks tty for the dates.

What would be interesting now is if somebody has the dates for the Moon large bassins. It is roughly the epoch they were lava filled, but I do not know when they formed.

These dates are in a relatively short span of time, suggesting a special event. But do we have a complete series of geological layers in the 3.8 - 3 Gyears ? It could happen that such large impacts were common at this epoch, but we do not notice them as we do not have the complete series, only a sample. After all, 4 ejecta layers in 200 Myears, it is not much more than today. Only the thickness of those layers would point at much larger events than the -64 Myeras Chixculub and the -225 Myears event.

If I remember well, into the South Africa green rocks belt, there is also a large impact. I have seen this recently in a science review.

[tty]

What would be interesting now is if somebody has the dates for the Moon large bassins. It is roughly the epoch they were lava filled, but I do not know when they formed.

They are thought to be mostly 3.8-4.0 bya old, though the lava filling is much younger

These dates are in a relatively short span of time, suggesting a special event. But do we have a complete series of geological layers in the 3.8 - 3 Gyears ? It could happen that such large impacts were common at this epoch, but we do not notice them as we do not have the complete series, only a sample. After all, 4 ejecta layers in 200 Myears, it is not much more than today. Only the thickness of those layers would point at much larger events than the -64 Myeras Chixculub and the -225 Myears event.

There are indeed few good sedimentary sections that old. They have mostly been eroded long ago. Pilbarra is really a quite remarkably old and stable area. There are even places in Pilbarra where there are still traces of glacial morphology from the Carboniferous-Permian glacial ages some 300 mya.

If I remember well, into the South Africa green rocks belt, there is also a large impact. I have seen this recently in a science review.

That's the ones we have been talking about! They are from the Barberton Mountais Greenstone Belt.

tty

[dvandorn]

They ((lunar basins)) are thought to be mostly 3.8-4.0 bya old, though the lava filling is much younger

The impact basins range from 3.55 to 4.2 billion years old, if memory serves, with Imbrium being the youngest of the large basins (even younger, apparently, than Orientale). Not all basins are mare-filled -- especially those on the far side. But of those that do have a mare fill, the basalts we've sampled are on the order of 3.1 to 3.8 billion years old (though there is some support for the concept of areas of western Procellarum being slightly less than a billion years old, and the lavas at the Surveyor 1 site may be less than two billion years old).

In general, though, I wouldn't call the mare basalts "much younger" than the basins themselves -- in most cases, the maria are less than a half-billion years younger than the basins they occupy. Over a 4.5-billion-year lifespan, that makes both of them similarly ancient -- nothing on the Moon can really be fairly called "young," I think. As of about three billion years ago, the Moon had undergone about 98% of all of the major activity it would ever see.

They've dated the basins directly by dating samples of the impact melt created by several basin-forming impacts. I believe they are pretty certain about having dated impact melts from the Imbrium, Serenatatis and Nectaris basins, and have tentatively dated others (such as the 4.2-billion-year-old South Pole-Aitkin Basin) from a few isolated samples returned from Apollo 16, plus analysis of crater counts and degradation of basin structures. IIRC, Imbrium is dated at about 3.55 billion years old, Serenatatis in the 3.8 billion range, and Nectaris being a little older than both, in the 3.9 billion range. (I'm speaking entirely from memory here, but I think I'm close...)

I recall that the lavas in Imbrium were apparently erupted over a period of around 300 million years, beginning hundreds of millions of years after the basin was formed. This effectively killed the theory that lunar lava eruptions were triggered by the basin-forming impacts, though the lava may well have seeped to the surface through cracks formed by basin-forming impacts. It just took hundreds of millions of years for the lavas to start to flow.

While all of the visible maria are no older than about 3.8 billion years, there are pieces of basalt found as clasts in some of the highland breccias that have been dated as old as 4.2 billion years -- so some of the basin-forming impacts must have wiped out very old maria that simply no longer exist.

-the other Doug