[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_*]

I do not believe too much in massive volcanic episodes following a large impact, such as the melting of the opposite side of the Earth (On Mercury, the opposite side of Mare Caloris did not melted, but id was severely shattered).

On medium impacts (Moon or Earth as well) we observe lava flows, but without roots, just the fallout of molten rocks in or around the crater. This is visible on Tycho, Moon, or in the Ries crater, Germany.

But what is possible anyway, on a planet with plate tectonics, is that a large impact may remove a part of the crust, and thus change the equilibrium of forces. This may result on the fast opening of distention fractures at a great distance of the original impact, such distention fractures producing large pouring of fluid basaltic lavas. But this is not what happened in Deccan. The Deccan traps started to form several millions years before Chixculub and they are the result of the emergence of a hot spot (diapir of hotter mantellic rock). This hotspot was also involved in the separation of India from Africa and it is now still active, although much weaker, resulting in a volcano in the french island "la reunion", Indian Ocean.

In the case of a very large impact, there may form distention fracturation all around, resulting in the formation of an ocean floor of basalt.


If extra large impacts happened into the 3.8-3.2 Billion years time, they had most chances of hitting oceanic plates, which are now absorbed from long ago into the depths of Earth mantle. Only small islands of continental plates existed at this time, so if a large impact occured right on one, it may have been completelly destroyed. If the margin of a crater intersected a continental plate, this margin was further extended in a way similar to continental margins when today an ocean opens into a continent. In both cases, no traces of the crater itself remain, only ejecta blankets or traces of far fallout. On the other hand, such fallout are visible everywhere (Chixculub's far fallout are findable all around the Earth). So infering large or extra-large craters from ejecta blankets or fallout layers is consistent. Just more accurate studies are needed to find the magnitude of the impact, and perhaps other data such as where the blanket rocks came from. (two Chixculub's rocks layers were dated and identified all around the globe).


But I shall abstain of very positive statements about corellations of volcanism with other phenomenon. There are large scale cycles in Earth magnetic field (>200 Myears) which are not yet completelly understood and which also resulted into variations of volcanism. And recently it was postulated a volcanic cycle of 25000 years in corellation with a climatic cycle!

In volcanism, there are things we can say we understand, and others we cannot.

[Bob Shaw]

Richard:

I think the Chixculub/Deccan Traps link always *was* rather speculative, although interesting. The facts-as-they-are-known appear not to support the idea, so let's leave it to the SF writers!

A comment made earlier, however, about the antipodal clustering of secondary impacts remains intriguing. Anyone know the figures for velocities of ejecta from big whacks? And, as Tycho's rays stretch so far round the Moon, is there any evidence for secondaries around it's antipode?

Bob Shaw

[elakdawalla]

I think the Chixculub/Deccan Traps link always *was* rather speculative, although interesting. The facts-as-they-are-known appear not to support the idea, so let's leave it to the SF writers!

I've been trying to track down a paper I wrote for an impact cratering class in grad school on this topic, antipodal effects of major impacts. I'm having no luck finding the paper, but dredging my memory, I think I remember a couple of interesting factoids: computer modeling of this question shows that there could indeed be huge shock pressures at the exact antipode of a major impact. And, in fact, now that there is very good coverage of the Earth with seismographs, there are now some actual data supporting the idea, seismographic records showing unexpectedly high shock pressures at the precise antipode to very large earthquakes. However, the key thing in the models and in the data is that the focusing really does take place only within less than one degree of the antipode. There are a lot of proposed pairings -- Deccan traps is just the famous one -- of impact sites with antipodal volcanism, but few of them can satisfy that criterion of being exactly antipodal, even when you take continental drift into account. And, as Mr. Trigaux pointed out, the physical mechanisms for how high shock pressures could cause a long-lasting volcanic episode are poorly defined.

As for where the ejecta falls, there are concentrations having to do with antipodal focusing, but the Coriolis effect also does some strange stuff, making really interesting patterns in where the ejecta falls, depending on the latitude the initial impact took place. Hopefully a little deeper trenching into my files will eventually recover the papers on this topic that I know I spent hours in the library photocopying!

But as far as I can tell, antipodal effects aren't what the Australian National University researchers were talking about. Basically they say that a bunch of impacts 3.2 billion years ago caused the Earth's volcano-tectonic style as a whole to change from a place where there was mostly ultramafic volcanism (which would look more like the Moon, Venus, and Mercury than it does now) to a place where you had continents (totally different rock composition) and plate tectonic activity. But I can't figure out whether or not they have any mechanism in mind by which major impacts could have caused that change, which makes it hard to assess the quality of the paper (which has been submitted for publication, but not reviewed or accepted, again as far as I can tell). And all the impact experts I know have been in Rio for a week at the IAU Symposium on Asteroids, Comets, and Meteors, where I am sure they have been poring over the Deep Impact results -- or just enjoying Rio -- and not paying attention to the claims of upstart Australian researchers!

Emily