Massive Asteroids Transformed The Earth's Surface Options

[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.

[blobrana]

Could one of the geologist interpret this please.

"The Pilbara Craton

in Western Australia has a domainal architecture, which has been interpreted to reflect a history of accretion. The Tabba Tabba Shear Zone is the major division between the East and West Pilbara blocks: this is based on significant differences in the tectono-thermal histories of the bordering terranes.

New laser ablation U-Pb zircon geochronological data, coupled with trace element data for the same core parts of the sampled mineral grains indicate a range of magmatic crystallization ages for representative igneous rocks emplaced before, during or after shearing.

The Tabba Tabba Shear Zone currently forms the eastern bounding fault of the Mallina Basin. The last major activity in the structure occurred during a major phase of oblique sinistral movement, corresponding to closure of the Mallina Basin. Ages of late syn-kinematic granitic intrusions indicate that this occurred at about 2940 Ma.

These unique locations have kept the formations from experiencing the depths that metamorphose rocks, obscuring their original sedimentary and volcanic features. "

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[Chmee]

You can imagine what Ceres and Vesta sized asteroids would do to the surface of the Earth: almost complete resurfacing of the upper crust. That was one heck of a bombardment that occured. Just looking at the Moon you can see the scars left behind. the Earth with its greater mass, would have attracted even more/larger asteroids.

[Guest_Richard Trigaux_*]

Fascinating prospect!

Why not? With more and more accurate geological data all around the world, we may find more and more unexpected things, such as super large meteorite impacts. But remember the debate about the 64 mllion years transition: at first it was supposed volcanic, but more and more evidences accumulated for an impact. But there are still some points unexplained, and thus still scientists who think about a volcanic origin. So what about 3 billion years?

Remember that there was not one large impact on the Moon, but several, and as far as we know they were not simultaneous. If others are found on Earth, that makes an unusual shower of large bodies with about the same orbit than Earth (The 3 billion years episode seems not to have taken place on other planets).
An explanation was proposed some years ago, I do not remember by who, as what there was an other Earth's satellite which formed at the same time than Earth and Moon. But its orbit was unstable, and it broke in several parts, explaining the shower of large bodies on the Moon (and Earth).

If a body as large as Vesta hit the Earth, I think that all the surface would melt, making life disappear, and even all fossil-bearing layers too. So what happened at that time was not so tremendous, as, as far as we know, life already existed before.

But 3 billion years is a kind of limit: we do not know much older rocks, and thus much older traces of life. So the first billion and a half of the geological evolution of Earth crust and Earth life are still largely unknown. The reason why older rocks are hard to find is that continents were smaller at that time (continents formed progressivelly by accretion of granitic rocks, a process which is still going on). But perhaps most of these ancient rocks still exist deeply buried under the continents.

[Bob Shaw]

Remember that there was not one large impact on the Moon, but several, and as far as we know they were not simultaneous.  If others are found on Earth, that makes an unusual shower of large bodies with about the same orbit than Earth (The 3 billion years episode seems not to have taken place on other planets).
An explanation was proposed some years ago, I do not remember by who, as what there was an other Earth's satellite which formed at the same time than Earth and Moon. But its orbit was unstable, and it broke in several parts, explaining the shower of large bodies on the Moon (and Earth).

I thought that Hellas and the other southern basins on Mars were also Late Heavy Bombardment candidates, too, and that even the Northern Plains had been put up as (first) impact basin then (second) ocean?

A second Terrestrial satellite which conveniently scarred both Earth and Moon seems a bit much like special pleading!

I wonder whether the putative second satellite was more-or-less co-planar with the Moon, and if so then perhaps the spatial distribution of (Imbrium and before) basins might give us a clue to any supposed orbital characteristics (or not!).

[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).

[tty]

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).

That's a nice theory killed by a set of brutal facts to cite H. L. Mencken. The Deccan eruptions started well before the Chicxulub impact as shown by the Maastrichtian biota found in the intertrappan beds.

There have been some interesting speculations that *very* large impacts might cause large scale melting and volcanism by removing the overburden and/or shock-melting large volumes of rock. It has been suggested that this might be the reason for the vast Siberian traps that erupted near the P-Tr boundary and for the huge mid-Cretaceous Ontong Java plateau in the Pacific. One would expect this to occur preferentially for ocean impacts since oceanic crust is much thinner than continental and more likely to be "punched through" by an impact. The mechanism as such seems quite reasonable but there is little concrete evidence yet.

tty

[Bob Shaw]

That's a nice theory killed by a set of brutal facts to cite H. L. Mencken. The Deccan eruptions started well before the Chicxulub impact as shown by the Maastrichtian biota found in the intertrappan beds.

I bow to your superior knowledge regarding the Deccan Traps - but the antipode to the Caloris Basin on Mercury was, I seem to remember, discovered to have been well-stirred by shockwaves, so presumably we on Earth could also see not just local (local! ha!) volcanic effects from big impacts but global ones.

[Guest_Richard Trigaux_*]

Thanks all for your precisions.




First the idea as what "something special" happened 3.2 billions years ago is the occurence of large (in the 1000kms) bassins on the Moon, which formed in a relatively short time. This simultaneousity hints at a special event, a group of large bodies with the same origin. This must not be confused with the initial bombing of all the planets at the origin of the solar system, which culminated 4.5 B years ago and decreased steadily since.

If the 3.2 B years event is not local to Earth, we may expect that Mare Caloris (Mercury) Hellas (Mars) and eventually others formed simultaneously. But we do not know the dates of these events.

But the fact that there is a group of large objects crashing on the Moon nearby in the same time makes that the idea of "A second Terrestrial satellite " not so "special pleading" as you say Bob Shaw. Of course other origins are possible, fo instance a large collision in the asteriod belt, and it is even more likely. (At the origin there was only some spherical asteroids, but numerous collisions occured which created the thousands angulous bodies we know today).

The original theory of the "second satellite of Earth" was in fact more complicated, it was rather a belt of debris: either an unique body never formed, or it broke appart. (Remember that at that time the Moon was much closer from Earth, so there was little place for the accretion of a third body, and matter rather formed a great number of small bodies, like today with the numerous little satellites at the rim of Saturn's ring). So when these bodies may have hit the Moon, due to the loss of mass, angular momentum rearanged the axis of rotation of the Moon, untill the next shock, so Bob Shaw, I think it would be rather difficult to infer orbital data of the impactors from the distribution of today marks.







Thank you tty for your precisions. The date you give (3.8 billion years) is what I was speaking about, but I did not remembered the figure. Perhaps this date is the date when the first continents began to form. Life seems much older, but this is known only from the evolution of DNA which has its own "clock" and we are not sure that this clock moves at a constant rate.






There was a discution about the effect of shockwaves on the opposite side of Mare Caloris, but these effects were purely tectonic, without melting of rock (But mountains were jumping and turned upside down like a pancake in its pan, if was rather better not to be here).




At last very large impact bassins like those we are discussing about (about 1000kms) could have formed in Earth past, but if they were into oceans, all the traces disappeared in 100 or 200 millions years, with the plate tectonics on the ocean floor. And 3.2 billions years ago, there was no large continents like today. So such impacts can be identified only by their ejecta (which may have covered the whole Earth). Are these ejecta basaltic/mantellic, or continental?





A last element is the evolution of life itself. It is now generally accepted that recent (less than Cambian time) impacts modeled the evolution of superior living beings. But 3.2 or 3.8 billons years ago, the skate of the evolution was rather setting the DNA code structure tself. Specialists have an hypothesis as what the genetic code is the result of an evolution in very ancient times. For instance there is a theory as what our three codons code evolved from a two codons code. So very ancien, but very large impacts may have driven the evolution of our DNA well before the appearance of our most ancient bacteria

Oooof record of the longest post to date!!!

[dvandorn]

I don't know, Richard -- I agree with Bob, I think it's a little bit of a stretch to posit a second terrestrial body as the source of the Heavy Bombardment impactors.

Instead, as has been discussed in the Jupiter board, it's more likely that Jupiter and Saturn moved into a gravitational resonance that disturbed "loose" bodies in the Asteroid Belt. Some of them were probably ejected from the Solar System altogether, but a lot came raining in towards the inner System, pelting the rocky planets with huge impacts. Even so, a lot of the bodies probably simply fell into the Sun, so a *lot* of bodies had to have been tumbled inward.

-the other Doug

[tty]

At last very large impact bassins like those we are discussing about (about 1000kms) could have formed in Earth past, but if they were into oceans, all the traces disappeared in 100 or 200 millions years, with the plate tectonics on the ocean floor. And 3.2 billions years ago, there was no large continents like today. So such impacts can be identified only by their ejecta (which may have covered the whole Earth). Are these ejecta basaltic/mantellic, or continental?
A last element is the evolution of life itself. It is now generally accepted that recent (less than Cambian time) impacts modeled the evolution of superior living beings. But 3.2 or 3.8 billons years ago, the skate of the evolution was rather setting the DNA code structure tself. Specialists have an hypothesis as what the genetic code is the result of an evolution in very ancient times. For instance there is a theory as what our three codons code evolved from a two codons code. So very ancien, but very large impacts may have driven the evolution of our DNA well before the appearance of our most ancient bacteria

The strange thing is that there seem to be no traces of large impact basins even in the oldest archean terranes. There have been a few suggestions (Central Hudson Bay, The Upland structure in Sweden, The Colorado plateau) but the evidence is slender at best. To me it looks as if the Late Heavy Bombardment more or less destroyed all evidence of older crust and that there hasn't been any really big impacts since then.

The only impact where there is strong evidence of global effects on life is Chicxulub which is also by far the greatest known impact during the Phanerozoic.
There is some evidence that there is a threshold at a crater diameter of ca 150 km, above which the energy deposited by fallback ejecta at the top of the atmosphere is sufficient to cause world-wide fires and mass-kill of unprotected land animals.

I bow to your superior knowledge regarding the Deccan Traps - but the antipode to the Caloris Basin on Mercury was, I seem to remember, discovered to have been well-stirred by shockwaves, so presumably we on Earth could also see not just local (local! ha!) volcanic effects from big impacts but global ones.

The Caloris impact was proportionally enormously bigger. The focused energy at the antipodes from Chicxulub is negligible in comparison. This is not the same as saying that the effects of the impact were negligible there. To the contrary the fallback ejecta is greatly concentrated near the antipodal point and India and Madagascar that were nearly antipodal position to Yucatan at 65 Mya were probably more or less sterilized.

Instead, as has been discussed in the Jupiter board, it's more likely that Jupiter and Saturn moved into a gravitational resonance that disturbed "loose" bodies in the Asteroid Belt. Some of them were probably ejected from the Solar System altogether, but a lot came raining in towards the inner System, pelting the rocky planets with huge impacts. Even so, a lot of the bodies probably simply fell into the Sun, so a *lot* of bodies had to have been tumbled inward.

That's not quite the way it works. When a major planet migrates its gravitational resonances migrates with it and remove smaller bodies from the space they move through since orbits become unstable near resonances ("resonance sweeping"). I don't think much of the material larger than dust actually hits the Sun. Most of the stuff that is deflected inward would be swept up fairly quickly by the inner planets hence the fairly brief "Late Heavy Bombardment" ca 3.8-3.9 Bya ago.

tty

[blobrana]

Hum,
There is also the complication of `coincidence`; with trying to tie in impacts with volcanism.
For example, the chances that asteroid impacts and huge bouts of volcanism coincide randomly to cause mass extinctions, is greater than previously imagined.

I believe that UK researchers conducted statistical tests to determine the probability of such catastrophic events happening at the same time in Earth history.
And they found massive releases of lava and space collisions should have overlapped three times in the last 300 million years. They found the probability of this happening at least once over a period of 300 million years was 57%.
Once the researchers reduced the size of the impact slightly, the probabilities increased sharply.
For craters exceeding 100km, the probability of at least three co-occurrences between flood basalts and impacts was 46%. For craters exceeding 60km, the probability of three or more was 97%!

Although the dates of the proposed impacts are ten times older, the probability that the Pilbara impacts and the volcanic eruptions were coincidences are probably equally high.

But having said that, it would be fair to say that any impact will transfer a tremendous amount of energy to the Earth, which if it were still in the process of `forming` would lead to increased volcanism.

For me, the jury is still out on this one….

[Bob Shaw]

Now here's a coincidence worthy of Arthur Koestler: I attended the World Science-Fiction Convention in Glasgow last week, and bought my usual fix of hardish SF.

One such was Ian McDonald's highly acclaimed 'River of Gods', which I began reading this week. Imagine my surprise when, after posting here yesterday, I lay in bed reading a bit more before going to sleep, and the first words I glanced at on p429 of the Pocket Books paperback edition (you'd need a BIG pocket!) were: 'Earth had survived the Chixulub impact and the resulting Deccan melt on the other side of the planet at the cost of twenty-five percent of its species...'.

Now is that spooky, or what?

[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.