Re: Hypothesis Involving Planetoid Impacts

Professor Gauss wrote:
> Not being a geologist, I am new to this group, but have a hypothesis and I was
> wondering if it has been discussed yet by geologists, either here or elsewhere.

On subjects like this, you'll find plenty of discussions here, some of 
them more serious and some less, but most of them in a quite emotional 
way, so expect the worst, then you may be positively surprised :-)

>  Here is the hypothesis:
>
> 1) The early earth was smaller than it is now.  It had a crust floating on top
> of a mantle which covered the entire earth.  The crust formed following an
> extended period of chemical reactions at the earth's surface.  A possible
> example of such a reaction is the hydration of minerals caused by the
> absorption of water from the atmosphere.
A similar theory is used by the so-called Earth Expansionists.

Many scientists think that the continental crust formed by magma 
differentiation on top of the mantle (which is not molten, just to be 
sure...).

> 2) Large planetoids impacted the earth.  The ejecta from one or more of these
> collisions coalesced to form the moon.  (This idea is not new.)  A large
> quantity of material was also added to the earth's mantle and/or core.  This
> caused the earth's volume to increase by a factor of about 5.  The crust was
> now distributed over a larger surface area.  Although land now comprises about
> 29% of the earth's surface, inclusion of continental shelf areas would
> correspond to a higher percentage, let's say 35%.  This would cause the radius
> of the earth to increase by about 70%.
The Earth probably grew like this some 4.5 Ga ago, but certainly, little 
crust was left after the bombardment. Most crust formed after the heavy 
bombardment era.

> 3) The relatively highly curved crust relieved itself on the now less highly
> curved surface by distorting.  Massive cracks may have given rise to long
> rivers such as the Mississippi.  In other places, more stress relief may have
> caused the formation of wrinkling and faulting to produce various types of
> mountain ranges.  The cracking process, in addition to the impacts themselves,
> may have caused the crust to break up into continents.
You got the distortion thing right (in contrast to most Earth 
Expansionists), but as I said, the crust is younger than the impact era.
And the Mississippi formed only a twinkling of an eye ago in relation to 
the Earth's history...
And I cannot recall any crack in the whole run of the Mississippi - if 
you would consider the African Rift valley, that's a different thing, 
but it's because of plate tectonics.

> I know that this hypothesis sounds far-fetched, especially considering the vast
> amount of material that needed to be added to the earth's mass without
> completely destroying it.  But isn't it plausible that high-density meteorites
> could have penetrated the crust and gravitated toward the earth's center, while
> the punctured crust floated back up to the surface?  If there are links to
> refutations of such ideas on the web, I would appreciate knowing about them. 
> Thank you for your consideration of this matter.

That's exactly the point - the crust cannot just float back, as most of 
the Earth is solid (only the outer core is liquid) The oldest rocks 
found so far have an age of some 3.8 Ga (I'm not sure on this), which is 
some 700 Ma after the time of heavy bombardment.

However, nothing certain is know about the size of continental crust at 
that time and how the oceanic crust looked like (there is very little 
rock left from that time).

Interesting idea, but most of it doesn't work, I'm afraid.

Have fun reading the flames that may follow your posting :-)

Cheers, Christof
--
Christof Kuhn
Inst. f. Angewandte Geologie,
Univ. f. BoKu Wien, Austria

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