🚀 go-pugleaf

On 8/26/03 10:25 PM, in article 3F4C410A.1020209@ix.netcom.com, "Robert
Casey" <wa2ise@ix.netcom.com> wrote:

> John Wilkins wrote:
> 
>> lumberjack <wfwoodland@yahoo.com.au> wrote:
>> 
>>  
>> 
>>> The following news item may be of interest ...
>>> 
>>> Volcano theories may need revision
>>> 
>>> A textbook example of how chains of volcanic islands like Hawaii form
>>> is based on false assumptions, U.S. scientists have argued.
>>> 
>>> http://www.abc.net.au/science/news/stories/s929822.htm
>>>    
>>> 
>> 
>> Here's a more detailed site:
>> 
>> http://www.geolsoc.org.uk/template.cfm?name=MovingStory
>>  
>> 
> I never did understand why a plume would have to be stationary especially if
> it is constantly ascending thru a moving fluid mantle.  And what is
> making it down there
> below the mantle anyway?  And what's to say that the source doesn't also
> wander
> over the eons as well?  The plume  might be drifting around a bit like a
> tornado
> does.  But maybe if the positional "noise" imposed by the wandering is
> low enough,
> one might still get away with using plumes as reference points.  But it
> might mean that
> one can only say that Antartica was at position "X" plus or minus 3
> megameters
> (thousand kilometers) 100 million years ago.
> 
> If the uncertainty is high enough, maybe one can place India on the
> exact opposite
> side of the Earth from where that asteroid hit at Chxxxxxxxxx(sp) 65
> million years
> ago.  The impact force getting focused thru the Earth to make lots of
> cracks in
> India making for lots of volcanic activity (Decan flats) there.  (Of
> course if the
> Decan flats happened before the asteriod hit, then forget this....).
> But if one can
> say for sure if the asteriod caused the Decan flats, then that might
> indicate how
> much error reference plumes can have.

Convection in the mantle of the Earth is time-dependent. We already knew
this before. The convective vigor is measure by a "Rayleigh number." Below a
certain critical value, no thermal convection occurs, and heat conduction
dominates. Just above this value, "steady" convection develops into regular
cells that eventually develop a stable geometric pattern. Another transition
occurs at even higher Rayleigh number, where the convection becomes vigorous
enough that the regular cellular pattern can no longer be sustained and the
positions of upwellings and downwellings begin to shift in a chaotic manner.
This is called the "transition to time-dependent convection." The critical
Rayleigh number for onset of convection is on the order of 1,000. The
transition to time dependence occurs when the Rayleigh number is on the
order of 100,000. Based on study of Earth's interior, we can estimate that
the Rayleigh number of the mantle is on the order of 10,000,000, which is
well above the transition to time-dependence.

Additional evidence of the time-dependence of mantle convection is readily
apparent in the geologic record. If mantle convection were in the steady
cellular regime, plate boundaries would never change location relative to
one another, and continents would be in a fixed formation and couldn't
collide or separate. But they do. That's a fact. This is more readily
apparent evidence that convection is time-dependent.

All that the new studies show is that assuming steady convective features in
an inherently time-dependent system was a bad assumption. Not surprising at
all, and in particular this observation has no bearing on the formation
mechanism of hotspots. It is a red herring from those wishing to de-bunk the
theory of plumes.

J