Space Forum / Astronomy / June 2006

Lord Rayleigh in *1906* calculated that the heat from radioactivity was
sufficient to persist for billions of years.

No one ever said that Don Findlay has any sense.

George

Don mentioned this business of the heat being used up back in March,
about 20  threads ago, and was demolished then, too. Here's the exchange
of posts

In message <1141771823.661981.85810@p10g2000cwp.googlegroups.com>, don
findlay <don@tower.net.au> writes

It's never "all used up". That's why it's called a half life. A thousand
million years ago (ten times earlier than the Mesozoic) there was only
twice the amount there is now.
Anyway, here's a link for you
<http://physicsweb.org/articles/news/7/5/4/1>

What this means, Don, is that radioactive materials keep adding *new* heat to
the core. Thus cyclic processes are quite plausible. So much for strike 5.

Quite obviously these are unfair questions and you're hurting his feelings!

Cooking?  DF?  He gets monthly care packages from Turdhard.

George

What he said was not true, even if there were only stable isotopes!
There still would have been some convection, formation of crust plates,
and plate movements. The difference is that the convection would not
have lasted as long.
Klaus

Filed under "Three thou shalt not count!"

I thought he was talking Bowling.

Calm down, Stu, this frothing at the mouth will only did you committed!

JT

Why?  For taking a swipe at the gift that keeps on giving?  Gift for
some..
(Stuart at ...   http://tinyurl.com/e57o4  ...   True Confessions)
...not for others ....
http://users.indigo.net.au/don/re/memory.html

I hate to do this, but you are laboring inder an illusion.  The mantle is
not liquid - the outer core *is*, but the mantle is not.  We know this
because the mantle transmits elastic shear waves (the 'S-wave' of
elementary geophysics), and liquids cannot transmit elastic shear waves.  
Thus, on time scales on the order of seconds, the mantle acts like an
elastic solid (elastic used here in the rheologic sense of something that
is springy).  However, on time scales that are longer by 12 to 16 orders
of magnitude the mantle acts as a viscous fluid (in fact, it acts like a
visco-elastic material, much like silly putty only moreso). We can
calculate the effective viscosity of the upper mantle based on measured
rates of uplift in response to unloading (resulting from the melting of
the really friggin big continental icecaps of the last ice age).   Using
these numbers, we can plug in and discover that the mantle is quite fluid
enough to convect under the thermal conditions that must occur within the
earth.    By the way, in general magma generation (that is, partial
melting of the mantle) occurs either when the introduction of aqueous
fluids reduces the melting point (such as when hydrated rocks are cycled
into the mantle in subduction zones) or when there is upwelling of hot
mantle material such as at spreading centers.

Don is also mistaken in the idea that formation of a crust 'uses up'
heat.  In fact, crystallization releases heat (the 'heat of
crystallization' of elementary chemsitry), and as I understand it, the
heat released by the ongoing crystallization of the outer core is a
significant source of heat within the earth.  He also seems to think that
the heavier elements like uranium are the only contrubutors to
radioactive heating of the earth, when in fact potassium-40 is a major
souce of heat within the crust (it's a lot more abundant than uranium).  
The fact that the crust produces heat is important because it acts sort
of like an electric blanket for the mantle, reducing the overall rate of
heat loss from the interior of the planet.

Because the temperature at the interface between two heat conductors at
different temperatures (earth and atmosphere) needs must be somewhere
between the two.  If the hot side is a liquid at crystallization
temperature, then by golly, a crystalline crust is gonna form at the
interface.  This is independent of the mode of heat transfer within the
liquid (convection or conduction).  Solidification happens from the
outside in, determined by the rate at which heat can be transported from
the inside out.  Now, rock is really, really lousy at conducting heat -
so lousy, in fact, that it is more efficient for the mantle to flow in
thermal convection than for the mantle to just sit there conducting.  
Notice that Don is basically engaging in an argument from incredulity
here - no numbers, no physics, just incredulity.

Further, the primary stratification of the earth is by density - the core
is really dense, the mantle less so, and the crust really not too
important.  The chemical stratification (increasing silicon content
outward) is really secondary.  Formation of the primary stratification of
the earth would result in production of heat (gravitational heating as
the core material falls inward) rather than loss of heat (to
crystallization? I'm not real clear on what Don means by stratifacation
losing heat, since both crystallization and falling release heat.)