From: cain@geomag.gly.fsu.edu (Joe Cain)
Subject: life and geology
Followup-To: sci.geo.geology
Keywords: melting point iron core inner core CO2 
Organization: Florida State University Geology Dept.
Date: Fri, 20 Nov 92 21:50:07 GMT

in Article 3239 of sci.geo.geology Richard Stead writes:
In article <1992Nov11.175421.19955@mailer.cc.fsu.edu>, cain@geomag.gly.fsu.edu (Joe Cain) writes:

>>... After life started on Earth so as to raise the oxygen level
>> from near zero creatures evolved with shells so the carbon dioxide
>> started to be captured in the shells that ended up as carbonate rocks.

>No.  oxygen levels were already high by the time multi-cellular organisms
>appeared.  3 billion years of algae produced oxygen by photosynthesis.
>The carbon was most probably locked up as organic molecules.  Carbonate
>deposits were probably inorganic.  Even stromatolitic limestones were
>a pretty late development.

Good point! However, I now hear from our paleoecologist here, that
there are some very old (> 2 Ga) stromatolites sitting on carbonate
rocks, and the present argument is that either they produced it, or
filtered what was already there. He also claims that not only most of
the carbonate now is organically produced by several algae, but that
he believes it always was so produced. If this is true there would
there be no need to have to wait for shelled critters to develop at
the beginning of the phanerozoic, What results have been published
either about the change of CO2 with time, or the reasons? I see in
such tomes as Hamblin and Christiansen's "Exploring the Planets" (p.
182) a plot of oxygen vs time that has it from 1% of present at 2 Ga
rising linearly on a log plot to a level present value near 300 My BP.
What would a similar curve for CO2 look like?

>> greenhouse affect of all the water vapor and CO2 was diminished. (The
>> increase in oxygen allowed ozone to form and organisms to survive in
>> the sunshine filtered of its hard UV). Was this decrease of
>> temperature a significant factor in allowing plate tectonics to
>> develop?

>Plate tectonics started as soon as the surface of the new earth was
>cool enough that some magma could solidify.  If anything, it has
>diminished with time. It is driven by the thermal gradient between the
>outer core and the surface. The outer core is at 12000K.  It really
>doesn't make much difference if the earth's surface is at 0K or 350K,
>the outer core temperature dominates. 

Although the strength of mantle convection indeed must be driven by
the temperature gradient, my thought was that the ease with which
subduction takes place would be related to how quickly the ocean floor
would cool as it came away from the ridges. I would think that if the
internal temperatures were some 300K hotter say 3 Ga ago, the crust
would have stayed thin and the pull part would be less or that the
crust might not have subducted so easily or done so at shallower angles.

 In any event your figures for internal temperature are way higher
than those quoted recently. For example, in the "Deep Earth Dialog"
reporting on the Mizusawa SEDI meeting, Loper reports regarding the
melting point of iron "The melting curve then must pass from the
hcp-bcc-liquid triple point (at 195 Gpa) to the solid-liquid
transition (at 224 GPa and 5000 deg K) observed by Brown and McQueen
(1986), which yields Tm ~5800K for pure iron at the liquid-solid core
boundary pressure (330 Gpa). If the inner core is nearly pure iron,
then the melting temperature at the Earth's center would be about 300
deg higher due to adiabatic compression, yielding Tm ~5800K at the
Earth's center. Thus the temperature at the Earth's center is less than
about 6000K."




Joseph Cain		cain@geomag.gly.fsu.edu
cain@fsu.bitnet		scri::cain