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Dear All, Oct 10, 2012
Over the years I have seen several articles about petrified wood/forests
in which the trunks of trees are shown (without comment or explanation)
petrifed in fairly short segments of relatively constant length
independently of diameter and typically the fracture plane appears to be a
right angles to the axis. I am drawing on memory so this may be an
over-simplification.
Because petrifaction will involve migration of dissolved SiO2,
presumably in alkaline soil or ash, it is logical that the initial deposits
would be highly hydrated (e.g. opal with density of 2.1-2.3 g/cm^3) and
because the final deposit is largely quartz (density 2.65-2.66) the
necessary dehydration might be expected to cause some shrinkage (the smaller
H2O being able to diffuse outward more rapidly in Opal than the larger SiO2
could diffuse inward).
And it is also logical that this shrinkage would proceed from the outer
surface inward and be constant on all sides of the cylinder. So this sets
the stage for a fracture plane that would be at right angles to the axis.
But what would determine the distance between fractures, i.e. length of
segments ? Perhaps the length at which the tensile force due to shrinkage,
cumulated along the length of an intact log, exceeds elasticity of quartz ?
Comments anyone ?
Dave Webster, Kentville
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