planes in granite

[Norman Watts <Norman_Watts@zzzzzzz>]

John, Daed,
Thanks for the petrology insights. Its really impressive that
exfoliation occurs in such thick layers (several feet) and down to such
depths (up to 100 feet). I always thought that exfoliation was just the
thin (fraction of an inch up to a couple of inches) layers that peel of
the surface  -and that creak so alarmingly when you put cams under them
when aiding!

So to sum it up, magma originally deep below the surface slowly sorts
itself due to gravity, heavier components sinking and lighter ones
floating upwards. During cooling the crystal domains form, slower
cooling leading to larger crystals. As the mass becomes exposed by
uplift and erosion the relief of the pressure above causes the surface
dome to split into shells, splitting occurring preferentially between
layers of differing grain size. This accounts for sheets of different
thicknesses, finer grained (faster cooled) layers near the surface
giving thinner sheets and larger grained (slower cooled ) sheets
further down being thicker. Thats pretty well what I see in the
picture.

I don't think the "diving board" was one of those resilient inclusions
though. Its just the furthest out of a set of exfoliation sheets that
are cantilevered out over each other. However, i have seen these
inclusions in other places. The ones I have seen looked like a dark
granite ball embedded in the normally light-colored granite of that
region.

n

Norman Watts, Ph. D.
National Institutes of Health
50 South Drive, Rm. 1509
Bethesda, MD 20892-8025
Phone: (301) 402-3418
Fax: (301) 480-7629