planes in granite

[Norman Watts <Norman_Watts@zzzzzzz>]

The wallpaper on my computer is a picture that I took in Yosemite,
California showing the "diving board" feature on Half Dome. Half Dome is
a huge granite dome with one half sliced off by glacial action. The top
edge sticks way out into space -the "diving board". The picture (you can
google similar ones) clearly shows the curved layers of the granitic
dome. I've seen this layering all over Yosemite and Tuolumne Meadows and
assume it came about through cooling of the molten stone while still
deep in the ground. Why the layers are different thicknesses is less
clear to me. However, the reason I'm describing all this is that just
recently I was reading again about the three splitting planes of granite
in a quarry. Its not clear to me why there are three (3) planes. Clearly
there are planes parallel to the surface of the dome, the same ones that
have split naturally in the picture.

These are probably also the ones easier to split with tools. But why
would there be two more planes? In principle there certainly are three
orthogonal planes, but why would the remaining two be different from
each other in terms of their ease of splitting? If granite were a huge
perfect crystal I could see why there might be three planes, but it
seems to be made of randomly arranged grains. Is this perception wrong?

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