Last Updated: Nov 22, 2016     Views: 175

From Dr. Robert Brill, Research Scientist Emeritus:

The answer starts with some glass science.  For strain to develop, the glass must have a substantial temperature gradient (temperature difference) between different parts of the glass while it cools through what is called its "annealing range".  The annealing range is a temperature interval between the softening point (where the hot glass will sag or flow slowly under its own weight) and the temperatures where the atoms and groups of atoms within the glass can move around just enough to rearrange themselves so as to release the forces that constitute strain.

With that behind us, I believe the answer (as far as obsidian is concerned) is that the glass actually doesn't cool all that quickly.  If you picture a flow of lava, it has an enormous mass, and heat is transferred only slowly from the interior outward.  Therefore, the main body of the glass cools slowly through its annealing range.  It is only at the surface that the lava chills and sets up more quickly.  So, if any strain develops, that would be where it will be concentrated: at the surface.  Over geological periods of time, the outer (strained) crust might well spall away.  In fact, large masses of obsidian probably do have fissures and cracks within them.

You might think that tektites would cool more rapidly, because they are smaller and exposed to low temperatures when the molten glass is tossed up into the upper atmosphere.  However, just as a space vehicle heats up during reentry (from friction caused by its movement through even a thin atmosphere), so also would a small mass of tektite glass heat up upon reentry because it is also moving at a great velocity.  I presume that it would hold its temperature (at red heat or hotter) and cool slowly enough through its annealing range that much of the strain would be released.  Tektites I have handled — and even sawed apart — do not appear to be heavily strained.

Fulgurites form quickly, but they are so thin (usually being hollow) that no temperature gradient is set up between the interior and surface walls of the glass, so little or no strain develops.  (Nevertheless, they are still very fragile.)

Prince Rupert's drops are not just cooled quickly, they are cooled very quickly or, as we say, "quenched" by being plunged into water while still molten.  In such a case, the glass cools and sets up immediately at the surface, while momentarily remaining red hot inside.  This means that when the surface sets up, the interior is still hot.  As the interior cools down, it shrinks.  Because the surface has already set up and become rigid, that tends to pull the surface glass inward toward the center causing strain to develop.  (We call that "being in tension", because the surface is being pulled inward.)  So both of the conditions we started out with are met— there is a temperature gradient in the glass ,and the glass cools quickly through its annealing range.  Prince Rupert's drops are an extreme example of highly-strained glass.