Corrected entry: When the lava is flowing down the streets, all the glass in the buildings shatter. At that heat, the gas would melt not shatter. The temperature that is used to melt and mould glass is a lot less than the temp of the lava flow.
J I Cohen
30th Oct 2003
Correction: The glass is shattering from the shock of the sudden air temperature change. It would be unlikely to melt instantly unless it was much closer to the lava.
J I Cohen