[Colin2B]

I once made the mistake of thinking that the explosion is caused by pressure building up inside the shell.

When the shell is intact it is usually a pressure buildup. What you have come across is a superheated area in the egg. You've probably noticed this when heating up liquids, especially thick ones that don't mix easily eg custard. The superheated zone is contained by the weight of the surrounding material until disturbed, perhaps by stirring, and there have been a number of reported scaldings. In your case the white was just strong enough to hold the pressure.


I don't envy you the cleanup job.

[vhfpmr]

With superheating, you get much the same effect without the need for anything to confine the pressure, a superheated glass of water will explode.

[Colin2B]

With superheating there are 2 confining pressures, the weight of the water and the surface tension around the bubbles. Although these are small they are enough to contain the micro bubbles until the temp is several degrees above boiling point, when slight disturbance can cause them to group and explode.

[pradeepkumar]

If you think your microwave's power is starting to wane, give it a test. Cook one cup of water on high power for two minutes. If it's not piping hot, it may be time to consider a new microwave.