Nice gallery of a microwaved CD – so you don’t have to… You can see the current vaporizing the aluminum in the CD as well as the arcs of microwaves, from what I understand the patches of aluminum become antenna-like.

Update: Bill writes:

The fractures first grow via a fascinating effect called “electromigration.” Once the aluminum layer has been spontaneously sliced into separate sections, high voltage appears across the fractures, and arcs start leaping across.

Try this: put one or more glasses of water in the microwave. It slows down the whole “zapping” process.

The arcing is not quite a “radio antenna” effect. An empty microwave oven contains an extreme e-field because of resonant rise. Any two conductive objects will be at two different voltages with respect to ground. The voltages can be pretty high (many kilovolts.) If two sharp-edged conductors such as the foil segments in a CD are very close together, the high voltage between them will cause arcing. It’s purely a voltage effect rather than the electric/magnetic effect present in a radio antenna. (An unloaded microwave oven is very much like a Tesla coil.)

But the aluminum layer gets sliced up before all this high-volt stuff happens. Why? It’s because there are large currents induced in the aluminum foil by the radio waves. The current isn’t nearly enough to hurt the foil. But if tiny defects exist in the foil, such as microscopic regions of insulator, these defects cause the “lines of current” to become compressed near the defect. It’s sort of like a lightning bolt, but rather than lines of e-field being attracted to the sharp tip of a plasma streamer, instead lines of charge-flow are pushed away by the sharp tip of a growing fracture in the aluminum. The lines are squeezed and the current density increases. If the tip of a fracture was infinitely sharp, the current density in the metal in front of the fracture would be infinitely high. A microscopic region of aluminum “goes away,” and the fracture gets longer. New fractures grow from the sides of old ones.

It’s like “inside out lightning bolts,” since the fractures are a network of insulators growing in a conductive environment, and the growth is caused by currents rather than voltage which concentrates at the tips of the growing fractures.