The reason that there is a pretty
light show when you
microwave a
compact disc is because of induced
surface currents on the
metallic layer in the disc. The time-varying
electric field excites high-frequency
currents in this metal layer, which very quickly heat the metal up to the melting point. At this point you see the wonderful
fireworks.
The radial and circumferential burn marks on the disc have to do with the electromagnetic modes that are excited in the microwave, and the resulting electric current distribution on the metal layer.
Consider http://home.c2i.net/metaphor/mvpage.html, where this guy has figured out how to use the microwave energy to melt and cast lots of different metals using a regular microwave oven. Don't try this at home, kids ...