The reason that there is a pretty light show
when you microwave
a compact disc
is because of induced surface current
s on the metallic layer
in the disc. The time-varying electric field
excites high-frequency current
s 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 ...